THE VARIOUS GROUPS OF PLANTS 59 



to the formation of coal. Otherwise their economic significance is 

 very hmited : some warmth-loving species are used as pot-plants, 

 and trailing Club-mosses are made into Christmas-wreaths (hence 

 another name, ' Christmas-greens '), while the minute spores of 

 members of the same genus are so highly inflammable owing to 

 stored oil that they can be used to produce ' stage lightning '. They 

 are still employed in dusting operations where a fine powder is 

 required, and for demonstrating sound-waves in physics. 



FiLiCiNEAE : These are the F'erns, and include the majority of 

 living Pteridophyta as well as some extinct forms. They are 

 perennial plants, sometimes small and moss-like but usually of at 

 least substantial size. The stems range from creeping and slender 

 to erect and stout, and from subterranean to aerial or occasionally 

 aquatic ; in Tree-ferns, the often massive, erect aerial stems may 

 be several yards high. There are usually plentiful fibrous roots 

 below, and, above, leaves (fronds) that characteristically are large 

 and compound, composed of more or less numerous segments. 

 Occasionally, however, the leaves are small and simple ; indeed the 

 Ferns are very varied in form, as may be seen from Fig. 17, which 

 shows a range of diff'erent types. These, of course, are all sporo- 

 phytes, the gametophytes being always small and insignificant. 



The spores are formed in sporangia which are commonly borne 

 in groups upon or partly within the lower surface of the ordinary 

 leaves, though in many cases the spore-producing leaves, or parts 

 of leaves, are modified — sometimes so drastically that they are 

 scarcely recognizable as leaf members. In most types the spores are 

 all of one kind and produce on germination a filamentous, or more 

 often a flat, green prothallus rather like a small unbranched thalloid 

 Liverwort, anchored to the ground by rhizoids and bearing both 

 male and female organs, though a few types have a subterranean 

 and saprophytic prothallus. However, in the small and usually 

 aquatic group known as Water-ferns, which have slender stems and 

 small and sometimes very simple leaves, separate microspores and 

 megaspores are formed, which produce male and female organs, 

 respectively, on germination. Following fertilization by the swim- 

 ming, corkscrew-like spermatozoid, the egg develops into a fresh 

 sporophyte plant which soon becomes photosynthetic and inde- 

 pendent. This is its primary mode of nutrition. Thus the life- 

 cycle, involving as usual in these vascular land-plants an alternation 

 of sexual and asexual generations, is completed. The sporophytes 



