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

Introduction Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems A seed consists of an embryo and nutrients surrounded by a protective coat

Concept 30.1 Seeds and Pollen In addition to seeds, the following are common to all seed plants 1. Reduced gametophytes The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte

2. Heterospory The ancestors of seed plants were likely homosporous, while seed plants are heterosporous Megasporangia produce megaspores that give rise to female gametophytes Microsporangia produce microspores that give rise to male gametophytes

3. Ovules An ovule consists of a megasporangium, megaspore, and one or more protective Integuments Gymnosperm megaspores have one integument Angiosperm megaspores usually have two integuments

4. Pollen Microspores develop into pollen grains, which contain the male gametophytes Pollination is the transfer of pollen to the part of a flower containing the ovules Pollen eliminates the need for water and can be dispersed great distances by air or animals If a pollen grain germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

The Evolutionary Advantage of Seeds A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat

Figure 30.3 From ovule to seed in a gymnosperm. 11

Advantage of the seed A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat Seeds provide some evolutionary advantages over spores: They may remain dormant for days to years, until conditions are favorable for germination They may be transported long distances by wind or animals

Concept 30.2 Gymnosperms Gymnosperms means “naked seeds” The seeds are exposed on sporophylls that form cones Angiosperm seeds are found in fruits, which are mature ovaries The gymnosperms consist of four phyla Cycadophyta (cycads) Gingkophyta (one living species: Ginkgo biloba) Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) Coniferophyta (conifers, such as pine, fir, and redwood)

Cycadophyta Cycads (Phylum Cycadophyta) superficially resemble palms. These thrived during the Mesozoic, but relatively few species exist today Palms are actually flowering plants.

Ginkophyta Phylum Ginkgophyta consists of only a single extant species, Ginkgo biloba. This popular ornamental species has fanlike leaves that turn gold before they fall off in the autumn. Landscapers usually only plant male trees because the seed coats on female plants decay, they produce a repulsive odor (to humans, at least). It has a high tolerance to air pollution and is a popular ornamental tree

Gnetophyta Phylum Gnetophyta consists of three very different genera. Weltwitschia plants, from deserts in southwestern Africa, have straplike leaves. Gentum species are tropical trees or vines. Ephedra (Mormon tea) is a shrub of the American deserts.

Coniferophyta This phylum is by far the largest of the gymnosperm phyla Most conifers are evergreens and can carry out photosynthesis year round Conifers include pines, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods.

Gymnosperm Life Cycle Three key features of the gymnosperm life cycle are: Dominance of the sporophyte generation Development of seeds from fertilized ovules The transfer of sperm to ovules by pollen The life cycle of a pine provides an example

Gymnosperm Reproduction 1. Most species produce both pollen cones and ovulate cones. 2. A pollen cone contains hundreds of microsporangia held on small sporophylls. Cells in the microsporangia undergo meiosis to form haploid microspores that develop into pollen grains. 3. An ovulate cone consists of many scales, each with two ovules. 4. During pollination, windblown pollen falls on the ovulate cone and is drawn into the ovule through the micropyle

5. The megaspore mother cell undergoes meiosis to produce four haploid cells, one of which will develop into a megaspore. 6. Two or three archegonia, each with an egg, then develop within the gametophyte. 7. At the same time that the eggs are ready, two sperm cells have developed in the pollen tube which has reached the female gametophyte. 8. The pine embryo, the new sporophyte, has a rudimentary root and several embryonic leaves.

Phylum Characteristics The term conifer comes from the reproductive structure, the cone, which is a cluster of scalelike sporophylls. Although there are only about 600 species of conifers, a few species dominate vast forested regions in the Northern Hemisphere where the growing season is short. Most conifers are evergreen, retaining their leaves and photosynthesizing throughout the year.

Coniferous trees are among the largest and oldest organisms of Earth. Much of our lumber and paper comes from the wood (actually xylem tissue) of conifers. Coniferous trees are among the largest and oldest organisms of Earth. Redwoods from northern California can grow to heights of over 110m. One bristlecone pine, also from California, is more than 4,600 years old.

Concept 30.3 Angiosperms Better known as flowering plants and these vascular seed plants have two key adaptations - flowers and fruits. Most diverse and geographically widespread of all plants. About 250,000 known species of angiosperms. All angiosperms are placed in a single phylum, the phylum Anthophyta.

