25: Seedless Plants Flashcards
Early Plant Life, Green Algae - Precursors of Land Plants, Bryophytes, Seedless Vascular Plants (185 cards)
1
Q
What are some examples of seedless plants and where are they found?
A
Mosses may grow on a tree trunk, and horsetails (Equisetum sp.) may display their jointed stems and spindly leaves across the forest floor, in damp, shaded environments under tree canopies where dryness is rare.
2
Q
How prevalent are seedless plants?
A
Today, seedless plants represent only a small fraction of the plants in our environment; yet, 300 million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.
3
Q
Which challenges did the ancestors of land plants overcome in order to colonize land?
A
Current evolutionary thought holds that all plants - green algae as well as land dwellers - are monophyletic. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants developed adaptations that allowed them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment.
4
Q
How many species of plants are there?
A
There are more than 300,000 species of catalogued plants. Of these, more than 260,000 are seed plants.
5
Q
Why is there disagreement among biologists about whether green algae should be classified as plants?
A
Most biologists consider green algae to be plants, although others exclude all algae from the plant kingdom. The reason for this disagreement stems from the fact that only green algae, the Charophytes, share common characteristics with land plants (such as using chlorophyll a and b plus carotene in the same proportion as plants). These characteristics are absent in other types of algae. Additionally, green algae lack alternation of generations, sporangia, gametangia, and apical meristem tissues in roots and shoots, as well as waxy cuticles and cell walls with lignin.
6
Q
Why is there disagreement among biologists whether all algae should be considered plants?
A
Some scientists consider all algae to be plants, while others assert that only the Charophytes belong in the kingdom Plantae. These divergent opinions are related to the different evolutionary paths to photosynthesis selected for in different types of algae. While all algae are photosynthetic - that is, they contain some form of a chloroplast - they didn’t all become photosynthetic via the same path.
7
Q
How did green algae evolve?
A
The ancestors to the green algae became photosynthetic by endosymbiosing a green, photosynthetic bacterium about 1.65 billion years ago. That algal line evolved into the Charophytes, and eventually into the modern mosses, ferns, gymnosperms, and angiosperms. Their evolutionary trajectory was relatively straight and monophyletic.
8
Q
How did non-green algae evolve?
A
Other algae - red, brown, golden, stramenopiles, etc. - all became photosynthetic by secondary, or even tertiary, endosymbiotic events; that is, they endosymbiosed cells that had already endosymbiosed a cyanobacterium. These latecomers to photosynthesis are parallels to the Charophytes in terms of autotrophy, but they did not expand to the same extent as the Charophytes, nor did they colonize the land.
9
Q
What challenges do land plants face that water plants do not?
A
Desiccation is a constant danger for an organism exposed to air. Even when parts of a plant are close to a source of water, the aerial structures are likely to dry out. Water also provides buoyancy to organisms. On land, plants need to develop structural support in a medium that does not give the same lift as water. The organism is also subject to bombardment by mutagenic radiation, because air does not filter out ultraviolet rays of sunlight. Additionally, the male gametes must reach the female gametes using new strategies, because swimming is no longer possible. Therefore, both gametes and zygotes must be protected from desiccation.
10
Q
What advantages did land offer to plants?
A
First, sunlight is abundant. Water acts as a filter, altering the spectral quality of light absorbed by the photosynthetic pigment chlorophyll. Second, carbon dioxide is more readily available in air than in water, since it diffuses faster in air. Third, land plants evolved before land animals; therefore, until dry land was colonized by animals, no predators threatened plant life.
11
Q
What is an example of tolerance to desiccation used by plants?
A
Many mosses can dry out to a brown and brittle mat, but as soon as rain or a flood makes water available, mosses will absorb it and are restored to their healthy green appearance.
12
Q
What is an example of plants colonizing humid environments to avoid desiccation?
A
Ferns, which are considered an early lineage of plants, thrive in damp and cool places, such as the understory of temperate forests.
13
Q
What is an example of resistance to desiccation used by plants?
A
Cacti, which minimize the loss of water to such an extent that they can survive in extremely dry environments.
14
Q
Which major adaptations are found in all terrestrial plants?
A
Alternation of generations, a sporangium in which the spores are formed, a gametangium that produces haploid cells, and apical meristem tissue in roots and shoots. The evolution of a waxy cuticle and a cell wall with lignin also contributed to the success of land plants.
15
Q
What is an antheridium?
A
Male gametangium.
16
Q
What is an archegonium?
A
Female gametangium.
17
Q
What is a charophyte?
A
Other term for green algae; considered the closest relative of land plants.
18
Q
What does it mean to be diplontic?
A
Diploid stage is the dominant stage.
19
Q
What is an embryophyte?
A
Other name for land plant; embryo is protected and nourished by the sporophyte.
20
Q
What does it mean to be extant?
A
Still-living species.
21
Q
What does it mean to be extinct?
A
No longer existing species.
22
Q
What is a gametangium?
