a. Kingdom Plantae includes the green photosynthetic organisms with cell walls made of cellulose, food reserves of starch, and the photosynthetic pigments chlorophylls a and b, along with certain carotenoids.

b. As molecular methods provide more information about the systematics of plants, their classifications have been constantly changing.

c. This text will use a modified version of the plant classification scheme proposed in 1966 by Cronquist, Takhtajan, and Zimmermann. (Table 33.1).

d. This classification scheme names two subkingdoms: Thallobionta (the green algae) and Embryobionta (all other plants).

e. The most significant changes in plants have occurred in their modes of reproduction and their life cycles.

f. The dominant plants all reproduce by means of seeds, and a major theme in this chapter concerns the evolution of seeds and pollen as a means of reproduction independent of water.

a. A unicellular flagellate such as Chlamydomonas (Figure 33.1), with a pair of flagella and a cup-shaped chloroplast, could be a model for a primitive green alga.

b. One trend in evolution has produced colonial flagellates (Figure 33.2) such as Gonium, with small colonies, and Volvox, with large colonies.

c. The life cycle of Volvox (Figure 33.3) includes mostly asexual reproduction interspersed with sexual reproduction involving sperm and ova, and zygotes that can develop heavy walls and survive harsh conditions.

d. A second trend in algal evolution has produced tubelike, coenocytic species like Acetabularia, which are also called siphonaceous because their tubes are like siphons (Figure 33.4).

e. In a third evolutionary trend, the cells remain uninucleate but have lost their flagella and form sheets and filaments of uniform cells, as in Ulva, the "sea lettuce" (Figure 33.5).

f. The Charophyta, or stoneworts (Figure 33.6), are quite different from other thallophytes, growing anchored to the substratum and extending upward filaments that are divided into nodes and whorls.

a. Green algae have evolved two different modes of cell division (Figure 33.7).

b. The cell plate mode of division is used by embryophytes, adding weight to the argument that they were derived from the few groups of algae that have this type of division.

a. Many plants have a haplodiplontic cycle, in which both haploid and diploid stages grow vegetatively in an alternation of generations.

b. A generalized life cycle of plants is illustrated in Figure 33.9, and includes the following stages and structures:

c. The trend in plant evolution has been toward dominance of the diploid sporophyte and reduction of the haploid gametophyte.

d. The most primitive vascular plants were all homosporous, with only a single type of sporangium that produced one type of spore.

e. Some plants became heterosporous, producing two kinds of spores, either megaspores from megasporangia, or microspores from microsporangia (Figure 33.8).

f. Each megaspore develops into a megagametophyte that produces eggs, and each microspore develops into a microgametophyte that produces sperm.

a. About 24,000 species of plants are called nontracheophytes (Atracheophyta), which have been classified in three divisions:

b. Atracheophyta lack efficient vascular tissue, grow only a few centimeters tall, have protective epidermal tissue that resists drying and rootlike rhizoids for anchoring and taking up water.

c. Nontracheophytes grow best in moist habitats, and can be found on the soil, on tree bark, on fallen logs, in wetlands, in fresh water, on bare rock, and on concrete structures.

d. Nontracheophytes are poikilohydric organisms that have a remarkable ability to withstand drying for long times.

e. The moss in Figure 33.10 illustrates the bryophyte life cycle.

f. Liverworts also have leafy gametophytes, but they are typically flat, fleshy-looking plates.

g. Liverworts develop two kinds of gametangia-bearing structures:

h. Liverworts produce elongated, dead cells called elaters, which act like tiny springs to disperse spores.

i. Liverworts also reproduce asexually by forming gemmae cups that can grow into new plants.

j. Hornworts include only about 100 species of plants, named for their tall, twisting psorophytes, which resemble horns. Mosses typically have leaves just one cell layer thick, though some have primitive vascular systems in the form of thickened midribs.

a. The remainder of the plant kingdom consists of plants with vascular tissue.

b. Several times during plant evolution, different groups developed tree forms soon after they arose, presumably due to the major advantages of this form of growth:

c. Vascular plant evolution is characterized by three major trends:

d. Vascular tissues evolved as xylem and phloem united to form a vascular bundle.

e. Leaves replaced stems as evolution of higher plants required increased surface areas for photosynthesis.

a. Two main patterns of branching and sporangia (Figure 33.13) have occurred in primitive vascular plants:

b. Two living genera, Psilotum (the tropical whiskferns) and Tmesipteris (Figure 33.14), are placed into division Psilophyta, which contains plants with no true leaves or roots, and a simple stele of xylem surrounded by phloem.

