32: Plant Reproduction Flashcards
Reproductive Development and Structure, Pollination and Fertilization, Asexual Reproduction (157 cards)
1
Q
What is the androecium?
A
The sum of all the stamens in a flower.
2
Q
What are the antipodals?
A
The three cells away from the micropyle.
3
Q
What is the exine?
A
The outermost covering of pollen.
4
Q
What is a gametophyte?
A
The multicellular stage of the plant that gives rise to haploid gametes or spores.
5
Q
What is the gynoecium?
A
The sum of all the carpels in a flower.
6
Q
What is the intine?
A
The inner lining of the pollen.
7
Q
What is megagametogenesis?
A
The second phase of female gametophyte development, during which the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, AKA the megagametophyte or embryo sac.
8
Q
What is a megasporangium?
A
Tissue found in the ovary that gives rise to the female gamete or egg.
9
Q
What is megasporogenesis?
A
The first phase of female gametophyte development, during which a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives.
10
Q
What is a megasporophyll?
A
A bract (a type of modified leaf) on the central axis of a female gametophyte.
11
Q
What is the micropyle?
A
The opening on the ovule sac through which the pollen tube can gain entry.
12
Q
What is a microsporangium?
A
Tissue that gives rise to the microspores or the pollen grain.
13
Q
What is a microsporophyll?
A
The central axis of a male cone on which bracts (a type of modified leaf) are attached.
14
Q
What is the perianth?
A
The part of the flower consisting of the calyx and/or corolla; forms the outer envelope of the flower.
15
Q
What are polar nuclei?
A
They are found in the ovule sac; fusion with one sperm cell forms the endosperm.
16
Q
What is a sporophyte?
A
The multicellular diploid stage in plants that is formed after the fusion of male and female gametes.
17
Q
What is a synergid?
A
A type of cell found in the ovule sac that secretes chemicals to guide the pollen tube towards the egg.
18
Q
What is the term for a flower that has all four whorls?
A
If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as complete. If any of the four parts is missing, the flower is known as incomplete.
19
Q
What are the terms for a flower that contains both an androecium and a gynoecium?
A
Perfect, androgynous, or hermaphrodites.
20
Q
What are the types of imperfect flowers?
A
Staminate flowers contain only an androecium, and carpellate flowers contain only a gynoecium.
21
Q
What are some examples of monoecious plants?
A
Corn and peas.
22
Q
What are some examples of dioecious plants?
A
C. papaya and Cannabis.
23
Q
What are the orientation-dependent terms for an ovary?
A
The ovary, which may contain one or multiple ovules, may be placed above other flower parts, which is referred to as superior; or, it may be placed below the other flower parts, referred to as inferior.
24
Q
Where does pollen develop?
A
The male gametophyte (the pollen grain) develops and reaches maturity in an immature anther. In a plant’s reproductive organs, development of pollen takes place in a structure known as the microsporangium. The microsporangia, which are usually bi-lobed, are pollen sacs in which the microspores develop into pollen grains. These are found in the anther, which is at the end of the stamen.
25
How are microspores developed?
Within the microsporangium, the microspore mother cell divides by meiosis to give rise to the four microspores, each of which will ultimately form a pollen grain.
26
What is the tapetum?
An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall.
27
What cells do mature pollen grains contain?
A generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells).
28
How is pollen released?
Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
29
What are the coverings of pollen grains?
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (thinner, inner layer).
30
How are pollen grains protected?
The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.
31
What are the components of an embryo sac and how do they develop?
Two of the nuclei–the polar nuclei–move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.
32
What is the role of the integument?
A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
33
Where are sporophytes and gametophytes found in conifers?
In conifers such as pines, the green leafy part of the plant is the sporophyte, and the cones contain the male and female gametophytes.
34
What prevents self-pollination in gymnosperms?
The female cones are larger than the male cones and positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. Because pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.
35
How does pollen develop in gymnosperms?
Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus, and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on the female cone.
36
How does the ovule develop in gymnosperms?
Within the megasporangium, the megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.
37
How does fertilization occur in gymnosperms?
Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After fertilization, the diploid zygote is formed, which divides by mitosis to form the embryo.
38
How do seeds develop in gymnosperms?
The scales of the cones are closed during the development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place because gymnosperm seeds have no covering.
39
How does the location of female gametophytes differ between angiosperms and gymnosperms?
