Plants for Midterm Flashcards
1
Q
• As plants have adapted to terrestrial environment complex bodies with extensive specialization have evolved
A
o A waxy cuticle
o CO2 and oxygen diffusion through stomata
o Photosynthetic cells contain chloroplasts with chlorophyll a and chlorophyll b
o Carbohydrates is stored in the form of starch, generally in chloroplast and other plastids
o Mitosis occurs in all plants proceeding through the typical phases
2
Q
• Plants produce their gametes
A
gametangia
3
Q
gametangia
A
(organs having protective jackets of sterile cells that prevent gametes from drying out)
4
Q
• In the life cycle of all plants, an alternation of generation, occurs in which
A
haploid gametophytes and diploid sporophytes take turns in producing one another
5
Q
• The life cycle of all plants features an alternation of generation
A
One generation in the gametophyte, a multicellular individuals with haploid cells, and the other generation is the sporophyte, a multicellular individual with diploid cell
6
Q
• There are four key traits that suggest that land plants relationship with charophyceans
A
o The cells of both lands plants and charophyceans have rosette cellulose synthesizing complexes that synthesize the cellulose microfibrils of the cell wall
o Peroxisomes enzyme of both land plants and charophyceans contain enzymes that help minimize the loss of organic products as a result of photorespiration
o The structure of the flagellated sperm in species of land plants that have flagellated sperm closely resemble that of charophycean sperms
7
Q
Formation of phragmoplast
A
• Certain details of cell division occur only in land plant and charophycean, during the synthesis of new cell wall between recently dividing cells
8
Q
The first period of evolution
A
during the Ordovician period of the plaeozoic era, 475 million years ago (m.y.a)
• The first terrestrial adaptation included spores toughened by sporopollenin, and jacketed gametangia that protected gametes and embryos
• Vascular tissue also evolved relatively early in the plant history
• Most bryophytes lack vascular tissues but present in some bryophytes
9
Q
The second major period of plant evolution
A
- Was diversification of vascular plants, during the early Devonian period, about 400 m.y.a
- The earliest vascular plant lack seeds, a condition still found in ferns and a few other groups of seedless vascular plants
10
Q
The third period of plant evolution
A
- Began with the origin of seeds, a structure that advanced to colonization of land by further protecting the plant embryos from desiccation and other hazards condition
- Early seed plants gave rise to many types of gymnosperms
- Vascular plants with seeds arose about 350 m.y.a, near the end of Devonian period
11
Q
The fourth period of plant evolution
A
• History was with the emergence of the flowering plants
12
Q
During the early cretaceous period
A
• In the Mesozoic era, about 130 m.y.a
• The flower is a complex reproductive structure that bear seed within protective chamber
•
13
Q
Bryophytes
A
• Non-vascular plants • Bryophytes are represented by three phyla or division o Phylum Hepatophyta (liverworts) Phylum Anthocerophyta (hornworts) Phylum Bryophyta (mosses)
14
Q
Bryophytes display two adaptations
A
first they are covered by waxy cuticle that helps the body to retain water, and second adaptation is their gametes develop gametangia, which is a multichamber that keep gametes moist
15
Q
male gametes produced
A
gametangia known as antheridia, produce flagellated sperm
16
Q
female gametangia
A
archegonia, in which one egg is produced
17
Q
how do byrophytes obtain water
A
- Since most byrophytes have no vascular tissue water move over the surface of plants
- They must imbibe (process of absorbing water) it like a sponge and distribute it throughout the plant
18
Q
Phylum Hepatophyta (liverworts
A
- Their bodies are divided into lobes giving an appearance of a liver, example of a liverwort is species known as Marchantia
- In those plants the gametangia looks like miniature tree
19
Q
Phylum Anthocerophyta (hornworts
A
• Hornworts resemble liver worts except their sporophyte are elongated capsules that grow like horns from the mat like gametophyte
20
Q
Phylum Bryophyta (mosses
A
- Each plant of the mate grips the substrate with a root-like structure called rhizoids
- Most photosynthesis occurs in the upper part of the plant
21
Q
Vascular Plants
A
• Seedless vascular plants • Phylum Lycophyta (Lycopodium) (club mosses or ground pine) • Phylum Pterophyta (Ferns) • Phylum Lycophyta o Lycopodium o Selaginella (spike mosses) o Isoetes (Quillworts)
22
Q
Phylum Pterophyta
• Ferns
• Equisetum (horse tail)
• Psiletum (whisk fern)
A
- they are the most diverse in temperate forest
- most ferns have leaves commonly called Fronds that are compound leaves, meaning that each leaf is divided into several leaflets
- the frond grows as its coiled tips the fiddlehead, unfurls
- the leaves may sprout directly from a prostate stem (Rhizome)
23
Q
Tropical Ferns
A
• which in of trees, by contrast have upright stem many meters tall
24
Q
how are sporangia of ferns arranged
A
- the sporangia of many ferns are arranged in clusters called sori (sorus)
- the spores are equipped with spring like devices that allow the spores to travel several meters, these devices called Elators
25
Seed Vascular Plants
| • Three life cycle modifications contributed to the success of seed plants as terrestrial organisms
o The gametophyte of seed plants became even more reduced then in ferns and other seedless plants
