614 - Heredity and Evolution 



elongate, leafy, prostrate stem, from which 

 numerous erect branches extend upwards 

 into the air, and numerous roots extend 

 downward into the soil. Also Lycapodium 

 spores, which are produced in great quantity, 

 are sometimes used as a dusting powder (ly- 

 copodium powder) in cases of skin irritation. 

 One of the spike mosses (Selaginella lepido- 

 phylla), which occurs in dry regions from 

 Peru to Texas, is called the resurrection 

 plant. During dry spells it rolls up into a 

 brownish, apparently lifeless ball; then 

 quickly and miraculously it unfolds into a 

 delicate green plant as soon as rainy weather 

 comes. 



Most Lycopsida produce only one kind of 

 spore (homospory), each of which may germi- 

 nate into a gametophyte in which both arche- 

 gonia and antheridia are formed. However, 

 a few species in the genus Selaginella display 

 heterospory. Selaginella produces two kinds 

 of spores — large macrospores, and small 

 microspores — as is shown in Figure 12-13. 

 This important feature seems to foreshadow 

 developments that are characteristic of the 

 highest plants (Gymnospermae and Angio- 

 spermae). 



In Figure 12-13, it may be noted that the 

 female gametophyte originates from the 

 macrospore; whereas a male gametophyte 

 comes from each microspore. The female 

 gametophyte not only gains protection from 

 the moisture-proof cover of the macrospore, 

 but also it receives nourishment from organic 

 materials previously stored up in this large 

 cell. Then in its turn, the female gameto- 

 phyte protects and nurtures the next genera- 

 tion, while the egg is being fertilized and 

 while the zygote is developing into the em- 

 bryo of the new sporophyte. 



Despite the fact that heterospory, which is 

 characteristic of the higher plants, had its 

 first incidence among the Lycopsida, the fos- 

 sil record and other evidence indicates that 

 this group did not give rise either to the 

 gymnosperms or to the angiosperms. The 

 Lycopsida arose, apparently, from an early 

 psilopsidan stock, the Psilophytales, of which 



a number of fossil forms have been found. 

 But after a great climax during the Car- 

 boniferous period, the Lycopsida have dwin- 

 dled, and now they seem well along the road 

 toward extinction. The Psilopsida and the 

 Lycopsida tend to resemble each other, espe- 

 cially in regard to the dichotomous branch- 

 ing of the stem and the primitive arrange- 

 ment of the vascular tissue in the stem. 



The Sphenopsida (Subphylum 3: Phylum Tra- 

 cheophyta). This group, which likewise is 

 represented mainly by fossil species, reached 

 a peak of size and abundance during the 

 Carboniferous period. At this time many 

 species were treelike plants attaining heights 

 up to 100 feet; and Sphenopsida had almost 

 a world-wide distribution. To some extent 

 the Sphenopsida took part in the formation 

 of coal, although most of our large deposits 

 were derived from the Lycopsida. 



Today there is only one surviving sphe- 

 nopsidan genus, namely Equisetum (Fig. 31- 



CONE 



LEAVES- 



7\ 



ADVENTITIOUS ROOTS 



Fig. 31-22. Horsetails (Genus Equisetum) are the 

 sole survivors from the ancient Sphenopsida, which in 

 ancient times included a wide variety of treelike spe- 

 cies. This species {Equisetum arvense) is less than 12 

 inches tall. Note the horizontal stem (rhizome); the 

 relatively small roots; the small, scalelike leaves; and 

 the conelike spore-forming strobilus. (From The Plant 

 World.) 