A
Structure on the (multicellular haploid) gametophyte in which gametes are produced by precursor cells via mitosis.
23
Q
What does it mean to be haplodiplodontic?
A
Haploid and diploid stages alternate.
24
Q
What does it mean to be haplontic?
A
Haploid stage is the dominant stage.
25
What does it mean to be heterosporous?
Produces two types of spores.
26
What does it mean to be homosporous?
Produces one type of spore.
27
What is a megaspore?
Female spore.
28
What is a microspore?
Male spore.
29
What is a non-vascular plant?
Plant that lacks vascular tissue, which is formed of specialized cells for the transport of water and nutrients.
30
What is a seedless vascular plant?
Plant that does not produce seeds.
31
What is a sporocyte?
Diploid cell that produces spores by meiosis.
32
What is sporopollenin?
A tough polymer found in the cell walls surrounding the spores of seedless plants.
33
What is the molecular makeup of sporopollenin?
Sporopollenin is characterized by long chains of organic molecules related to fatty acids and carotenoids; hence the yellow color of most pollen.
34
What is a vascular plant?
A plant containing a network of cells that conducts water and solutes through the organism.
35
What are the steps of the alternation of generations between the 1*n* gametophyte and the 2*n* sporophyte?
First, there are spores, which undergo mitosis to form a haploid multicellular gametophyte. The gametophyte gives rise to gametes, which undergo syngamy (fusion) with other gametes to form a zygote, which undergoes mitosis to form a diploid multicellular sporophyte. The sporophyte bears the sporangia and undergoes meiosis to form spores.
36
How noticeable is the gametophyte phase in plants?
It can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain, in the higher (vascular) plants.
37
How noticeable is the sporophyte phase in plants?
The sporophyte stage is barely noticeable in lower plants (e.g. mosses, liverworts, and lichens). Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines.
38
How does embryo protection distinguish land plants?
Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered from desiccation and other environmental hazards. In both seedless and seed plants, the female gametophyte provides protection and nutrients to the embryo as it develops into the new generation of sporophyte. This distinguishing feature of land plants gave the group its alternate name of embryophytes.
39
In which major plant group did sporangia first appear?
Land plants.
40
What does "sporangia" mean?
Literally, "spore in a vessel", because it is a reproductive sac that contains spores.
41
What are some examples of polyploid sporophytes?
Durum wheat is tetraploid, bread wheat is hexaploid, and some ferns are 1000-ploid.
42
How many types of spores are produced in plants?
Two different types of spores are produced in land plants, resulting in the separation of sexes at different points in the lifecycle. Seedless non-vascular plants produce only one kind of spore and are called homosporous. Gametophytes that result from the germination of spores of homosporous plants produce both male and female gametangia, usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores.
43
Which phase is dominant in seedless non-vascular plants?
The gametophyte phase.
44
In which types of plants is heterospory found?
Heterospory is observed in a few seedless vascular plants and in all seed plants.
45
In what ways is sporopollenin tough?
Sporopollenin is unusually resistant to chemical and biological degradation. In seed plants, which use pollen to transfer the male sperm to the female egg, the toughness of sporopollenin explains the existence of well-preserved pollen fossils.
46
In which major plant groups is sporopollenin found?
Sporopollenin was once thought to be an innovation of land plants; however, the green algae *Coleochaetes* forms spores that contain sporopollenin.
47
How do antheridia interact with archegonia in seedless plants?
The antheridium releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia. The embryo develops inside the archegonium as the sporophyte. Gametangia are prominent in seedless plants, but are very rarely found in seed plants.
48
How do meristems contribute to plant growth?
Shoots and roots of plants increase in length through rapid cell division in a tissue called the apical meristem, which is a small zone of cells found at the shoot tip or root tip. The apical meristem is made of undifferentiated cells that continue to proliferate throughout the life of the plant. Meristematic cells give rise to all the specialized tissues of the organism. Elongation of the shoots and roots allows a plant to access additional space and resources: light in the case of the shoot, and water and minerals in the case of roots. A separate meristem, called the lateral meristem, produces cells that increase the diameter of tree trunks.
49
What is the function of a root cap?
The root cap protects the fragile apical meristem as the root tip is pushed through the soil by cell elongation.
50
Why did vascular systems evolve in plants?
In small plants such as single-celled algae, simple diffusion suffices to distribute water and nutrients throughout the organism. However, for plants to evolve larger forms, the evolution of vascular tissue for the distribution of water and solutes was a prerequisite.
51
Which tissues are found in plant vascular systems?
The vascular system contains xylem and phloem tissues. Xylem conducts water and minerals absorbed from the soil up to the shoot, while phloem transports food derived from photosynthesis throughout the entire plant.
52
Why did root systems evolve?
Root systems evolved to take up water and minerals from the soil, and to anchor the increasingly taller shoot in the soil.
53
What are some of the pros and cons of cuticles?
In land plants, a waxy, waterproof cover called a cuticle protects the leaves and stems from desiccation. However, the cuticle also prevents intake of carbon dioxide needed for the synthesis of carbohydrates through photosynthesis. To overcome this, stomata or pores that open and close to regulate traffic of gases and water vapor appeared in plants as they moved away from moist environments to drier habitats.
54
How do land plants protect themselves from UVB light?
Water filters ultraviolet-B (UVB) light, which is harmful to all organisms, especially those that must absorb light to survive. This filtering does not occur for land plants. This presented an additional challenge to land colonization, which was met by the evolution of biosynthetic pathways for the synthesis of protective flavonoids and other compounds: pigments that absorb UV wavelengths of light and protect the aerial parts of plants from photodynamic damage.
55
How do plants deter predation?
Plants cannot avoid being eaten by animals. Instead, they synthesize a large range of poisonous secondary metabolites: complex organic molecules such as alkaloids, whose noxious smells and unpleasant taste deter animals. These toxic compounds can also cause severe diseases and even death, thus discouraging predation. Humans have used many of these compounds for centuries as drugs, medications, or spices.
56
How was plant evolution influenced by animal evolution?
As plants co-evolved with animals, the development of sweet and nutritious metabolites lured animals into providing valuable assistance in dispersing pollen grains, fruit, or seeds.
57
What are the periods of the Paleozoic era?
It starts with the Cambrian period, followed by the Ordovician, Silurian, Devonian, Carboniferous, and Permian.
58
When did land plants evolve?
The major event to mark the Ordovician, more than 500 million years ago, was the colonization of land by the ancestors of modern land plants. Fossilized cells, cuticles, and spores of early land plants have been dated as far back as the Ordovician period in the early Paleozoic era. The oldest-known vascular plants have been identified in deposits from the Devonian.
59
What is one of the most important sources of information regarding the evolution of land plants?
One of the richest sources of information is the Rhynie chert, a sedimentary rock deposit found in Rhynie, Scotland, where embedded fossils of some of the earliest vascular plants have been identified.
60
What are the extinct vascular plants?
The extinct vascular plants, classified as zosterophylls and trimerophytes, most probably lacked true leaves and roots and formed low vegetation mats similar in size to modern-day mosses, although some trimerophytes could reach one meter in height.
61
What is *Cooksonia*?
The later genus *Cooksonia* (beyond zosterophylls and trimerophytes), which flourished during the Silurian, has been extensively studied from well-preserved samples. Imprints of *Cooksonia* show slender branching stems ending in what appear to be sporangia. From the recovered specimens, it is not possible to establish for certain whether *Cooksonia* possessed vascular tissues.
62
Which land plants were found by the end of the Devonian period?
Fossils indicate that by the end of the Devonian period, ferns, horsetails, and seed plants populated the landscape, giving rise to trees and forests. This luxuriant vegetation helped enrich the atmosphere in oxygen, making it easier for air-breathing animals to colonize dry land. Plants also established early symbiotic relationships with fungi, creating mycorrhizae.
63
What is paleobotany?
The study of extinct plants.
64
What are the goals of paleobotany?
Paleobotany addresses the questions of how organisms acquired traits that allow them to colonize new environments - and how the contemporary ecosystem is shaped - through the analysis of fossilized specimens retrieved from field studies, reconstituting the morphology of organisms that disappeared long ago. Paleobotanists trace the evolution of plants by following the modifications in plant morphology: shedding light on the connection between existing plants by identifying common ancestors that display the same traits. This field seeks to find transitional species that bridge gaps in the path to the development of modern organisms
65
How are fossils formed?
Fossils are formed when organisms are trapped in sediments or environments where their shapes are preserved.
66
In what ways do paleobotanists work with fossils?
Paleobotanists collect fossil specimens in the field and place them in the context of the geological sediments and other fossilized organisms surrounding them. The activity requires great care to preserve the integrity of the delicate fossils and the layers of rock in which they are found.
67
What is required for the fossilization of molecular structures?
Preservation of molecular structures requires an environment free of oxygen, since oxidation and degradation of material through the activity of microorganisms depend on its presence.
68
What is an example of the use of analytical chemistry and molecular biology in paleobotany?
The identification of oleanane, a compound that deters pests. Up to this point, oleanane appeared to be unique to flowering plants; however, it has now been recovered from sediments dating from the Permian, much earlier than the current dates given for the appearance of the first flowering plants.
69
In what ways do paleobotanists work with DNA?
Paleobotanists can study fossil DNA, which can yield a large amount of information, by analyzing and comparing the DNA sequences of extinct plants with those of living and related organisms. Through this analysis, evolutionary relationships can be built for plant lineages.
70
Why are some paleobotanists skeptical of the conclusions drawn from the analysis of molecular fossils?
The chemical materials of interest degrade rapidly when exposed to air during their initial isolation, as well as in further manipulations. There is always a high risk of contaminating the specimens with extraneous material, mostly from microorganisms.
71
What are streptophytes?
The green algae and land plants are grouped together into a subphylum called the Streptophytina, and thus are called streptophytes.