o Pollination replaced something as the mechanism for delivering sperm nuclei to the egg
o The seed evolved instead of the zygote developing directly into a young sporophyte the zygote of a seed plant develops into an embryo that is packaged with a food supply within a seed coat
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Gymnosperms(phylums)
* Phylum cycodophyta
* Phylum ginkgophyta
* Phylum gnetophyta
* Phylum coniferophyta ( largest phylum)
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Phylum coniferophyta
* The name conifer comes from the reproductive structures of these plants, the cones
* Pines , fir, spruce, larches, junipers, cedars, cypresses, and redwood all belong to this phylum coniferophyta
* There are about 550 species
* The needle shaped leaves of pines and ferns are adapted to dry conditions
* We get most of our lumber and paper pulp from the wood of conifer
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Phylum Ginkgophyta
* This phylum contains one species Ginkgo biloba
* It has fan-like leaves that turn gold and are deciduous in autumn
* Ginkgo is a source of a popular medicine that is believed to improve memory
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Phylum Cycadophyta
* Superficially resemble palms
| * Cycods seeds develop on the surface of the reproductive leaves
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Phylum: Gnetophyta
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• it contains three genera
• 1) Welwitchia
o has giant strap like leaves
• 2) Gnetum
o grows in the Tropics as tree vines
• 3) Ephedra
o is a shrub of American deserts
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The flowering plant(phylum)
Phylum: anthophyta
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Phylum: anthophyta
* Today the flowering plants are the most diverse and geographically wide spread of all plants
* About 300,000 species are known, compared to 550 species of gymnosperms
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The angiosperms
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• The angiosperm are split into two classes
o Monocotyledon (monocot)
o Dicotyledon (dicot)-Eudicots
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example of monocot angiosperms
lilies, orchids, palm, grasses, sugar cane, grain crops ( corn, wheat, rice others)
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Example dicot angiosperms
roses, peas, buttercups, sunflower, oak, maple
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how angiosperms transfer pollen
• Many angiosperms employ insects and other animals to transfer pollen from male to female gametophyte
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• Also during the development of angiosperms the xylem became more specialized for water transport
• The xylem in conifers mainly consist of tracheids, while xylem in angiosperm consists of tracheids and vesicle elements
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The flower
* the outstanding characteristics of angiosperms is the flower
* flowers may be clustered in various ways into aggregation called inflorescence
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sepal
usually green in colour and collectively called calyx
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petal
the bright coloured part of the flower, collectively called corolla
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stamen
anther, filament(male gametophyte)
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carpel
stigma, style and ovary including the ovules (female gametophyte)
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complete flower
• flower that contains the four parts
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incomplete flower
• flower missing any of the four parts
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perfect flower
• flower contains both the male and female gametophyte
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imperfect flower
• flower only with stamens ( stamenata flower) or only with carpel ( carpellate flower)
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monoecious plant
• if then stamenata flower and the carpellate flower are on the same individual plant
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dioecious plant
• if the stamenata and carPellate flowers on separate individual plants
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Point of insertions
- if the sepals, petals and stamens are attached to the receptacle below the ovary
, the ovary is said to be superior and the flower is said to be hypogenous flower
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perigenous flower
• in which the stamens and petals are attached to form the middle of the ovary
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epignous
• (inferior ovary) the sepals, petals and stamens apparently grow from the top of the ovary
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The positions of ovules attachment to the ovary(3 types)
axial,free central, basal
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axial placentation
• occurs in flowers with compound ovary and the ovules attached to the central column of tissue
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free central placentation
• in which the ovules attached to the central column of tissue but also without the ovary without septa
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basal placentation
• found in some flowers with a single ovule at the very base of unilocular ovary
