Plant Diversity II

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Exam 1 (Ch 30) for Prof Hoffman at UofL

What are the five adaptations common to all seed plants that ensure their success in the adaptation to land?

Heterospory, highly reduced gametophytes, ovules, pollen, seeds

Explain the difference between homospory and heterospory.

Homospory characterizes plants that produce spores that are all the same size (as in bryophytes and most seedless vascular plants). Homospory can result in bisexual gametophytes (as in ferns, pg 611) or unisexual gametophytes (male or female) as in mosses - see pg 607 in the text.
Heterospory characterizes plants that produce spores of different sizes: megaspores and microspores which give rise to separate female and male gametophytes, respectively.

Contrast sperm delivery in seedless plants with sperm delivery in seed plants.

Sperm delivery in seedless plants is dependent upon the presence of water. The sperm of these organisms have flagella and require a pool in which to swim in order to get to the egg, which is usually not too far away. Sperm delivery in seed plants depends upon the pollen grain, an invention that obviates the need for water. The pollen grain delivers the sperm (nucleus) to the ovule through the air, thus eliminating the need for water for this purpose.

What features not present in seedless plants have contributed to the enormous success of the seed plants on land?

Three things: The gametophytes of seed plants are reduced and dependent upon the parent sporophyte which protects them (particularly in the case of the female gametophyte). Pollen is covered with sporopollenin which is resistant to physical factors in the environment. The structure of the seed protects the next generation sporophyte and allows it to remain dormant until conditions are right for germination, development and growth.

Explain how the pine life cycle in Figure 30.6 reflects the five adaptations common to all seed plants.

The ovule contains the megasporangium which gives rise to the female gametophyte; this structure remains protected within the sporangium and integument. Archegonia develop within each gametophyte, each containing an egg cell. Pollen is produced in the microsporangia of the male cones. Each pollen grain contains a generative nucleus, tube nucleus and two body cells. Pollen is delivered via the air (wind, usually) to the opening of the ovule (micropyle). The tube cell forms a pollen tube through which the sperm nucleus (derived from one division of the generative nucleus) is delivered to the ovule. After fertilization the embryo develops within the ovule and a seed is formed from the integument (seed coat), nutritive tissue (gametophyte) and embryo (next generation plant or sporophyte).

How do the life cycles of seedless vascular plants differ from that of gymnosperms?

Seedless vascular plants require water for the sperm to get to the egg; this takes place on an independent (stranded) gametophyte. Most seedless plants are also homosporous. Seed plants are heterosporous and therefore have different sized spores. The female gametophyte of seed plants is derived from a functional megaspore and is retained on the parent plant until seed dispersal. This provides a protected environment for the developing young sporophyte.

Describe how the parts of an ovule (integument, megaspore, megasporangium) correspond to parts of a seed (seed coat, embryo) in gymnosperms; include ploidy levels.

The integument (2n) surrounds the megasporangium (2n); the megasporangium (2n) contains the megasporocyte (2n) which undergoes meiosis to give rise to the functional megaspore (n). The megaspore (n) divides by mitosis to produce the female gametophyte (n) and several archegonia (n), each containing one egg (n). The mature seed has a seed coat derived from the integument (2n), nutritive material derived from the female gametophyte (n), and the embryo (2n) derived from the zygote after fertilization.

Describe the pine cone in terms of structure and function.

The female pine cone is a strobilus formed from a central axis with whorls of sporophylls attached. The sporophylls are scale-like structures with two ovules at the base of each. The scales are open when the female cone is young in order to accept pollen delivered by the wind. After pollination the scales of the female cone close. The cone develops and enlarges over the next year or so as the pollen tube grows through the micropyle towards the archegonium. After fertilization the female cone becomes even larger and woody. When the seeds are completely developed the pine cone opens up to release the seeds.

In the pine life cycle, where can you find the antheridium?

You cannot. The male gametophyte in seed plants is so reduced there is no antheridium. In gymnosperms, the 4-celled pollen grain is what remains of the male gametophyte - a sperm delivery device floating in the air.

Distinguish between pollination and fertilization.

Pollination is the delivery of pollen and its deposition on the recipient (micropyle of the ovule in the case of gymnosperms). Fertilization is what it is always is: union of egg and sperm, wherever that may be.

What is (are) the benefit(s) of seed dormancy?

Dormancy allows a plant to remain protected and inactive until an appropriate time arrives for germination.

Does the life cycle of animals have any structure analogous to plant gametophytes? Explain.

No, there is no gametophyte equivalent in animals. Animals have a gametic life cycle which does not include a multicellular haploid stage or phase.

pollen

The fine dust-like grains or powder formed within the anther of a flowering plant that become the sperm cells.

ovule

A structure that develops within the ovary of a seed plant and contains the female gametophyte.

strobilus

Compact cluster of spore-bearing structures found in seedless vascular plants

integument

a layer of sporophyte tissue envelops and protects the megasporangium.

homospory

one kind of spore is produced, typically resulting in a bisexual gametophyte

heterospory

two types of spores develop into separate male and female gametophytes, all seed plants

sporangium

spore capsule in which haploid spores are produced by meiosis

sporophyll

modified leaves that bear sporangia

megasporocyte

diploid reproductive cell enlarges and undergoes meiosis; producing four haploid megaspores; only one survives; the others disintegrates

microsporocyte

diploid reproductive cells in the stamen (male reproductive part), underfoes meiosis and produces 4 haploid microspores --> 4 pollen grain

female gametophyte

the gametophyte that produces an egg

archegonium

a female sex organ occurring in mosses, ferns, and most gymnosperms

male gametophyte

gametophyte that produces sperm

artheridia

male gametangia; produce sperm and release them into the environment

pollen grain

Structure consisting of a few haploid cells surrounded by a thick protective wall that contains a male gametophyte of a seed plant

pollination

the transfer of pollen from male reproductive structures to female reproductive structures in plants

fertilization

process in sexual reproduction in which male and female reproductive cells join to form a new cell

egg

haploid female sex cell produced by meiosis

sperm

haploid male sex cells produced by meiosis

generative cell

in a pollen grain, the cell that forms two sperm

tube cell

the cell from a male plant that grows down the style to the opening of the micropyle; grows a pollen tube

pollen tube

slender tubular outgrown from a pollen grain that penetrates the ovule and releases male gametes

micropyle

in the ovules of seed plants, the opening in the integuments through which the pollen tube usually enters

seed

a mature fertilized plant ovule consisting of an embryo and its food source and having a protective coat or testa

embryo

the young plant that develops from the zygote

seed coat

structure that surrounds and protects a plant embryo and keeps it from drying out

Four Gymnosperms

Cycadophyta, Ginkophyta, Gnetaphyta, Coniferophyta

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