swim through water in reaching the eggs. 

 The sporophyte generation, on the other 

 hand, is a well-adapted land plant. Often the 

 sporophyte reproduces vegetatively by send- 

 ing up new clusters of leaves (fronds) and 

 roots from the underground horizontal 

 rhizome (Fig. 12-4). Indeed, the rhizome, dur- 

 ing successive seasons of growth, may con- 

 tinue to reach out from the place where the 

 sporophyte first established itself, thus giving 

 rise to a number of sporophytes within a cer- 

 tain area. 



All available evidence indicates that the 

 Filicineae originated from the Psilophytales 

 (p. 563), and in fact, the earliest fossil ferns 

 bear a striking resemblance to this group. 

 Also the evidence indicates that the early 

 ferns, now extinct, represent the stock from 

 which the Gymnospermae and Angiospermae 

 arose (Fig. 29-11). 



The Gymnospermae (Class 2; Phylum 

 Tracheophyta; Subphylum Pteropsida). 

 This important group (about 750 living spe- 

 cies) includes all the conifers (pines, spruces, 

 hemlocks, ginkgoes, cycads, etc.). Among the 

 gymnosperms, the reproductive organs are 

 cones, rather than flowers. Essentially, each 

 cone is a cluster of sporophylls (modified 

 spore-bearing leaves). However, the gymno- 

 sperms are heterosporous; that is, they pro- 

 duce both microspores and macrospores. 

 Also the gymnosperms bear two kinds of 

 cones: (1) smaller staminate cones, which 

 produce microspores (pollen); and (2) larger 

 pistillate cones, which produce macrospores 

 (Fig. 31-25). 



All Gymnospermae (literally, "naked 

 seeds") produce seeds. Among Gymnosper- 

 mae, however, the seeds occupy an exposed 

 position — two on the upper surface of each 

 scale of the pistillate cone. 



The life cycle of a gymnosperm, such as 

 the pine, resembles that of the angiosperm, 

 which was described in Chapter 12. Typi- 

 cally, two ovules (macrosporangia) are borne 

 on the upper surface of each scale of the 

 pistillate cone (Fig. 31-26); and one micro- 

 scopic female gametophyte develops from 



The Plant Kingdom -617 



the macrospore at the center of each ovule. 

 Here it is protected and nurtured by the sur- 

 rounding tissues. Each microspore (pollen 

 grain), if it is carried by the wind to an ovule, 

 develops into a microscopic male gameto- 

 phyte, which likewise is protected and nur- 

 tured by the ovule tissues. Fertilization oc- 

 curs when the tip of the male gametophyte 

 (pollen tube) grows into contact with the egg 

 cell — after the female gametophyte (mega- 

 gametophyte) in the ovule has reached ma- 

 turity (Fig. 31-26). Finally, each ovule, as a 

 whole, gives rise to one seed and an embryo 

 of the next sporophyte generation lies at the 

 center of each seed (Fig. 12-22). 



This type of life cycle has many advan- 

 tages for land plants — as was explained in 

 Chapter 12. It enables the species to achieve 

 fertilization without requiring the sperm to 

 swim through water. Also the protected and 

 nurtured gametophytes are not required to 

 face the hazards of independent life, such 

 as are encountered by the poorly adapted 

 gametophytes of lower land plants. And 

 finally, the new sporophyte, at the center of 

 the seed, is carried to a relatively advanced 

 stage of development before it is forced to 

 shift for itself. Consequently it is not sur- 

 prising to find that the seed plants (Gymno- 

 spermae and Angiospermae) have come to 

 dominate the land environment. 



It is difficult to evaluate the tremendous 

 importance of the vast forests of pines and 

 other gymnosperms. They prevent soil ero- 

 sion; they provide food and shelter for wild 

 animals; they yield vast quantities of lumber 

 and wood products (paper, wood alcohol, 

 turpentine, etc.), and they have many other 

 values. All gymnosperms are woody plants, 

 chiefly trees. Some, such as the California 

 redwoods and big trees, achieve gigantic size. 

 The wood of most species consists mainly of 

 tracheids (p. 244); and most species are ever- 

 greens, with needlelike leaves. The adapta- 

 tion of the sporophyte generation to land 

 life in relatively dry cold regions is excellent; 

 and since sexual reproduction, including seed 

 production, can continue under such condi- 



