MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 17. VASCULAR CRYPTOGAMS

Reproductive Systems

Oogamy and Morphology/12.html">ALTERNATION OF GENERATIONS, in which the gametophyte depends on the sporophyte for Nutrition, are typical of all vascular plants (Fig. 17-6). Clearly, oogamy is best suited for land plants, as it requires only one type of gamete rather than both to travel through an alien medium outside the plant body. The sporophyte dominates The life cycle of vascular plants; it is larger and structurally much more complex than the gametophyte. In bryophytes, as discussed in Chapter 16, the gametophyte is almost always larger than the sporophyte, although, as in vascular plants, it is usually simpler in Structure.

Class="center">Fig. 17-6. Generalized Life Cycle of a vascular plant

Homospory and Heterospory

Early vascular plants produced only one type of spore; they are termed homosporous. Among modern vascular plants, homospory occurs in whisk ferns, horsetails, some lycophytes, and almost all ferns. As a result of Meiosis, homosporous plants produce a single type of spore that develops into bisexual gametophytes—that is, those bearing both antheridia and archegonia.

Heterospory—The production of Two Types of spores in two distinct types of sporangia—is found in some lycophytes, a few ferns, and all seed plants. It arose multiple times during the evolution of unrelated vascular plant lineages and was already commonplace in the Devonian period, more than 360 million years ago. These spores are designated as microspores and megaspores, and are produced in microsporangia and megasporangia, respectively. Although "micro" implies small and "mega" large, megaspores are not always larger than microspores, particularly in seed plants. However, Functional differences between them are always present. Microspores give rise to male gametophytes (microgametophytes), and megaspores to female ones (megagametophytes). Both types of unisexual gametophytes are much smaller than the gametophytes of homosporous vascular plants. In heterosporous plants, they develop within the spore wall, whereas in homosporous plants, they develop outside it.

In horsetails, all spores are morphologically identical yet physiologically distinct—some give rise to unisexual gametophytes. It can therefore be assumed that horsetails exhibit physiological heterospory, which likely represents an evolutionary stepping stone toward the morphological heterospory discussed above.

Gametophytes and Gametes

The relatively large gametophytes of homosporous plants are nutritionally independent of the sporophyte, although the subterranean gametophytes of certain species, such as whisk ferns and several clubmosses, are heterotrophic and obtain nutrients via endomycorrhizal Fungi. In other clubmosses, as well as in most ferns and horsetails, the gametophytes are free-living and photosynthetic. In contrast, the gametophytes of heterosporous plants derive their nutrients from the sporophyte.

The Evolution of the vascular plant gametophyte is characterized by a gradual reduction in size and structural complexity, with this reduction being most pronounced in angiosperms (see p. 332). Archegonia are present in all vascular cryptogams and most gymnosperms, but are absent in all angiosperms; antheridia, which are typical of cryptogams, are entirely lacking in gymnosperms and angiosperms. In cryptogams, including ferns, the motile sperm swims through an aqueous medium to the archegonium; consequently, these plants must inhabit environments where Water is abundant, at least periodically.

The seed is a unique structure in which the embryo is shed from the parent plant, enclosed in a tough seed coat along with a food supply that AIDS its establishment. Seeds appeared in several vascular plant lineages at least 360 million years ago. Among these groups are lycophytes and horsetails, which are discussed in this chapter on cryptogams because all living representatives of these phyla lack seeds. The evolution of the seed—one of the primary factors underlying the dominance of seed plants—and the KEY FEATURES OF such plants will be examined in Chapter 18.