138 



GENERAL BIOLOGY 



at the top of the stem, as indicated. The outer cover- 



ing of each spore is spHt into four long 

 involute strips, which serve as elaters, 

 and, being exceedingly hydroscopic, 

 these extend and roll up again with slight 

 changes of moisture and push the spores 

 out of the sporangia. 



Two kinds of Gametophytes. — AVhile 

 the spores all look alike, some of them 

 on germination develop into small pro- 

 thallia which produce only antheridia 

 and others grow into larger prothallia 

 which produce the archegonia. 



The prothallia (fig. 87) are therefore 

 unisexual, as in some of the mosses, but 

 each thallus and especially the male, is 

 of very small size. Fertilization occurs 

 with the aid of water for transport of 

 the sperms as in the fern, and the devel- 

 opment of the sporophyte from the 

 fertilized egg follows the same general 

 course. 



Selaginella (fig. S8,) is another pteridophyte with a trailing 

 stem bearing small two-ranked leaves and lesser scales. It 

 is especially interesting on account of the development of its 

 spores. The sporangia are located in the axils of scales 

 aggregated in several terminal laterally flattened spikes 

 and are of two sorts large (macrosporangia) , and small 

 (microsporangia) . The spores within them are of two 

 sorts, large ones (macrospores) , four in number developed 

 in the macrosporangia, and small ones (microspores) 

 developed in large numbers in the microsporangia. It is 

 less surprising, therefore, that there should develop from 

 them two sorts of prothallia ; but the prothallia themselves 



Fig. 87. Male and 

 female prothallia of 

 equisetum (after 

 Goebel). a, the 

 male prothallium, 

 with marginal an- 

 theridia; 6, a sperm 

 cell; c, the female 

 prothallium bearing 

 three archegonia. 