In angiosperms, the female gametophyte exists in an enclosed structure–the ovule–which is within the ovary; in gymnosperms, the female gametophyte is present on exposed bracts of the female cone.
40
How does fertilization differ between angiosperms and gymnosperms?
Double fertilization is a key event in the lifecycle of angiosperms, but is completely absent in gymnosperms.
41
How does the location of gametophytes differ between angiosperms and gymnosperms?
The male and female gametophyte structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are part of the flower.
42
How does pollination differ between angiosperms and gymnosperms?
Wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common.
43
What is accessory fruit?
Fruit derived from tissues other than the ovary.
44
What is aggregate fruit?
Fruit that develops from multiple carpels in the same flower.
45
What is the aleurone?
The single layer of cells just inside the seed coat that secretes enzymes upon germination.
46
What is the coleoptile?
The covering of the shoot tip, found in germinating monocot seeds.
47
What is the coleorhiza?
The covering of the root tip, found in germinating monocot seeds.
48
What is the cotyledon?
The fleshy part of the seed that provides nutrition to the embryo.
49
What is cross-pollination?
Transfer of pollen from the anther of one flower to the stigma of a different flower.
50
What is dormancy?
A period of no growth and very slow metabolic processes.
51
What is double fertilization?
Two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote, whereas the other sperm fuses with the polar nuclei, forming endosperm.
52
What is the endocarp?
The innermost part of the fruit.
53
What is the endosperm?
The triploid structure resulting from fusion of a sperm with polar nuclei, which serves as a nutritive tissue for the embryo.
54
What is an endospermic dicot?
A dicot that stores food reserves in the endosperm.
55
What is the exocarp?
The outermost covering of a fruit.
56
What is the epicotyl?
The embryonic shoot above the cotyledons.
57
What is gravitropism?
Response of a plant growth in the same direction as gravity.
58
What is the hypocotyl?
The part of the embryonic axis below the cotyledons.
59
What is the mesocarp?
The middle part of a fruit.
60
What is multiple fruit?
Fruit that develops from multiple flowers on an inflorescence.
61
What is a nectar guide?
A pigment pattern on a flower that guides an insect to the nectaries.
62
What is a non-endospermic dicot?
A dicot that stores food reserves in the developing cotyledon.
63
What is the pericarp?
A collective term describing the exocarp, mesocarp, and endocarp; the structure that encloses the seed and is a part of the fruit.
64
What is a plumule?
A shoot that develops from the germinating seed.
65
What is pollination?
Transfer of pollen to the stigma.
66
What is the radicle?
The original root that develops from the germinating seed.
67
What is scarification?
Mechanical or chemical processes to soften the seed coat.
68
What is a scutellum?
A type of cotyledon found in monocots, as in grass seeds.
69
What is self-pollination?
Transfer of pollen from the anther to the stigma of the same flower.
70
What is simple fruit?
Fruit that develops from a single carpel or fused carpels.
71
What is the suspensor?
The part of the growing embryo that makes connection with the maternal tissues.
72
What is the tegmen?
The inner layer of the seed coat.
73
What is the testa?
The outer layer of the seed coat.
74
What is vernalization?
Exposure to cold required by some seeds before they can germinate.
75
How is corn an example of selective breeding?
Today's corn is a result of years of breeding that started with its ancestor, teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds–vastly different from today's relatively giant ears of corn. Though these two plants appear to be entirely different, the genetic difference between them is miniscule.
76
Why does self-pollination occur?
Self-pollination occurs in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower's stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.
77
What are some ways that plants avoid self-pollination?
In some species, the pollen and the ovary mature at different times. By the time pollen matures and has been shed, the stigma of the flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. The primrose is one such flower. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube's halfway point, and the thrum-eyed flower's stigma is likewise located at the halfway point. Insects easily cross-pollinate while seeking the nectar at the bottom of the pollen tube. This phenomenon is known as heterostyly. Many plants, such as cucumber, have male and female flowers located on different parts of the plant, making self-pollination difficult. In other species, the male and female flowers are borne on different plants (dioecious).
78
How do incompatibility genes prevent pollen germination?