The Flower The flower is an angiosperm structure specialized for reproduction. In many species, insects and other animals transfer pollen from one flower to female sex organs of another. Some species that occur in dense populations, like grasses, rely on the more random mechanism of wind pollination.

A flower is a specialized shoot with four circles of modified leaves: sepals, petals, stamens, and carpals.

Parts of the Flower The sepals at the base of the flower are modified leaves that enclose the flower before it opens. The petals lie inside the ring of sepals. These are often brightly colored in plant species that are pollinated by animals. They typically lack bright coloration in wind-pollinated plant species. Neither the sepals or petals are directly involved in reproduction.

Carpals are female megaspores and their products, female gametophytes. Stamens, the male reproductive organs, produce microspores that will give rise to gametophytes. A stamen consists of a filament and anther where pollen is produced. Carpals are female megaspores and their products, female gametophytes. At the tip of the carpal is a sticky stigma that receives pollen. A style leads to the ovary at the base of the carpal.

Fruit A fruit is a mature ovary. Fruits protect dormant seeds and aid in their dispersal. If a flower is not pollinated, the fruit does not develop, and the entire flower withers and falls away. In some plants, such as dandelions and maples, the fruit functions like a kite or propeller, enhancing wind dispersal. Many angiosperms use animals to carry seeds.

Various fruit adaptations help disperse seeds Seeds can be carried by wind, water, or animals to new locations

Angiosperm Reproduction All angiosperms are heterosporous, producing microspores that form male gametophytes and megaspores that form female gametophytes. The immature male gametophytes are contained within pollen grains and develop within the anthers of stamens. Each pollen grain has two haploid cells. Ovules, which develop in the ovary, contain the female gametophyte, the embryo sac. It consists of only a few cells, one of which is the egg. - Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of the same species

A pollen grain that has landed on a stigma germinates and the pollen tube of the male gametophyte grows down to the ovary The ovule is entered by a pore called the micropyle Double fertilization occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule

One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing endosperm The triploid endosperm nourishes the developing embryo Within a seed, the embryo consists of a root and two seed leaves called cotyledons

Angiosperm Diversity The two main groups of angiosperms are monocots (one cotyledon) and eudicots (“true” dicots) The clade eudicot includes some groups formerly assigned to the paraphyletic dicot (two cotyledons) group

Monocots More than one-quarter of angiosperm species are monocots

Eudicots More than two-thirds of angiosperm species are eudicots

Concepts 30.4 Our Need for Plants No group of plants is more important to human survival than seed plants Plants are key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical

Most of our food comes from angiosperms Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% of the calories consumed by humans Modern crops are products of relatively recent genetic change resulting from artificial selection Many seed plants provide wood Secondary compounds of seed plants are used in medicines

Problems Facing Plants Demand for space and natural resources resulting from the exploding human population is extinguishing plant species at an unprecedented rate. In the tropics 55,000 km2 are cleared each year At this rate, the remaining tropical forests will be eliminated in 200 years Loss of plants = loss of potential that plant could have at impacting human life Almost all of our food is based on cultivation of only about two dozen species.

A satellite image from 2000 shows clear-cut areas in Brazil surrounded Figure 30.16 A satellite image from 2000 shows clear-cut areas in Brazil surrounded by dense tropical forest. By 2009, much more of this same tropical forest had been cut down. Figure 30.16 Impact: Clear-Cutting of Tropical Forests 4 km 47

Five Derived Traits of Seed Plants Figure 30.UN04 Five Derived Traits of Seed Plants Reduced gametophytes Microscopic male and female gametophytes (n) are nourished and protected by the sporophyte (2n) Male gametophyte Female gametophyte Heterospory Microspore (gives rise to a male gametophyte) Megaspore (gives rise to a female gametophyte) Ovules Integument (2n) Megaspore (n) Ovule (gymnosperm) Megasporangium (2n) Figure 30.UN04 Summary figure, Concept 30.1 Pollen Pollen grains make water unnecessary for fertilization Seeds Seeds: survive better than unprotected spores, can be transported long distances Seed coat Food supply Embryo 48