72
What are the major groups in land plant classification?
Land plants are classified into two major groups according to the absence or presence of vascular tissue (vascular and non-vascular plants).
73
What are some examples of seedless, non-vascular plants?
Liverworts, mosses, and hornworts are seedless, non-vascular plants that likely appeared early in land plant evolution.
74
What are some examples of seedless vascular plants?
Lycophytes and pterophytes.
75
What are some examples of lycophytes?
Club mosses (not to be confused with the mosses), quillworts, and spike mosses.
76
What are some examples of pterophytes?
Ferns, horsetails, and whisk ferns.
77
When did vascular plants evolve?
The first vascular plants appeared in the late Ordovician and were probably similar to lycophytes and pterophytes.
78
What is a spermatophyte?
Seed plants, which forms the largest group of all existing plants, and dominate the landscape.
79
What are the major divisions of seed plants?
Seed plants include gymnosperms and angiosperms.
80
What is a gymnosperm?
Division of seed plants which produce "naked seeds". This most notably includes conifers.
81
What is an angiosperm?
A major division of seed plants that produces flowers, and is the most successful of all plants.
82
How do angiosperms protect their seeds?
Angiosperms protect their seeds inside chambers at the center of a flower; the walls of the chamber later develop into a fruit.
83
What are bryophytes?
Seedless, non-vascular (land) plants.
84
Why did land plants lose some of the accessory pigments present in brown and red algae?
Sunlight is not filtered by water or other algae on land; therefore, there is no need to collect light at additional wavelengths made available by other pigment coloration.
85
Why are brown, red, and gold algae not considered plants?
Apart from their ability to capture light energy and fix CO2, they lack many structural and biochemical traits that distinguish plants from protists. They have different accessory pigments and types of chlorophyll molecules in addition to chlorophyll *a*.
86
What are some similarities between green algae and land plants?
Green algae contain the same carotenoids and chlorophyll *a* and *b* as land plants. Cells in green algae divide along cell plates called phragmoplasts, and their cell walls are layered in the same manner as the cell walls of embryophytes.
87
What is Chlorophyta?
Green algae.
88
How many species of Chlorophyta are there and where are they found?
Chlorophyta includes more than 7000 different species that live in fresh or brackish water, in seawater, or in snow patches. A few green algae even survive on soil, provided it is covered by a thin film of moisture in which they can live. Periodic dry spells provide a selective advantage to algae that can survive water stress.
89
What are some examples of green algae?
*Spirogyra* and desmids, whose cells contain chloroplasts that display a large variety of shapes, and their cell walls contain cellulose. Some green algae are single cells, such as *Chlorella* and *Chlamydomonas*, which adds to the ambiguity of green algae classification, because plants are multicellular. Other algae, like *Ulva* (commonly called sea lettuce), form colonies.
90
How do green algae reproduce?
Green algae reproduce both asexually, by fragmentation or dispersal of spores, or sexually, by producing gametes that fuse during fertilization. In a single-celled organism such as *Chlamydomonas*, there is no mitosis after fertilization. In the multicellular *Ulva*, a sporophyte grows by mitosis after fertilization. Both *Chlamydomonas* and *Ulva* produce flagellated gametes.
91
What are coleochaetes?
Microscopic green algae that enclose their spores in sporopollenin.
92
What are the closest living relatives of embryophytes?
Green algae in the order Charales, and the coleochaetes. The Charales can be traced back 420 million years.
93
What are Charales?
An order of green algae. They live in a range of fresh water habitats and vary in size from a few millimeters to a meter in length. The representative species is *Chara*, often called muskgrass or skunkweed because of its unpleasant smell. Large cells form the thallus: the main stem of the alga. Branches arising from the nodes are made of smaller cells. Male and female reproductive structures are found on the nodes, and the sperm have flagella. Unlike land plants, Charales do not undergo alternation of generations in their lifecycle. They produce the compounds lignin and sporopollenin, and form plasmodesmata that connect the cytoplasm of adjacent cells. The egg and later, the zygote, form in a protected chamber on the parent plant.
94
What has been discovered through DNA sequence analysis of green algae?
Information from extensive DNA sequence analysis of green algae indicates that the Zygnematales are more closely related to the embryophytes than the Charales. The Zygnematales include the familiar genus *Spirogyra*. As techniques in DNA analysis improve and new information on comparative genomics arises, the phylogenetic connections between species will change. Plant biologists have not yet solved the mystery of the origin of land plants.
95
What is a capsule?
The case of the sporangium in mosses.
96
What is a gemma?
Leaf fragment that spreads for asexual reproduction. Plural: gemmae.
97
What are hornworts?
Group of non-vascular plants in which stomata appear.
98
What are liverworts?
The most primitive group of the non-vascular plants.
99
What are mosses?
Group of bryophytes in which a primitive conductive system appears.
100
What is a peristome?
Tissue that surrounds the opening of the capsule and allows periodic release of spores.
101
What is a protonema?
Tangle of single-celled filaments that forms from the haploid spore.
102
What are rhizoids?
Thin filaments that anchor the plant to the substrate.
103
What is a seta?
A stalk that supports the capsule in mosses.
104
When did bryophytes evolve?
Bryophytes are the group of plants that are the closest extant relative of early terrestrial plants. The first bryophytes (liverworts) most likely appeared in the Ordovician period, about 450 million years ago. Because of the lack of lignin and other resistant structures, the likelihood or bryophytes forming fossils is rather small. Some spores protected by sporopollenin have survived and are attributed to early bryophytes. By the Silurian period, however, vascular plants had spread through the continents. This compelling fact is used as evidence that non-vascular plants must have preceded the Silurian period.
105
How many species of bryophyte are there and where can they be found?
More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They constitute the major flora of inhospitable environments like the tundra, where their small size and tolerance to desiccation offer distinct advantages.
106
How do bryophytes resist desiccation?
They generally lack lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more accurate, bryophytes are commonly called nonvascular plants.
107
What are some features of bryophyte gametophytes and sporophytes?
In a bryophyte, all the conspicuous vegetative organs - including the photosynthetic leaf-like structures, the thallus, stem, and the rhizoid that anchors the plant to its substrate - belong to the haploid organism or gametophyte. The sporophyte is barely noticeable. The gametes formed by bryophytes swim with a flagellum, as do gametes in a few of the tracheophytes. The sporangium - the multicellular sexual reproductive structure - is present in bryophytes and absent in the majority of algae. The bryophyte embryo also remains attached to the parent plant, which protects and nourishes it. This is a characteristic of land plants.
108
What are the phyla of bryophytes?
The liverworts or Hepaticophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.
109
What species are viewed as the plants most closely related to the ancestor that moved to land?
Liverworts (Hepaticophyta).
110
How many species of liverworts are there and where are they found?
Liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species.
111
How do liverworts get their name?
Some gametophytes form lobate green structures. This shape is similar to the lobes of the liver, and hence provides the origin of the name given to the phylum.
112
How do liverworts manage water intake and gas exchange?
Openings that allow the movement of gases may be observed in liverworts. However, these are not stomata, because they do not actively open and close. The plant takes up water over its entire surface and has no cuticle to prevent desiccation.
113
How does sexual reproduction occur in liverworts?
The cycle starts with a release of haploid spores from the sporangium that developed on the sporophyte. Spores disseminated by wind or water germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Male and female gametangia develop on separate, individual plants. Once released, male gametes swim with the aid of their flagella to the archogonium, and fertilization ensues. The zygote grows into a small sporophyte, a slender stalk called a seta, still attached to the parent gametophyte. It will give rise, by meiosis, to the next generation of spores.
114
How do liverworts reproduce using gemmae?
Liverwort plants can reproduce asexually, by the breaking of branches or the spreading of leaf fragments called gemmae. In this latter type of reproduction, the gemmae - small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus - are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes.
115
Where are hornworts found?
They have colonized a variety of habitats on land, although they are never far from a source of moisture.
116
What are some characteristics of gametophytes and sporophytes in hornworts?
The short, blue-green gametophyte is the dominant phase of the lifecycle of a hornwort. The narrow, pipe-like sporophyte is the defining characteristic of the group. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant.
117
What are some characteristics of hornworts?
Stomata appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus contain a single chloroplast. Meristem cells at the base of the plant keep dividing and adding to its height. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.
118
What is the lifecycle of hornworts?
The lifecycle of hornworts follows the general pattern of alternation of generations. The gametophytes grow as flat thalli on the soil with embedded gametangia. Flagellated sperm swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that eventually splits open, releasing spores. Thin cells called pseudoelaters surround the spores and help propel them further in the environment. Unlike the elaters observed in horsetails, the hornwort pseudoelaters are single-celled structures. The haploid spores germinate and give rise to the next generation of gametophyte.
119
How many species of mosses have been identified and where are they found?
More than 10,000 species of mosses have been catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests.
120
How do mosses live in the tundra and what role do they play?
In the tundra, the mosses' shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox.
121
In what ways are mosses sensitive?
Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.
122
What are some characteristics of moss leaves?
Green, flat structures - resembling true leaves, but lacking vascular tissue - are attached in a spiral to a central stalk. The plants absorb water and nutrients directly through these leaf-like structures. Some mosses have small branches.
123
What reproductive characteristics of green algae are found in mosses?
Some primitive traits of green algae, such as flagellated sperm, are still present in mosses that are dependent on water for reproduction.
124
What are some characteristics of moss sporophytes?
Stomata are present on the stems of the sporophyte, and a primitive vascular system runs up the sporophyte's stalk.
125
How do mosses anchor themselves to a substrate?
Mosses are anchored to the substrate - whether it is soil, rock, or roof tiles - by multicellular rhizoids. These structures are precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk.
126
How do mosses reproduce?