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Actinomorphic flower (regular)
• the flower in which the corolla is made of petals of similar shape that radiate from the center of the flower
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Zygomorphic flower
• the flower in which one or more members of the flower are different from other members of the flower (bilateral symmetrical)
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Microsporogenesis (development of pollen grain)
* in the sporogenous tissue the microsporoge mother cell undergoes meiosis producing tetrads of four haploid cells by cytokinesis producing four separate cells, later develop thick walls (exinc) represent then outer shell of the pollen grain
* each mature pollen grain contains a tube nucleus and generate a cell which will form the sperm nuclei
59
Megasporogenesis (development of ovules)
* Development of female gametophyte
* Within a flower there is a diploid megaspore mother cell (megapsorocyst) in each ovule of the ovary
* Then the megaspore mother cell undergoes meiosis
* Four haploid megaspores, three of these disintegrated leaving one, the functional megaspore, whose nucleus undergoes mitotic division three times, producing 8 haploid cells
* This is the female gametophyte (also called embryo sac)
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Pollination
* Pollination is simply the transfer of the pollen rain from the anther to the stigma
* Pollination is brought about by wind or with assistants of particular pollination
* Most of the flowering plants, their flowers, are adapted to animal pollinators
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Self Compatibility/ Self- incompatibility
* Some flowers such as garden peas, self-fertilize
* This process is called “selfing”
* However, many angiosperms species, have mechanisms that make it difficult or impossible for a flower to fertilize itself (self-incompatible)
* The most common anti-selfing mechanism in flowering plants known as self-incompatibility
* The ability of a plant to reject its own pollen and some times the pollen of other closely related individuals
* The various barriers that prevent self-fertilization contribute to genetics variety by ensuring that the sperm and eggs come from different parents
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Fertilization
* Once the pollen rains land on the stigma, its germinates forming the pollen tube
* The germinated pollen grain containing a tube nucleus and the 2 sperm nuclei, this is a mature male gametophyte
* Then the pollen tube grows through the style and reaches to the micropyle
* Now double fertilization occurs (this is a characteristic of the flowering life cycle)
* One sperm nucleus unites with the egg →zygote (2N)
* The other sperm unite with the polar nuclei forming 3N endosperm nucleus
* Then tube nucleus disintegrates the endosperm nucleus→endosperm
* The zygote(2n)→ embryo
* The integument →seed coat
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Development of the embryo
* The early division of the zygote produces two parts, the embryo and the lower part is the suspensor, which anchors the embryo and transfers nutrients to it from the sporophyte plant
* Soon the cotyledons or seed leaves can be seen
* At this point the dicot embryo is a heart shaped and alter it comes torpedo shaped
* At this stage it is possible to see the shoot apex and the root apex
* These two areas contain the two meristomatic region which brings the primary growth in a plant
* Monocot differ from the dicot by having only one cotyledons
* Another important difference between monocot and dicot is the manner in which nutrients are stored in the seed
* In a monocot seed, the cotyledons rarely store food, rather than absorb it from the endosperm and passes them to the embryo
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epicotyle
is the portion of the stem above the attachment of the cotyledons
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Hypocotyle
is the motion of the stem below the attachment of the cotyledon
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radicle
is the part of the embryo that contains the root apical meristem and becomes the first (primary) root of the seedling
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growth of seed depend on 3 factors
• Among the external factors, 3 are specifically important
o Water
o Oxygen
o Temperature
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early stages of germination
* During the early stages of germination respiration may be entirely anaerobic, but as soon as the seed coat is ruptured, the seed switches to aerobic respiration
* Most seeds germinate at optimum temperature of 25-30 degrees
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Dormancy
is of great survival value to the plant, as a means of ensuring that conditions will be favorable for growth of the seedling when germination occurs
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* Some seeds must pass through the intestine of birds or mammals before they will germinate
* Resulting in wider dispersal of the species
* Some desert species will germinate only when inhibitors in their seeds coats are leached away by rainfall
* Still other seeds lie dormant until the intense heat of a fire cracks the seed coat
* This promoting survival of the species in areas frequently swept by fires
as above
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First step in germination
inhibition, • Hydration causes the seed to expand and rupture its coat and also trigger a series of metabolic activities that cause the seed to resume growth
• The first organ to emerge from germinating seed is the radicle (embryonic root)
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second step in germination
* Next the shoot tip must break through the soil surface, in many dicots a hook forms in the hypocotyl and the growth pushes the hook above the ground
* Stimulated by light, the hypocotyle straightens raising the cotyledons and epicotyle
* The eliminated epicotyle now spread it’s plumule and the first foliage leaves expand making photosynthesis
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Fruit Development
* Fertilization triggers hormonal changes that cause the ovary to begin its transformation into a fruit
* Sometimes after portions of the flower or the whole inflorescence develop into a fruit
* As the ovary develops into a fruit, its wall often thickens and becomes differentiated into distinct layers
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Fruits are generally classified as
simple, multiple, or aggregate
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Simple fruits
develop from one carpel or several united carpels
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Aggregate fruits
consist of a number of separate carpel of one gynoecium ex. Raspberries, or strawberries
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Multiple fruits
consist of ovaries of more than one flower ex. Pineapple, is a multiple fruit consisting of an inflorescence with many separate ovaries fused on the axis
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* The simple fruit are by far the most diverse of the three groups
* They may be
soft and fleshy. Dry and woody, papery
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Soft and fleshy
3 main types known as berry, drupe, pome
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Berry Type
inner coat of the fruit is flesh ex. Tomatoes, grapes
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Drupe
there are one to several carpels, but each contains a single seed ex. Peach, cherry, olive, plums
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Pome
derived from a compound ovary, it is inferior ex. Apples
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Asexual reproduction in plants
• In asexual reproduction the offspring derived from a single parent without genetic recombination (genetically identical to parents)
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Fragmentation
• Separation of a parent plant into parts that develop into a whole plant
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Apomixes
* In apomixes throe is no joining of sperm and egg
* In a stem a diploid cell in the ovule gives rise to the embryo and the ovules mature into seeds without fertilization ex. Dandelion plants
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The tap root system found in most dicot plants
* Consist of one main vertical root (tap root) that develops from an embryonic root
* The tap root gives rise to lateral roots
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Feverous root system
consists of a mat of generally thin roots spreading out below thin roots spreading below the soil surface, with no roots standing out as the main one
• This type of root: mostly found in seedless vascular plants and monocot (grasses)
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Fibrous root system are also said to be “adventitious
• This term given tot any root not only originating form the root radical
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Prop roots
these roots emerge from the stem node to support tall top-heavy plants ex. Corn
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Storage root
root store food and water as in sweet potatoes
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Buttress root
aerial root that look like buttresses support the tall trunk of some tropical trees (cesba tree)
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Pneumatophores
also known as air roots are produced by mangroves that inhabit tidal swamp by projecting above the surface, they enable the root system to obtain oxygen
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Stem
* Is an organ consisting of alternating system of nodes (the points at which leaves are attached)
* Internode: stem segment between the nodes
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Stolons
horizontal stem that grows along the surface ex. Strawberries
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Bulbs
vertical underground shoot mostly of enlarged bases of leaves that store food ex. Onion bulbs
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Rhizome
a horizontal stem, that grows just below the surface, and aerial stems develop from the nodes
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Tubers
enlarged stem specialized for storing food, such as potatoes
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3 primary meristems: develop from the apical meristems are
o Protoderm →epidermal tissues or epidermis
o Ground meristem → ground tissues
o Procambium → primary vascular tissue (primary xylem and primary phloem)
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Secondary growth
o Which occurs in woody plants such as trees and shrubs results from lateral meristem
• Vascular cambium → secondary xylem and phloem
• Cork cambium →new cork cells
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Life span of Plants
* Annual
* Biennial
* Perennials
