CH 30: Plant Diversity II: The Evolution of Seed Plants

Seed Plants

• There are two major groups of seed plants:
1. Gymnosperms ("gymno" - naked + "sperm" - seed) - seeds are totally exposed to environment or held on "scales" of "cones".
2. Angiosperms ("angio" - vessel) - seeds enclosed within the remnants of enclosed chambers called ovaries. These are associated with a structure called fruit.
• These terms are not taxa, but refer to biological groupings.

Gymnosperms

• Category II B - The more primitive of the two groups of seed plants, these produce ovules and seeds on specialized leaves (sporophylls).
• Includes about 600 recognized species in 4 phyla (we'll examine 3 in lab):
1. Phylum Coniferophyta - Most of the species occur in this phylum.
• All are woody plants that are collectively referred to as the "evergreens", e.g. cedars, hemlocks, spruces, firs, pines, cypress, larches. (All of the woody species that produce 'pine cones'). Woods are referred to as 'soft woods'.
• Most conifers usually are monoecious ("one" + "house"), having both male and female reproductive organs on/in the same individual.
• Reproductive Cycle - The conifer trees or shrubs we see are all sporophytes (2n). When sexually mature, these will produce spores by heterospory in 2 structures.
• These structures are referred to as "(pine) cones", especially the female structure (male is usually very small and inconspicuous).
• Strobilus - can be directly translated as "pine cone".
• MALE "CONE" - Possesses bracts (sporophylls), which are the "sections" of the cone. Each contains two 2n microsporangia.
• These undergo meiosis to produce 4 haploid microspores, which are the male gametophytes.
• The male gametophytes in conifers are referred to as pollen grains. They are so reduced in size there are no antheridia produced.
• Instead, these (pollen grains) produce sperm cells by mitosis. Pollen grains are produced in huge numbers, carried by wind.
• FEMALE "CONE" - Possesses bracts (sporophylls), each with 2n megasporangia -> meiosi -> four haploid megaspores, 3 of which die, 4th becomes the female gametophyte, which produces haploid eggs in several archegonia by mitosis.
• When pollen grain comes into contact with the ovule (archegonium) during pollination, a pollen tube grows out of the pollen grain into the archegonium.
• the pollen tube digests its way to the egg.
• Inside pollen grain, a haploid cell divides to produce two sperm nuclei. One of these will fertilize the egg --> 2n zygote.
• Female gametophyte tissue nourishes the developing embryo which is enclosed in a seed coat and develops a thing "wing" for wind dispersal.
• It may take as much as a year between pollination and fertilization.
• Seed maturation may take months to years.
• No H2O needed for fertilization! Sperm are non-motile and pass through pollen tube. Adaptation to true terrestrial existence!
2. Phylum Cycadophyta ("cycad" - palm) - Very few species still extant. This phylum was most diverse about 250 million years ago.
   • Unlike conifers, cycads are dioecious ("two houses"), i.e., male and female plants are different individuals.
   • Although pollen grains are produced, sperm are flagellated. Still need water to fertilize the female gametophyte.
   • Seeds are primitive, but form in cones.
3. Phylum Ginkgophyta - This is a monotypic ("one" + "type") phylum, represented by a single species, Ginkgo biloba.
• The ginkgo is a "living fossil", structurally identical to fossil plants of 200 million years ago.
• Like cycads, sperm is flagellated, although produced in pollen grains.
• The species is dioecious.
• Seeds on the female tree are completely exposed to environment, although surrounded by a temporary fleshy covering (NASTY smelling!).
• The ginkgo is a cultivated plant. There are no naturally occurring ginkgoes anywhere on Earth!

Angiosperms

• The most diverse group of plants on the planet, > 240,000 extant species.
• All belong to the Phylum Anthophyta ("flower" + "Plant"), the flowering plants.
• Additional angiosperm characteristics:
• Flower - highly modified leaves that house the sexual reproductive organs and serve as an attractant for pollinators. Non-mobile organisms putting mobile organisms to work for reproductive processes.
• Unique double fertilization process.
• Fruit - The seed is enclosed in a vessel of tissue usually formed by tissues of the ovary.
• The fruit functions in both protection and dispersion of the seeds.
• The phylum was originally divided into two classes, the monocots and dicots, based on many structural differences.
• Currently, the monocots (Monocotyledones) are still recognized as a single monphyletic class.
• It includes about 65,000 species of monocots, including the lilies, grasses, palms, orchid and the crop plants we refer to as "grains" (rice, oats, wheat, corn, etc.).
• The dicots have been subdivided into several clades, with the largest, the eudicots containing the most advanced and familiar flowering plants (about 165,000 species).
• It is the development of the flower and the fruit that probably allowed the high diversity of angiosperms to become possible.
• In addition, the vascular tissues of the xylem in particular became modified to carry water even more efficiently over longer distances.

The Flower

• The flower is the reproductive structure of angiosperms. It is NOT a separate organ, but is actually a highly modified leaf.
• A flower may have up to 4 different parts:
1. Sepal - The outer, leaflike layer, which is usually green and encloses and protects the young flower before it opens (bud).
2. Petal - This is often brightly colored and is the most "recognizable" part of the flower. Its function is to attract animals to the flower to aid in pollination.
3. Stamen - The male reproductive structure. Composed of a stalk-like filament, plus a sac-like anther in which pollen are produced by meiosis.
4. Carpel - This is the female reproductive structure which usually lies in the center of the flower. The carpel consists of:
• Stigma, which typically has a sticky surface that may produce attractive odors;
• Style, a long tubular structure;
• Ovary, encloses one or more ovules, the female gametophyte containing the egg.
• Multiple carpels may fuse together to form a pistil.
• Each ovule develops into a seed after fertilization.

The Fruit

• The fruit is a post-fertilization structure.
• After fertilization, the ovule(s) in the ovary turn into the seed(s).
• In most cases, the tissues of the ovary develop into the fruit.
• Different types of flowers give rise to different types of fruit.
• There are three basic types of fruit:
1. Simple - develop from a single carpel of a single flower. Most fruits are of this type.
• Simple fruits may be fleshy, meaning the fruit tissues are moist when the seed(s) is/are mature (e.g. apple), or dry, where the fruit flesh becomes dehydrated and hardened (most beans).
2. Aggregate - develop from one flower that has multiple carpels. each ovary produces a fruit which then fuse together.
3. Multiple - formed from the ovaries of many flowers that grow close together then fuse to form one fruit.
• Fruits are designed to assist in widespread dispersal of seeds by a variety of means.

Angiosperm Reproduction

• All visible angiosperms are sporophytes, the gametophyte generation is reduced to a few cells (pollen and ovule).
• Although heterosporous, they also have an unique "double" fertilization process:
• FEMALE - Ovule contains 2n megasporocytes --> Meiosis --> 4 n megaspores. Three disintegrate, 4th mitotically divides into an embryo sac with one egg plus two polar nuclei.
• MALE - anther contains 2n microsporocytes --> meiosis --> 4 n microspores which become 4 male gametophytes (pollen grains).
• Pollination - Adherence of pollen grains to the stigma of carpel. Accomplished by wind, but especially by animal carriers (pollinators).
• Pollen grain develops a pollen tube which grows down through style to ovary.
• Pollen mitotically divides to produce two non-flagellated n sperm nuclei
• One sperm drops down pollen tube and fertilizes the egg --> 2n zygote (embryo).
• Second sperm drops down pollen tube and fuses with both polar nuclei forming a 3n cell which develops into solid or liquid endosperm to nourish embryo.
• "Pop" corn is solid endosperm, coconut "milk" is liquid endosperm.

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