336 THE PTERIDOPHYTES 



into the split megasporangia below. The sperms are formed 

 and set free in the moisture of such situations, and the eggs of 

 the gametophytes may be fertilized while the megaspores are 

 still retained within the megasporangium. The young sporo- 

 phytes as they develop are thus actually held by the sporophylls 

 of the parent sporophyte (Fig. 290, Z>) until they reach a con- 

 siderable size and fall off. These habits should be noted and 

 this paragraph read again after the life history of the seed plant 

 is thoroughly understood. 



The development of the young sporophytes of Selaginella 

 and also of Lycopodium has features resembling those of the 

 seed plants. The early divisions of the egg establish a structure 

 called the suspensor (Fig. 290, (7, Sus), which carries the devel- 

 . oping embryo down into the midst of the tissue of the gameto- 

 phyte, where it can draw nourishment from all of the cells 

 around it. A large foot is developed (Fig. 290) which absorbs 

 food from that portion of the gametophyte which lies in the 

 megaspore, so that the embryo sporophyte is actually nourished 

 with food stored in the megaspore by the sporophyte of the 

 previous generation. 



326. Life history of Selaginella. Selaginella is an excellent 

 type with which to illustrate the life history of a heterosporous 

 pteridophyte. Since two forms of spores, microspores and mega- 

 spores, are present, there are two forms of gametophytes, male 

 and female, and this feature complicates the relatively simple 

 life-history formulae of bryophytes and homosporous pterido- 

 phytes (Sees. 285, 310). 



The life history of Selaginella is as follows : 



~ , ^microspore Male Gametophyte sperm . 

 P P y ^megaspore Female Gametophyte egg - 



Sporophyte, etc. 

 This in abbreviated form becomes 



mi sp M G s . 