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Epidermis
• The entire body of a non woody plant (herbaceous) and young woody plants are covered in epidermis
102
epidermal cells characterize by the following
o Closely packed
o The walls outer epidermal cells covered by caticel (waxy material)
o Epidermal cell contains an opening known as stomata, surrounded by guard cells
o The root epidermal cells contain root hairs that increase the surface area for absorption of water and minerals
o In older woody plants, the epidermis of the stem is replaced by cork tissue
103
Ground tissue
: forms the bulk of the plants and contains 3 types of cells
• Parenchyma cell
• Collenchyma cell
• Sclerenchyma cell
104
Parenchyma
* Are least specialized
* Found in all organs of the plant
* May Contain chloroplast and carry photosynthesis
* Parenchyma cells can divide and give rise to more specialized cells
• Are least specialized
• Found in all organs of the plant
• May Contain chloroplast and carry photosynthesis
• Parenchyma cells can divide and give rise to more specialized cells
Collenchyma cells
105
• Are least specialized
• Found in all organs of the plant
• May Contain chloroplast and carry photosynthesis
• Parenchyma cells can divide and give rise to more specialized cells
Collenchyma cells
* Like parenchyma except they have thicker walls at their carriers
* Collenchyma cells often form bundles
* Their function for support
* Like parenchyma living cell at maturity (nucleus and cytoplasm)
106
Sclerenchyma cells
• Have thick secondary cell wall impregnated with lignin (an organic substance that makes the wall tough and hard)
107
Fibers
type of sclernchyma, • Are generally long, slender cells and they commonly occur in strands or bundles
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Sclerids
type of sclerenchyma, • Are variable in shape and they are often branched, short cell, they make seed coat, shell of nuts and the endocarp
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There are two types of xylem cells
* tracheid’s
| * vessel elements
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Tracheids
* Are found in all vascular plants
* They are long, thin cells with tapered ends
* Water moves from cell to cell though pits
* Tracheids cells are hardened with lignin
111
vessel elements
* vessel elements are found in most angiosperms and in a few seedless vascular plants, and in a few gymnosperms
* they are wider, shorter, thin-walled and less tapered then tracheids
* the end walls of vessels elements have perforation enable water to flow freely through the vessel
112
sugar conducting cells (phloem
* phloem cells are alive at maturity
* in seedless vascular plants and gymnosperms sugars and other nutrients are transported through narrow long cells, called sieve cells
* phloem of angiosperms the nutrients are transported though the sieve tube cell, though alive , sieve tube member lack nucleus, ribosomes, and vacuoles
* however, these organelles are found in newborn cells called companion cell
113
Primary growth of the root
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• Though a longitudinal section of the root one can see the following
o root cap
o apical meristem
o zone of elongation
o zone of maturation
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114
Epidermis (root hairs)
* A single layer of cells covering the root
| * Water and minerals absorbed by plants through epidermis and the root hairs
115
Cortex
* The region between the epidermis and the vascular cylindrical (stele)
* The ground tissues of cortex are mostly parenchyma (for storage)
* Some species may include collenchyma cells under the epidermis
* The inner most layer of the cortex is the endodermis (cells w/ bonds of suberin)
* Beneath it is a layer of cells called pericycle
116
Pericycle
• From which later roots will develop
In the most roots, the stele or the vascular cylinder form a solid core of xylem and phloem (in dicots)
In many monocots the vascular tissue consists of a central core of parenchyma (pith) , surrounded by alternating rings of xylem and phloem
117
Tissue organization of stem in primary growth
• the epidermis covers the stem as part of the dermal tissue
• in gymnosperms and the most dicots the vascular tissue consists of vascular bundles arranged in a form of a ring
• the xylem faces the pith and the phloem faces the cortex
• the ground tissue between the epidermis and the vascular bundles forms the cortex
• the center of the stem consists of mainly of parenchyma called the pith
• in monocots plants, the vascular bundles are scattered through the ground tissue and thus through the ground tissue and thus no cortex and pith can be specified and called the ground tissue
•
118
Growth and Development
* the vascular cambium and secondary vascular tissue
* in a typical gymnosperm of wood dicots stem
* the vascular cambium form a layer between the primary xylem and primary phloem of each vascular bundle an din the ground tissue between the bundles
* the meristematic bonds within and between the bundles unite to become a continuous cylinder of dividing cells
* as secondary growth continues over the years then layers of secondary xylem (wood) accumulate, consisting of mainly of tracheids vessel elements and fibers in angiosperms while in gymnosperms mainly of tracheids
* tracheids and vessel elements that develop early in the growing season (early spring)
* usually with larger diameter and known as early wood or spring wood
* tracheids and vessel elements develop later in the season are with smaller diameter and known as summer wood or fake food