The moss lifecycle follows the pattern of alternation of generations. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema - usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Rhizoids form at the base of the gametophore. Gametangia of both sexes develop on separate gametophores. The antheridium produces many sperm, whereas the archegonium forms a single egg. At fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
127
What is the function of setae?
The slender seta (plural, setae) contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule.
128
How do peristomes work?
A structure called a peristome increases the spread of spores after the tip of the capsule falls off at dispersal. The concentric tissue around the mouth of the capsule is made of triangular, close-fitting units, a little like "teeth"; these open and close depending on moisture levels, and periodically release spores.
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What does it mean to be adventitious?
It describes an organ that grows in an unusual place, such as a roots growing from the side of a stem.
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What are club mosses?
Members of the phylum Lycopodiophyta, and the earliest group of seedless vascular plants.
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What is a fern?
A seedless vascular plant that produces large fronds; the most advanced group of seedless vascular plants.
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What is a horsetail?
A seedless vascular plant characterized by joints.
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What is lignin?
A complex polymer that is impermeable to water and confers mechanical strength to vascular tissue.
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What is a lycophyte?
Club moss.
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What is a megaphyll?
A “big leaf” with a pattern of branching veins.
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What is a microphyll?
A “little leaf” of small size and simple vascular system with a single unbranched vein - a bundle of vascular tissue made of xylem and phloem - that runs through the center of the leaf.
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What is peat moss?
*Sphagnum*.
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What is phloem?
Tissue responsible for the transport of sugars, proteins, and other solutes.
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What is a sporophyll?
A leaf modified structurally to bear sporangia.
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What are strobili?
Cone-like structures that contain the sporangia.
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What is a tracheophyte?
A vascular plant.
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What is a vein?
A bundle of vascular tissue made of xylem and phloem.
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What is a whisk fern?
A seedless vascular plant that lost roots and leaves by reduction.
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What is xylem?
Tissue responsible for long-distance transport of water and nutrients.
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How prevalent are tracheophytes?
Tracheophytes are the dominant and most conspicuous group of land plants. More than 260,000 species of tracheophytes represent more than 90% of Earth's vegetation.
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What differences between tracheophytes and bryophytes explain their ability to colonize dry environments?
Bryophytes may have been successful at the transition from an aquatic habitat to land, but they are still dependent on water for production, and absorb moisture and nutrients through the gametophyte surface. The lack of roots for absorbing water and minerals from the soil, as well as a lack of reinforced conducting cells, limits bryophytes to small sizes. Although they may survive in reasonably dry conditions, they cannot reproduce and expand their habitat range in the absence of water. Vascular plants, on the other hand, can achieve enormous heights, thus competing successfully for light. Photosynthetic organs become leaves, and pipe-like cells or vascular tissues transport water, minerals, and fixed carbon throughout the organism.
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What is the dominant phase of the lifecycle of seedless vascular plants?
Diploid sporophyte. The gametophyte is now an inconspicuous, but still independent, organism. Throughout plant evolution, there is an evident reversal of roles in the dominant phase of the lifecycle.
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What is required for fertilization in seedless vascular plants?
Seedless vascular plants still depend on water during fertilization, as the sperm must swim in a layer of moisture to reach the egg. This step in reproduction explains why ferns and their relatives are more abundant in damp environments.
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When did vascular tissue evolve?
The first fossils that show the presence of vascular tissue date to the Silurian period, about 430 million years ago.
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How is vascular tissue arranged?
The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem.
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What is the role of xylem tissue?
Xylem is the tissue responsible for the storage and long-distance transport of water and nutrients, as well as the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. Xylem conductive cells incorporate the compound lignin into their walls, and are thus described as lignified. With their rigid cell walls, the xylem cells provide support to the plant and allow it to achieve impressive heights.
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What are the advantages of height in plants?
Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores further away, thus expanding their range. By growing higher than other plants, tall trees cast their shadow on shorter plants and limit competition for water and nutrients in the soil.
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What is the role of phloem tissue?
It transports sugars, proteins, and other solutes throughout the plant. Phloem cells are divided into sieve elements (conducting cells) and cells that support the sieve elements.
154
When did roots evolve?
Roots are not well preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution than vascular tissue. The development of an extensive network of roots represented a significant new feature of vascular plants.
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What are the benefits of roots over rhizoids?
Thin rhizoids attach bryophytes to the substrate, but these rather flimsy filaments do not provide a strong anchor for the plant, and do not absorb substantial amounts of water and nutrients. In contrast, roots, with their prominent vascular tissue system, transfer water and minerals from the soil to the rest of the plant. The extensive network of roots that penetrates deep into the soil to reach sources of water also stabilizes trees by acting as a ballast or anchor. The majority of roots also establish a symbiotic relationship with fungi, forming mycorrhizae, which benefit the plant by greatly increasing the surface area of absorption of water and soil minerals and nutrients.
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How are leaves beneficial to plants?