Incompatibility genes, which prevent pollen from germinating or growing into the stigma of a flower, have been discovered in many angiosperm species. If plants do not have compatible genes, the pollen tube stops growing. Self-incompatibility is controlled by the S (sterility) locus. Pollen tubes have to grow through the tissue of the stigma and style before they can enter the ovule. The carpel is selective in the type of pollen it allows to grow inside. The interaction is primarily between the pollen and the stigma epidermal cells. In some plants, like cabbage, the pollen is rejected at the surface of the stigma, and the unwanted pollen does not germinate. In other plants, pollen tube germination is arrested after growing one-third the length of the style, leading to pollen tube death. Pollen tube death is due either to apoptosis or to degradation of pollen tube RNA. The degradation results from the activity of a ribonuclease encoded by the S locus. The ribonuclease is secreted from the cells of the style in the extracellular matrix, which lies alongside the growing pollen tube.
79
What are some characteristics of bee pollinators?
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees. The most common species of bees are bumblebees and honeybees. Since bees cannot see the color red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy-rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, and not to humans; it helps to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees' fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second flower.
80
What are some characteristics of fly pollinators?
Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. The nectar provides energy, whereas the pollen provides protein.
81
What are some examples of flowers that use fly pollination?
The corpse flower or voodoo lily (*Amorphophallus*), dragon arum (*Dracunculus*), and carrion flower (*Stapelia*, *Rafflesia*).
82
Other than bees, what other stinging insect pollinates flowers?
Wasps are also important insect pollinators, and pollinate many species of figs.
83
What are some characteristics of butterfly pollinators?
Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to make access to nectar easier. The pollen is picked up and carried on the butterfly's limbs.
84
What are some characteristics of moth pollinators?
Moths pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white and are flat, enabling the moths to land.
85
What are some characteristics of the symbiotic relationship between the yucca plant and the yucca moth?
The shape of the flower and moth have adapted in such a way as to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and developing seeds.
86
Which moth and plant have a similar relationship to the yucca moth and plant?
The corn earworm moth and Gaura plant have a similar relationship.
87
What are some characteristics of bat pollinators?
In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored; thus, they can be distinguished from the dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally large and wide-mouthed to accommodate the head of a bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.
88
What are some characteristics of bird pollinators?
Many species of small birds, such as the hummingbird and sun birds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access for the bird's beak. Brightly colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird's head and neck and is then transferred to the next flower it visits.
89
How can the range of plants be determined by bird pollination?
Botanists have been known to determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.
90
What are some characteristics of pollination by wind?
Most species of conifers, and many angiosperms, such as grasses, maples, and oaks, are pollinated by wind. Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green, small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it. The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feather stigma of the flower.
91
What are some characteristics of pollination by water?
Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water, and when it comes into contact with the flower, it is deposited inside the flower.
92
How abundant are orchids?
Orchids are highly valued flowers, with many rare varieties. They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.
93
How is food deception used for pollination?
Some species of orchid have evolved a pollination method known as food deception, in which bright colors and perfumes are offered, but no nectar. *Anacamptis morio*, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent–which usually indicates food for a bee–and in the process, picks up the pollen to be transported to another flower.
94
How is sexual deception used for pollination?
Some orchids use sexual deception for pollination. *Chiloglottis trapeziformis* emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and in the process, transfers pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in a different sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, and in the process, picks up pollen, which it then transfers to the next counterfeit mate.
95
During double fertilization, how does the pollen tube grow through the style?
The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube's growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac, and it enters the ovule sac through the micropyle. After fertilization is complete, no other sperm can enter.
96
How does cell division occur after double fertilization?
After fertilization, the zygote divides to form two cells: the upper cell, or terminal cell, and the lower, or basal, cell. The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo.
97
How does the embryo in a seed develop in dicots?
In dicots (eudicots), the developing embryo has a heart shape, due to the presence of the two rudimentary cotyledons. In non-endospermic dicots, such as *Capsella bursa*, the endosperm develops initially, but is then digested, and the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they run out of room inside the developing seed, and are forced to bend. Ultimately, the embryo and cotyledons fill the seed, and the seed is ready for dispersal.
98
How does the embryo develop after seed maturity?
Embryonic development is suspended after some time, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
99
How does the embryo access food reserves in monocots?
In monocots, such as corn and wheat, the single cotyledon is called a scutellum; the scutellum is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.
100
How does the embryo access food reserves in endospermic dicots?
