Introduction to Seed Plants I: Gymnosperms

The life cycle of vascular cryptogams is an alternation of independent, heteromorphic generations. A disadvantage of this life cycle is that the new sporophyte is temporarily dependent upon a tiny gametophyte for its start in life. Consequently, many new sporophytes perish. All the genes of each gametophyte and half the genes of each new sporophyte embryo are identical to those of the maternal sporophyte, so any mutation that improves the survival of the gametophytes or embryos contributes to the reproductive success of the maternal sporophyte. It would be advantageous if the embryo could use the photosynthetic and absorptive capacity of the leaves and roots of the previous sporophyte. In order £r this to happen, the megagametophyte and embryo must be retained inside the maternal sporophyte; this is accomplished by retaining the megaspore and allowing the megagametophyte to develop within the sporangium (Fig. 1a to d). Such an arrangement requires some alteration in the microgametophyte as well, because retention of the megagametophyte changes its position. Free-living gametophytes develop on the soil, whereas retained ones develop high on the plant in strobili, a position that cannot be reached by swimming sperm cells produced by microgametophytes living at the soil surface (Fig. 1e). The modifications that overcame this problem are simple: The megasporophylls with the megagameto- phytes were arranged in upright cone-like structures, allowing microspores to be claimed by wind and dropped into the megasporangiate cone. There they would germinate into a microgametophyte, produce their antheridia and sperm cells, and carry out fertilization. Fertilization itself has changed little, with a moist sporophyll replacing moist soil. These changes produced the first seed plants, the division Pteridospermophyta or seed ferns and the early members of division Coniferophyta, known as class Cordaitles. These two groups are now extinct, but their descendants still exist as the living seed plants. In several lines of evolution, those ending in the cycads and in Ginkgo, this method of fertilization still exists, and beautiful swimming sperms are produced. In other lines, those leading to conifers and to flowering plants, sperms have become nonmotile and are carried to the egg by growth of the microgametophyte as a pollen tube.

FIGURE .1: (a and b) Plants that release megaspores. Only limited amounts of nutrient can be placed in a spore, and after spore release (b) the sporophyte can do nothing more to nourish or protect the gametophytes or new sporophytes. All the genes of each gametophyte and half the genes of each new sporophyte are identical to those of the parental sporophyte, so any mutation that improves the survival of the gametophytes or young progeny sporophytes contributes to the reproductive success of the parental sporophyte. (c and d) Plants that retain megaspores. Mutations that cause a reduction in the number of megaspores and their retention in the sporangium can be selectively advantageous. The sporophyte can nourish and protect its gametophytes and also the subsequent progeny sporophytes, at least for a while. Whereas the free gametophytes in (b) might be killed by a brief drought, those of (d) are provided with water by the root and vascular system of the parent. Also, the sporangium wall may have a sclerenchyma sheath or antiherbivore chemical defenses that protect the spores, gametophytes, and new sporophytes; such protection is not available in free-spormg species once the spores are released. In (d), the embryo is shown embedded in tissue of the megagametophyte; this occurs in gymnosperms—seed plants that are rot flowering plants. In flowering plants, the tiny megagametophyte is quickly replaced by endosperm, (e) Megasporangia that are packed together in a cone composed of sporophylls automatically have a means of catching and retaining microspores. These germinate into microgametophytes with antheridia that release sperms within a millimeter or two of the megagametophyte and its archegonia.

An old classification from the 1800s grouped all the seed plants together in a single division, Spermatophyta, with two classes, class Gymnospermae and class Angiospermae. The gymnosperms are those plants with "naked ovules," that is, ovules located on flat sporophylls, not enclosed in carpels. Angiosperms are the flowering plants, those with carpels, which are believed to be sporophylls that form a tublelike, closed structure. This classification emphasized both the close relationship of all seed plants and the idea that the evolution of the flower was the most significant event in this group. But new fossil evidence indicates that seed plants are a much more ancient group than previously thought, and many evolutionary developments were occurring: the origin of seeds and cones, the origin of a vascular cambium and secondary growth, the origin of vessels, and the origin of many defensive chemical compounds. The different types of gymnosperms are more distinct than we realized before, and some must have evolved to be the ancestors of flowering plants (Fig.2). Consequently, many botanists believe that the old division "Spermatophyta" and classes "Gymnospermae" and "Angiospermae" should be abandoned, and each major group should be elevated to division status. Other botanists believe that these various lines are still rather closely related and form a natural group and should therefore be held together in one division. Either way, the concept of "gymnosperm" is still so useful that it is constantly employed, at least informally, and the word "angiosperm" is still used to refer to flowering plants. The divisions of living seed plants commonly accepted now and used in this book are (1) division Coniferophyta, (2) division Cycadophyta, (3) division Ginkgophyta, (4) division Gnetophyta (these four are gymnosperms), and (5) division Magnolio- phyta (the flowering plants). Numerous extinct groups exist, the most significant for us being progymnosperms and seed ferns.

FIGURE .2:Proposed phylogeny of seed plants based on the most recent analysis of fossils and living plants. Rhyniophytes are the basic group, and then trimerophytes, which gave rise to several lines, He most important for us being Aneurophytales. An early split occurred into an evolutionary line (conifers and related groups) that would never form flowers and a second line (flowering plants and related groups) that would. The many dashed lines indicate uncertainty; many of these plant have so many similar, derived features that it is not easy to be certain how they are interrelated. Many important evolutionary advances involved subtle changes in metabolism and embryo nutrition, characters not preserved in fossils. (See also Paleobotany and the Evolution of Plants by W. N. Stewart and G. W. Rothwell, 1993, Cambridge University Press; and Paleobotany: An Introduction to Fossil Plant Biology by T. N. Taylor and E. L. Taylor, 1993, Prentice-Hall.).