a. Figure 33.15 summarizes the probable phylogeny of vascular plant groups.

b. The club mosses, the simplest plants with roots, leaves, and stems, evolved along an independent evolutionary branch.

c. The main evolutionary line has three branches: one leading to the equisetophytes (e.g. modern horsetails), one leading to the ferns, and one leading to ancient progymnosperms, which were apparently ancestral to all other plants.

d. The progymnosperms diverged into two major groups:

a. Lycopsids include the club mosses (Figure 33.16), which have microphylls arranged spirally around the stem.

b. The lycopsid sporangia became associated with microphylls, with each leaf bearing a single sporangium on its upper surface.

c. These structures aggregate at the tips of the stems in Lycopodium to make a conelike structure.

d. The living lycopsids are all small and herbaceous, but their ancestors in the Carboniferous period included trees like Lepidodendron (Figure 33.17), which grew as high as 35 meters, and which formed modern day coal beds.

a. Figure 33.18 shows the equisetophytes or horsetails.

b. The cells of these plants have walls impregnated with silica, and the plants were called scouring rushes by American pioneers.

c. The central plant stem has a series of joints and a central canal surrounded by multiple vascular bundles.

d. The Equisetum life cycle is illustrated in Figure 33.19.

a. Ferns (Figure 33.20) do not bear seeds and require water for reproduction, thus they are confined to moist habitats.

b. The fern life cycle (Figure 33.21) follows the familiar gametophyte to sporophyte cycle, and is distinguished by free-swimming sperm, sporophyte fronds, and sporangia clustered into sori on the undersides of the fronds.

a. The evolution of pollen to carry sperm and seeds to protect the embryo were major steps in allowing terrestrial plants to become independent of water.

b. The essential feature of seed formation is the fertilization of the megaspore inside the megasporangium, where the embryo remains (Figure 33.22).

c. The megasporangium is reduced to an ovule, which consists of a thin covering around the megaspore with a tough integument covering.

d. The seed is the ripened, fertilized ovule containing the embryo.

e. Seeds protect the embryo from dessication and also from being eaten in some cases.

f. The pollen grain is formed from a microgametophyte made of only a few cells.

g. Pollen grains stick to female structures, and grow pollen tubes down to the ovule, so free-swimming sperm are not released

h. In a seed plant, nearly the entire plant is sporophyte and the gametophyte is reduced to a few cells.

i. The first known seed plants were the seed ferns (Figure 33.23).

j. Plants of this kind are called gymnosperms, literally "naked seed" plants, since their ovules are naked and exposed.

k. Flowering plants are called angiosperms, literally "seeds in vessels," whose ovules develop inside an ovary.

l. The living cycads (Figure 33.24) are most similar to the seed ferns, with separate female and male reproductive structures called cones.

a. The trees now classified as Cordaitales (Figure 33.25) had separate male and female cones and are among the most primitive of modern conifers (Figure 33.26).

b. The pine life cycle (Figure 33.27) involves a visible sporophyte plant that typically bears female and male cones on the same plant.

c. The development of the female cone is a slow process of repeated cell division that begins after a pollen grain has landed on the ovule.

d. After the sperm reaches the megagametophyte through a pollen tube, one sperm nucleus fertilizes a single egg and an embryo develops within the cone.

e. The seeds are released from the cone and carried by the wind to new destinations; seeds may lie dormant for long periods of time until conditions (including forest fires) stimulate their opening and growth.

f. Conifer needles are highly reduced megaphylls that are adapted to dry conditions as well as short growing seasons and cold temperatures.

g. The Ginkgo biloba species (Figure 33.28) represents one small plant group allied to the conifers, and is actually an intermediate stage in the evolution of plant reproduction.

h. Male ginkgo trees produce air-borne pollen granules that produce pollen tubes and motile, flagellated sperm.

a. The Anthophyta (or Magnoliophyta) are the most prominent of all modern plants, and they include almost all modern fruit and vegetable species.

b. Their distinctive reproductive structure, the flower (Figure 33.29), is made of male and female parts that are actually fertile leaves.

c. Several flowers may be grouped into an inflorescence.

d. A perfect flower has both stamens and pistils; one lacking either structure is imperfect.

e. Monoecious species have the female and male parts on the same plant; dioecious species have male and female parts on separate plants.

f. Each fertilized ovule develops into a seed and the ovary develops into a fruit that surrounds and protects the seed.

g. The angiosperm life cycle (Figure 33.30) has the following features: