216 - Multicellular Plants 



little chlorophyll, and is dependent for its 

 growth upon substances initially present in 

 the macrospore. 



The many sperm, which are liberated from 

 the male gametophyte, swim in the soil 

 water, and each may succeed in fertilizing an 

 egg. The zygote gives rise to the young sporo- 

 phyte, which at first grows inside the female 

 gametophyte, at the expense of organic mate- 

 rials present in the gametophyte cells (Fig. 

 12-13). Finally, however, the sporophyte de- 

 velops its own root-stem-leaf system and be- 

 comes an independent, self-sustaining plant 

 (Fig. 12-13). 



In Selaginella and other club mosses, the 

 gametophyte has lost the stattis of an inde- 

 pendent plant. It is reduced to a mere repro- 

 ductive mechanism, which, however, assures 

 the species of fertilization and biparental in- 

 heritance. The sturdy land-adapted sporo- 

 phyte provides the gametophyte with a food 

 supply adequate for the production of gam- 

 etes, and thus the hazards experienced by a 

 poorly adapted gametophyte are reduced. 

 This eliminates one vulnerable feature of 

 the cycle, which tended to prevent the spread 

 of more primitive plants to drier regions of 

 the earth. But only among the seed plants 

 has another vulnerability of the cycle been 

 eliminated, since Selaginella still requires a 

 water-flooded soil at the time when the deli- 

 cate sperm are swimming to the eggs. 



LIFE CYCLE OF THE SEED PLANTS 



The seed-bearing plants are by far the 

 most successful groups in the plant kingdom. 

 All common trees, shrubs, grasses, weeds, 

 and "garden plants" are of this kind. In 

 number of species, seed plants far exceed all 

 other plants combined, even though their 

 evolutionary history — as indicated by the 

 absence of their fossils in all earlier geologi- 

 cal formations — has been a comparatively 

 short one. The seed plants are exceedingly 

 well adapted to land conditions, and some 

 species have spread to virtually all habitats. 



A seed, essentially, is a dormant embryo 



sporophyte, protected by several enveloping 

 tissues. But the true nature of seeds, and the 

 importance of seeds in the evolutionary suc- 

 cess of the higher plants can be appreciated 

 only in relation to the life cycle (see later). 



Gymnosperms vs. Angiosperms. The Gym- 

 nospermae (literally, naked seeds) include 

 the pines, spruces, hemlocks, ginkgoes, cy- 

 cads, and all other plants having seeds that 

 are formed in cones (rather than true flowers) 

 and that are not contained within an ovule 

 chamber (see below). 



The Angiospermae (literally, covered seeds) 

 include a wide variety of trees, shrubs, grasses, 

 and all other plants possessing true flowers, 

 and seeds that develop inside of the cap- 

 sular ovule chamber of the flower. 



The Gymnospermae, while less numerous 

 in species than the Angiospermae, show more 

 variability in the reproductive cycle (Chap. 

 31). The Gvmnospermae exhibit many transi- 

 tions between the rather primitive cycle of 

 Selaginella (p. 214) and the well-established 

 cycle of the Angiospermae. But it is neces- 

 sary to pass over these transitional types and 

 to deal mainly with the flowering plants 

 (Angiospermae), which make up a great ma- 

 jority of familiar plants, especially in tem- 

 perate zones. 



The Sporophyte Generation. Among the 

 Angiospermae the whole visible plant — con- 

 sisting of roots, stem, leaves, and flowers — 

 represents the sporophyte generation. The 

 sporophyte varies in size in different species, 

 from the tremendous bulk of larger trees to 

 the inconspicuous size of many grasses. But 

 both the male and the female gametophytes 

 ol the angiosperms are reduced to micro- 

 scopic dimensions. In fact these haploid 

 sexual stages are only found by dissecting 

 into the tissues of the flower. In the flower 

 the gametophytes receive protection and 

 nourishment during the critical period when 

 the eggs and sperm are formed, and while 

 fertilization is occurring. 



The Flower. A flower represents an organ- 

 ized group of modified leaves, which fulfills 

 the reproductive functions of the plant. A 