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leaves
* The leaf is the main photosynthetic organ of the most vascular plants
* It consists of a flatbread blade, and a stalk (petiole) which joins the leaf to the node
* A grass leaf lack petioles instead the base of the leaf forms a sheath that envelops the stem
120
dicot leaf veniation
• Dicot leaves show netted or reticulate venation
121
monocot leaf veniation
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monocot leaves (grasses) shows parallel venation
• Simple leafs
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122
Simple leafs
o is a single, consisting of a blade and petiole
123
Compound leaf
o the blade consists of multiple leaflets, Rachis and petiole
124
Doubly compound leaf
o in which each leaflet is divided into smaller leaflets
125
Tendril
form a coil that brings the plant closer to the support (ex. Pea plant)
126
spines
o the spines of cacti
127
storage leaf
o most succulent plants have leaves modified for storing water
128
reproductive leaf
o the leaves of some succulent plants produce adventious plantlets which fall off the leaf and take roots in the soil forming a new plant
129
Mesophyte
o that grow where it is neither too wet nor too dry
130
Hydrophytes
o plants that grow wholly or partly submerged in water
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Xerophytes
o plants that grow in a dry or arid habitat
• such distinctions are not harp, however, leaves exhibit a combination of features of different ecological
• regardless of their shape and sizes, all leaves are composed of
o epidermis
o mesophyll
o vascular bundles (veins)
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Epidermis
* Epidermal cells are similar to those of stem and compactly arranged
* Covered with cuticle and also stomata may occur on both sides, upper and lower epidermis, most mesophytes stomata occur on the lower epidermis
* In hydrophytes that float on the surface of the water
* Stomata may occur on the upper epidermis
* In submerged plant no stomata
* In xerophytes contain sunken stomata
133
Mesophyll
• in most plants the mesophyll consists of two layers
o palisade parenchyma
o spongy parenchyma
134
palisade parenchyma
• palisade parenchyma are columnar cells and are specialized in photosynthesis
•
135
spongy parenchyma
spongy parenchyma cells are irregular with intercellular spaces
136
Vascular bundles
* The veins containing xylem and phloem
| * Commonly the xylem occurs in the upper side of the leaf, the phloem on the lower side
137
Leaf abscission
• In many plants the normal separation of the leaf from the stem ( the process of abscission ) is proceeded by certain structure and chemical changes near the base of the petiole which result in the formation of an abscission zone
138
Transport of water and solutes in vascular plants occurs on three scales
Transport of water and solutes by individual cells, ex. By root hair
o 2. Short distance transport of substances from cell to cell at the level of tissues and organs
o 3. Long distance transport within the xylem and phloem
139
the central role of proton pumps
proton pumps play a central role in transport across plant membranes)
140
how does proton pump work
* the proton pump transport mechanism hydrolyzes ATP and uses the released energy to pump hydrogen ion (H+) out of the cell
* because proton pump removes positive charges in the form of H+ out of the cell it generates a membrane potential, because inside of the cell becomes negative in charge relative to the outside
* their membrane potential help drive K+ in the cell, because K+ diffusing down its electrochemical gradient
* accumulation of the ions by this mechanism represents a passive transport
* but it is the active transport of the H+ ion out of the cell that maintains the membrane potential, makes it possible for the cell to accumulate potassium (K+)
141
both the symplast and apoplast function in transport within tissues and organs
• there are 3 roots available for lateral transport
142
by the first route transport
such substances move out of one cell, across the cell wall and into the neighboring cells
143
by the second route
via the symplast, the continuum of cytosol within a plant issue , requires only one crossing if plasma membrane, after entering one cell
• solutes exist in one cell (through the plasma membrane) then move from cell to cell via plasmodesmata
144
the third route
is for lateral transport within a plant tissue or organ is along the apoplast, the extracellular pathway consisting of cell wall
• the solute move in the extracellular spaces between the cells
• As the water and minerals move all the way to the endodermis, the apoplastic roots cannot continue into the stele, due to the presence of the endodermal cosponian scrips
• Only mineral and solutes that are in the symplastic route, or those through crossing the plasma membrane of each cell, can reach to the stele
• Once the water reaches the xylem in the stele, the water enters the vessel element and for tracheid via the apoplast route, because xylem cells are dead cells at maturity
145
Root pressure
When transpiration is very slow or ascent, as at night for example, the root cells may still accumulate ions in the xylem because the vascular tissue of the root is surrounded by the epidermis, ions tend not to lead out of the xylem
• Therefore the water potential of the xylem become more negative and water moves into the xylem by osmosis through the surrounding cells
• In this manner, root pressure is created it forces both water and dissolved ions up by the xylem