In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves, much like in monocots (which, by definition, also have endospermic seeds).
101
What are some examples of endospermic dicots?
Tobacco (*Nicotiana tabaccum*), tomato (*Solanum lycopersicum*), and pepper (*Capsicum annuum*).
102
How does the embryo access food reserves in non-endospermic dicots?
In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage.
103
What are some examples of non-endospermic dicots?
The two halves of a peanut seed (*Arachis hypogaea*) and the split peas (*Pisum sativum*) of split pea soup are individual cotyledons loaded with food reserves.
104
What are some characteristics of the seed coat?
The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen.
105
What are the parts of the embryonic axis?
The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The embryonic axis terminates in a radicle (the embryonic root), which is the region from which the root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots, the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the embryonic axis that projects above the cotyledons is known as the epicotyl. The plumule is composed of the epicotyl, young leaves, and the shoot apical meristem.
106
How does germination occur in dicot seeds?
Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards. This shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate. During this time, the radicle is also growing and producing the primary root. As it grows downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system.
107
How does germination occur in monocot seeds?
In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering: the coleorhiza. Next, the primary shoot emerges, protected by the coleoptile: the covering of the shoot tip. Upon exposure to light (when the plumule has exited the soil and the protective coleoptile is no longer needed), elongation of the coleoptile ceases and the leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other, adventitious roots emerge from the base of the stem. This gives the monocot a fibrous root system.
108
How does dormancy increase seed viability?
Dormancy helps keep seeds viable during unfavorable conditions. Upon a return to favorable conditions, seed germination takes place. Dormancy may last for months, years, or even centuries.
109
What are some examples of favorable conditions?
Favorable conditions could be as diverse as moisture, light, cold, fire, or chemical treatments. After heavy rains, many new seedlings emerge. Forest fires also lead to the emergence of new seedlings.
110
How are temperature treatments used in seed germination?
Some seeds require vernalization (cold treatment) before they can germinate. This guarantees that seeds produced by plants in temperate climates will not germinate until the spring. Plants growing in hot climates may have seeds that need a heat treatment in order to germinate, to avoid germination in the hot, dry summers.
111
How does the seed coat affect germination?
In many seeds, the presence of a thick seed coat retards the ability to germinate. Scarification, which includes mechanical or chemical processes to soften the seed coat, is often employed before germination. Presoaking in hot water, or passing through an acid environment, such as an animal's digestive tract, may also be employed.
112
How does seed size affect germination?
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have enough food reserves to germinate deep below ground, and still extend their epicotyl all the way to the soil surface. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath the surface, the developing seedling would not have enough food reserves to reach the sunlight.
113
Where do fruits develop?
After fertilization, the ovary of the flower usually develops into the fruit. In most cases, flowers in which fertilization has taken place will develop into fruits, and flowers in which fertilization has not taken place will not. Some fruits develop from the ovary and are known as true fruits, whereas others develop from other parts of the female gametophyte and are known as accessory fruits.
114
What are some examples of fruit?
The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn).
115
How may fruits be classified?
Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin.
116
What are some examples of simple fruits?
Nuts and beans.
117
What is an example of an aggregate fruit?
Raspberries.
118
What is an example of a multiple fruit?
Pineapples, where the flowers fuse together to form the fruit.
119
What is another term for accessory fruit?
False fruit.
120
What are some examples of accessory fruits?
Strawberries, which are derived from the receptacle, and apples and pears, which are derived from the hypanthium.
121
What are the parts of a fruit?
Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the endocarp is the edible part. In many fruits, two or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy.
122
What are dehiscent and indehiscent fruits?
Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds.
123
How are fruits adapted for seed dispersal?
Some fruits have built-in mechanisms so they can disperse by themselves, whereas others require the help of agents like wind, water, and animals. Modifications in seed structure, composition, and size help in dispersal.
124
How are some fruits adapted for dispersal by wind?
Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits–for example, the dandelion–have hairy, weightless structures that are suited to dispersal by wind.
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How are some fruits adapted for dispersal by water?
Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water.
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How are some fruits adapted for dispersal by animals?
Animals eat fruits, and the seeds that are not digested are excreted in their droppings some distance away. Some animals, like squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate. Some fruits, like the cocklebur, have hooks or sticky structures that stick to an animal's coat and are then transported to another place. Humans play a big role in dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds.
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What is apomixis?