Accompanying the prominence of the sporophyte and the development of vascular tissue, the appearance of true leaves improved their photosynthetic efficiency. Leaves capture more sunlight with their increased surface area by employing more chloroplasts to trap light energy and convert it to chemical energy, which is then used to fix atmospheric carbon dioxide into carbohydrates. The carbohydrates are exported to the rest of the plant by the conductive cells of phloem tissue.
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How did microphylls evolve?
The microphyll, or "little leaf", can be dated to 350 million years ago in the late Devonian. Microphylls may have originated from the flattening of lateral branches, or from sporangia that lost their reproductive capabilities. Microphylls are present in club mosses and probably preceded the development of megaphylls.
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How did megaphylls evolve?
Megaphylls most likely appeared independently several times during the course of evolution. Their complex network of veins suggest that several branches may have combined into a flattened organ, with the gaps between the branches being filled with photosynthetic tissue.
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What are some examples of sporophylls?
Pine cones, mature fronds of ferns, and flowers are all sporophylls - leaves that were modified structurally to bear sporangia. Strobili are cone-like structures that contain sporangia, and are prominent in conifers and are commonly known as pine cones.
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When did swamp forests appear?
By the late Devonian period, plants had evolved vascular tissue, well-defined leaves, and root systems. With these advantages, plants increased in height and size. During the Carboniferous period, swamp forests of club mosses and horsetails - some specimens reaching heights of more than 30m - covered most of the land. These forests gave rise to the extensive coal deposits that gave the Carboniferous its name.
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How large are club mosses?
Club mosses dominated the landscape of the Carboniferous, growing into tall trees and forming large swamp forests. Today's club mosses are diminutive, evergreen plants consisting of a stem (which may be branched) and microphylls.
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How many species of Lycopodiophyta have been identified and what are some examples?
The phylum Lycopodiophyta consists of close to 1200 species, including the quillworts (*Isoetales*), the club mosses (*Lycopodiales*), and spike mosses (*Selaginellales*), none of which are true mosses or bryophytes.
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What are some characteristics of the lifecycle of lycophytes?
Lycophytes follow the pattern of alternation of generations seen in bryophytes, except that the sporophyte is the major stage of the lifecycle. The gametophytes do not depend on the sporophyte for nutrients. Some gametophytes develop underground and form mycorrhizal associations with fungi. In club mosses, the sporophyte gives rise to sporophylls arranged in strobili, cone-like structures that give the class its name. Lycophytes can be homosporous or heterosporous.
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How are horsetails, whisk ferns, and ferns classified?
Horsetails, whisk ferns, and ferns belong to the phylum Monilophyta.
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How are horsetails classified?
Horsetails are placed in the class Equisetopsida. The single genus *Equisetum* is the survivor of a large group of plants, known as Arthrophyta, which produced large trees and entire swamp forests in the Carboniferous.
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What are some characteristics of horsetails?
Horsetails are usually found in damp environments and marshes. The stem of a horsetail is characterized by the presence of joints or nodes, hence the name Arthrophyta (arthro- = "joint"; -phyta = "plant"). Leaves and branches come out as whorls from the evenly-spaced joints. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem. Silica collects in the epidermal cells, contributing to the stiffness of horsetail plants. Underground stems known as rhizomes anchor the plants to the ground. Modern-day horsetails are homosporous and produce bisexual gametophytes.
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What are some characteristics of whisk ferns?
While most ferns form large leaves and branching roots, the whisk ferns, Class Psilotopsida, lack both roots and leaves, probably lost by reduction. Photosynthesis takes place in their green stems, and small yellow knobs form at the tip of the branch stem and contain the sporangia. Whisk ferns were considered an early pterophyte, but recent comparative DNA analysis suggests that this group may have lost both vascular tissue and roots through evolution, and is more closely related to ferns.
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What are some general characteristics of ferns?
With their large fronds, ferns, also Class Psilotopsida, are the most readily recognizable seedless vascular plants. They are considered the most advanced seedless vascular plants and display characteristics commonly observed in seed plants. More than 20,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist, shaded places. Ferns made their appearance in the fossil record during the Devonian period and expanded during the Carboniferous.
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What are some physical characteristics of ferns?
The dominant stage of the lifecycle of a fern is the sporophyte, which consists of large compound leaves called fronds. Fronds fulfill a double role; they are photosynthetic organs that also carry reproductive organs. The stem may be buried underground as a rhizome, from which adventitious roots grow to absorb water and nutrients from the soil; or, they may grow above ground as a trunk in tree ferns.
170
What is a fiddlehead?
The tip of a developing fern frond is rolled into a crozier, or fiddlehead. Fiddleheads unroll as the frond develops.
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What are some characteristics of sporophytes in ferns?
Most ferns produce the same type of spores and are therefore homosporous. The diploid sporophyte is the most conspicuous stage of the lifecycle. On the underside of its mature fronds, sori (singular, sorus) form as small clusters where sporangia develop.
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How are fern gametophytes produced?
