MARSILIA 323 



clear that the food of this much-reduced prothallium is fur- 

 nished chiefly by the sporophyte by means of the megaspore. 

 The gametophyte of Marsilia has therefore degenerated from 

 the independent condition in the common ferns and is now 

 no longer self-supporting, but is dependent upon food stored by 

 the sporophyte, a relation which is exactly the reverse of that 

 in the bryophytes. 



The microspore develops a very small male gametophyte even 

 more quickly than the megaspore develops the female one. This 

 structure consists of a lens-shaped sterile cell called the prothal- 

 lial cell (Fig. 282, A, p), together with a group of cells which 

 probably represent a single much-reduced antheridium. The 

 sperms are formed within this group. They are remarkably long, 

 coiled bands covered with cilia (Fig. 282, B), and are among the 

 largest sperms in the pteridophytes. 



The young sporophyte develops within the archegonium, fol- 

 lowing essentially the same history as that of the common 

 ferns, and is consequently attached to the megaspore (Fig. 282, Z>). 

 But there is an important peculiarity in its relation to food 

 supply. This sporophyte makes use of considerable food that 

 remains in the megaspore after the development of the female 

 gametophyte. The Marsilia plant, therefore, actually provides 

 for the next sporophyte generation by storing food in the mega- 

 spore. This provision is strikingly similar, as will appear later, 

 to the conditions in the seed where the embryo (young sporo- 

 phyte) is nourished by food stored in the seed by the sporophyte 

 of the preceding generation. 



Marsilia illustrates exceptionally well three important prin- 

 ciples in the evolution of pteridophytes and seed plants, namely : 

 (1) the establishment of heterospory, resulting in the separation 

 of male and female gametophytes, (2) the reduction or degenera- 

 tion of the gametophytes, which become dependent upon food 

 stored in the microspores and megaspores, and (3) provision in 

 the megaspore for the nourishment of the embryo of the suc- 

 ceeding sporophyte generation. 