The process by which seeds are produced without fertilization of sperm and egg.
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What is cutting?
The method of asexual reproduction where a portion of the stem that contains nodes and internodes is placed in moist soil and allowed to root.
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What is grafting?
The method of asexual reproduction where the stem from one plant species is spliced to a different plant.
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What is layering?
The method of propagating plants by bending a stem under the soil.
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What is micropropagation?
Propagation of desirable plants from a plant part; carried out in a laboratory.
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What are monocarpic plants?
Plants that flower once in their lifetime.
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What are polycarpic plants?
Plants that flower several times in their lifetime.
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What is a scion?
The part of a plant that is grafted onto the root stock of another plant.
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What is senescence?
The process that describes aging in plant tissues.
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Why does asexual reproduction occur in plants?
Many plants are able to propagate themselves using asexual reproduction. This method does not require the investment to produce a flower, attract pollinators, or find a means of dispersal. Traditionally, these plants survive well under stable environmental conditions when compared with plants produced from sexual reproduction because they carry genes identical to those of their parents.
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What are some different types of root modifications?
The corm is used by gladiolus and garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples. A potato is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an adventitious root, and the strawberry plant has a stolon, which is also called a runner.
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How does apomixis occur?
Either the ovule or part of the ovary, which is diploid in nature, gives rise to a new seed.
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What are some advantages of asexual reproduction?
An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is arising from an adult plant or plant parts, it will also be sturdier than a seedling.
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What are some natural methods of asexual reproduction?
Many plants–like ginger, onion, gladioli, and dahlia–continue to grow from buds that are present on the surface of the stem. In some plants, such as the sweet potato, adventitious roots or runners can give rise to new plants. In *Bryophyllum* and kalanchoe, the leaves have small buds on their margins. When these are detached from the plant, they grow into independent plants; or, they may start growing into independent plants if the leaf touches the soil.
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What are some artificial methods of asexual reproduction in plants?
Grafting, cutting, layering, and micropropagation.
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How is grafting performed?
In grafting, two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than a right angle), placed in close contact with each other, and are then held together. Matching up these two surfaces as closely as possible is extremely important because these will be holding the plant together. The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts producing shoots, and eventually starts bearing flowers and fruits.
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Where is grafting used?
Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. Grafting is widely used in viticulture (grape growing) and the citrus industry. Scions capable of producing a particular fruit variety are grated onto root stock with specific resistance to disease.
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How is cutting performed?
Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of the African violet will root if kept in water undisturbed for several weeks.
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How is layering performed?
Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this way. In some plants, a modified form of layering known as air layering is employed. A portion of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some gardeners also apply rooting hormone. After some time, roots will appear, and this portion of the plant can be removed and transplanted into a separate pot.
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Where is micropropagation used?
Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions. This method allows propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important, or are in demand as disease-free plants.
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How is micropropagation performed?
To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass known as callus, from which individual plantlets begin to grow after a period of time. These can be separated and are first grown under greenhouse conditions before they are moved to field conditions.
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What is the difference between life span and life cycle?
The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant.
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How long are plant life spans?
Some plants, such as annuals, only need a few weeks to grow, produce seeds and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4500 years.
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How do the cells of a plant grow over its life span?
Some parts of a plant, such as regions containing meristematic tissue, continue to grow, and others undergo apoptosis. The cork found on stems, and the water-conducting tissue of the xylem, for example, are composed of dead cells.
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How are plants categorized by seasonal lifecycles?
Plant species that complete their lifecycle in one season are known as annuals, an example of which is *Arabidopsis*, or mouse-ear cress. Biennials such as carrots complete their lifecycle in two seasons. In a biennial's first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth, and do not allow the plants to flower. Perennials, such as the magnolia, complete their lifecycle in two years or more.
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What are some characteristics of monocarpic plants?
Monocarpic plants flower only once in their lifetime; examples include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually and accumulate a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die.
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What are some characteristics of polycarpic plants?
Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this means, the plant does not require all its nutrients to be channeled towards flowering each year.
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What factors influence plant survival?
Genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival.
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Why does leaf fall occur?
In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency, due to shading by upper leaves, or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling.
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How does senescence occur?
Senescence is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues.
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What role do hormones play in senescence?
The complex pathways of nutrient recycling within a plant are not well understood. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.