Inside the sori, spores are produced by meiosis and released into the air. Those that land on a suitable substrate germinate and form a heart-shaped gametophyte, which is attached to the ground by thin filamentous rhizoids.
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How are fern sporophytes produced?
The inconspicuous gametophyte harbors both sex gametangia. Flagellated sperm released from the antheridium swim on a wet surface to the archegonium, where the egg is fertilized. The newly-formed zygote grows into a sporophyte that emerges from the gametophyte and grows by mitosis into the next generation sporophyte.
174
What are some jobs of a landscape designer?
A landscape designer will plan traditional public spaces - such as botanical gardens, parks, college campuses, gardens, and larger developments - as well as natural areas and private gardens. The restoration of natural places encroached on by human intervention, such as wetlands, also requires the expertise of a landscape designer.
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What are the educational requirements of a landscape designer?
A landscape designer's education includes a solid background in botany, soil science, plant pathology, entomology, and horticulture. Coursework in architecture and design software is also required for the completion of the degree.
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What are some considerations in landscape design?
The successful design of a landscape rests on an extensive knowledge of plant growth requirements, such as light and shade, moisture levels, compatibility of different species, and susceptibility to pathogens and pests. The future growth of individual plants must be taken into account, to avoid crowding and competition for light and nutrients. The appearance of the space over time is also of concern. Shapes, colors, and biology must be balanced for a well-maintained and sustainable green space. Art, architecture, and biology blend in a beautifully designed and implemented landscape.
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What are the first organisms to colonize an area?
Mosses and liverworts are often the first macroscopic organisms to colonize an area, both in a primary succession - where bare land is settled for the first time by living organisms - or in a secondary succession, where soil remains intact after a catastrophic event wipes out many existing species. Their spores are carried by the wind, birds, or insects. Once mosses and liverworts are established, they provide food and shelter for other species.
178
What is the importance of mosses in tundra?
In a hostile environment, like the tundra where the soil is frozen, bryophytes grow well because they do not have roots and can dry and rehydrate rapidly once water is again available. Mosses are at the base of the food chain in the tundra biome. Many species - from small insects to musk oxen and reindeer - depend on mosses for food. In turn, predators feed on the herbivores, which are the primary consumers.
179
How do bryophytes improve soil quality?
Some reports indicate that bryophytes make the soil more amenable to colonization by other plants. Because they establish symbiotic relationships with nitrogen-fixing cyanobacteria, mosses replenish the soil with nitrogen.
180
Why are mosses and lichens not as prevalent in urban and suburban environments?
At the end of the nineteenth century, scientists observed that lichens and mosses were becoming increasingly rare in urban and suburban areas. Since bryophytes have neither a root system for absorption of water and nutrients, nor a cuticle layer that protects them from desiccation, pollutants in rainwater readily penetrate their tissues; they absorb moisture and nutrients through their entire exposed surface. Therefore, pollutants dissolved in rainwater penetrate plant tissues readily and have a larger impact on mosses than on other plants. The disappearance of mosses can be considered a bioindicator for the level of pollution in the environment.
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How do ferns contribute to the environment?
Ferns contribute to the environment by promoting the weathering of rock, accelerating the formation of topsoil, and slowing down erosion by spreading rhizomes in the soil. The water ferns of the genus *Azolla* harbor nitrogen-fixing cyanobacteria and restore this important nutrient to aquatic habitats.
182
What are some uses of seedless plants for humans?
Seedless plants have historically played a role in human life through uses as tools, fuel, and medicine.
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What are some uses of *Sphagnum*?
Dried peat moss, *Sphagnum*, is commonly used as a fuel in some parts of Europe and is considered a renewable resource. *Sphagnum* bogs are cultivated with cranberry and blueberry bushes. The ability of *Sphagnum* to hold moisture makes the moss a common soil conditioner. Florists use blocks of *Sphagnum* to maintain moisture for floral arrangements.
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What are some uses of ferns?
The attractive fronds of ferns make them a favorite ornamental plant. Because they thrive in low light, they are well-suited as house plants. More importantly, fiddleheads are a traditional spring food of Native Americans in the Pacific Northwest, and are popular as a side dish in French cuisine. The licorice fern, *Polypodium glycyrrhiza*, is part of the diet of the Pacific Northwest coastal tribes, owing in part to the sweetness of its rhizomes. It has a faint licorice taste and serves as a sweetener. The rhizome also figures in the pharmacopeia of Native Americans for its medicinal properties and is used as a remedy for sore throat.
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How do seedless vascular plants impact human life via coal?
By far the greatest impact of seedless vascular plants on human life comes from their extinct progenitors. The tall club mosses, horsetails and tree-like ferns that flourished in the swampy forests of the Carboniferous period gave rise to large deposits of coal throughout the world. Coal provided an abundant source of energy during the Industrial Revolution, which had tremendous consequences on human societies, including rapid technological progress and growth of large cities, as well as the degradation of the environment. Coal is still a prime source of energy and also a major contributor to global warming.