• Root pressure causes “guttation” the exudation of water droplets, that can be seen in the morning on tips of grass blades or leaf margins or some herbaceous plants
146
adhesion
o Holding together of unlike substances
147
cohesion
holding together of like substances
148
Transpiration
• Transpiration can be defined as the loss of water vapour by any part of the plant body
149
Surface tension is caused by the cohesiveness of water molecules
* The reason for this extraordinary cohesiveness lies in the structure of the molecule
* As a consequence of the water molecule, it tends to form a hydrogen bond with four other molecules
* The water molecule is a tetrahedron (four corned) with two positive corners and two negative corners
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The Cohesion-Adhesion-Tension Mechanism
• When the cells of the leaf lose water to the air during transpiration, their ions and molecules become more concentrated
• The water potential of the cells becomes increasingly more negative
• Because the intercellular water is in direct connection, through the water permeability of the plasma membranes, a pull is exerted on this water when water evaporates from the walls, increasing the negative water potential
• The water of the wall is in connection with the water is in connection with the water of the xylem cells
• So the negative potential is transmitted to the centrals of the xylem as well which causes a build-up of tension (negative water potential) there
o Because of the extraordinary cohesiveness of water, this tension is transmitted all the way down the xylem of the stem to the xylem of the root
• This lost makes the water potential of the root more negative, this increases the ability of the root to extract water from the soil
• This theory is known as cohesive-tension theory because it depends on the great cohesiveness of water
• It is the cohesion of the water, due to the hydrogen bonds, that makes it possible to pull a column of sap from above without the water separating
• The transpiration pull can extend down the root only through unbroken chains of water molecules
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How Stomata Open and Close
• Each stoma is flanked by a pair of guard cells which are kidney-shaped in dicots and dumbbell shaped in most grasses (monocots)
• The guard cells are suspended by subsidiary cells (epidermal cells attached to the guard cells) over an air chamber that leads to air spaces within the leaf
• Guard cells control the diameter of the stoma be changing shape, which widens or narrows the gap between the guard cells
• When guard cells takes in water and swells, the size of the gap increases (stomata opens)
• When guard cells loose water, they become flaccid, thus closing the gap (stomata closed)
• Changes in the turgor pressure results primarily from reversible absorption and less potassium ions by the guard cells
• Stomata opening results from potassium ion accumulation which results in negative water potential and movement of water from nearby cells by osmosis
• Stomata closure parallels an exodus of potassium from guard cells, which leads to osmotic loss of water
• Three factors that contribute to stomata opening
o Light stimulates guard cells to accumulate potassium and become turgid
• Depletion of carbon dioxide within the air spaces of leaf, which occurs during photosynthesis, stimulates opening of the stomata
• Circadian rhythmis (the daily rhythm of opening and closing the stomata)
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Movement of Substances in the Phloem
• According to the pressure flow hypothesis assimilates move through the sieve tube along the concentration gradient between the sources of the assimilates and the site of utilization or sinks of these assimilates
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Sugar manufactured in mesophyll cells of the leaf is actively secreted or pumped into the sieve tube of the vein by neighboring parenchyma cells, such as
companion cells
* This decreases the water potential in the sieve tube and cause water to move into the sieve tube from the xylem with the movement of the water into the sieve tube, the sugar is carried passively to the growing root tip, when the sugar is removed from the sieve tube through the expenditure of energy be neighboring parenchyma cells
* This results in increased water potential in the sieve tube and the eventual movement of water out of the sieve tube at the sink
* Note that the role of the sieve tube is relatively passive, as energy is expended by parenchyma cell as the source and sink
* The pressure flow mechanism is an ex. Of “mass form” which depend on the differences in the water potential on either side of the differentially permeable membrane
* These differences in water potential results in osmosis and provides the driving force for the pressure flow mechanism
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A hormone
hemical messenger produced in a small amount by one part of the body that is active in a different part of the body
• Reaction to a hormone is usually dependent not so much on the absolute amount f that hormone as on its relative concentration to the other hormones
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5 classes hormones identified
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o Auxin
o Gibberellins - hormones that promote growth
o Cytokinins
o Abscisic acid
o Eythylene
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Hormones that promote growth
auxin,
