548 LIFE-HISTORIES 



The Lj^copodinaD, which comprise in a very few genera about 

 600 species, occupy an intermediate place betiveen Filicina; and Equise- 

 tinm in the relative size of their leaves and in their arrangement which 

 may he either alternate or whorled; but, ichile having the sac-leaves or 

 sacs often in terminal cones, the sporangia are always solitary and 

 never on the under side of a leaf; and there are no elaters. 



195. The pteridophyte division, fernworts (Pteridophyta) 



is made up of the three classes above named. Ferns being 

 especially typical of them all, the plants of this division are 

 conveniently designated as fernworts. 



While, as we have seen, mossworts were perhaps the first green 

 plants out of water to succeed in standing upright, it is among 

 fernworts that we first find vertical growth producing lofty trunks. 

 Spores formed near the top of such a trunk are plainlj^ given an 

 immense advantage since they may be dispersed over an extensive 

 area. This highly beneficial provision for the welfare of offspring 

 was made possible by the development of roots able to absorb 

 subterranean moisture, and of leaves that could utilize it in con- 

 nection with the air and sunlight. A protective function with 

 reference to the sporangia or to tender parts of the stem was easily 

 assumed by these lateral appendages, and in some cases became 

 their chief or only office, as happened with the sheathing whorls of 

 Equisetum or the cone-scales of Selaginella. Various differentia- 

 tions of the stem-parts, gave rise to more or less branched ascending 

 axes, either independent or climbing, or to more or less horizontal, 

 often subterranean stems in which the capacity for storing food was 

 often especially developed, and from which vertical branches or 

 vertical leaves arose during seasons favorable for growth. For the 

 bearing of spore-sacs either leaf-parts or stem-parts were available; 

 and sometimes the one, sometimes the other was used. Nurse- 

 plants were depended upon to foster the embryo and prepare it for 

 independent, vigorous life; and the nurse itself was so well provided 

 with reserve food that it could afford to dispense with food-making 

 organs of its own to a considerable extent. It is thus perhaps of 

 evolutionary significance that the gametophyte of fernworts is 

 commonly much simpler in form and of less vegetative importance 

 in the life-history of the plant than is the case in mossworts. An 

 extreme of specialization in the reproductive function of the game- 

 tophyte is found in those fernworts which have male and female 

 si)ores, the latter having in general such a large amount of reserve 

 food that the nurse-plant does not need to make anv for itself and 

 scarcely protrudes beyond the spore, while the former having no 

 embryo to nurse reduces its vegetative part to a single cell. Such 

 gametophytes are virtually hysterophytic, and it is interesting to 

 observe that types which do not produce macrospores but are 



