ON THE PROTHALLIUM AND EMBRYO OF MARSILIA 

 VESTITA. 



BY DOUGLAS HOUGHTON CAMPBELL. 



In Studying the evolution of the higher cryptogams, 

 perhaps the most striking peculiarity noticed is the pro- 

 gressive reduction of the sexual generation. This has 

 evidently gone on along several distinct lines, and has 

 resulted in at least one of them in the production of seed- 

 bearing plants (spermaphytes), or as they are more popu- 

 larly called, phanerogams. Whether or not all the sper- 

 maphytes have had a common origin it is not the purpose 

 of the present paper to discuss. 



As we survey the great group of Pteridophytes we 

 find remnants among our living flora of four groups 

 where the preliminary step toward the formation of seeds 

 has been taken, but there is strong reason to believe that 

 in two, at least, of these, the process has not gone any 

 further. This preliminary step is heterospory or the 

 formation of two sorts of spores, large and small, giving 

 rise respectively to female and male prothallia. These 

 are very much reduced, and sometimes almost completely 

 included within the spore. 



Among living Pteridophytes, as already stated, are 

 four groups in which heterospory is present, viz : Mar- 

 siliacete, Salviniace^e, Selaginellese, and Isoeteee, all of 

 which are represented in the flora of California. It is 

 with the first of these that we have to deal here. 



The MarsiliacccE, including the two genera Marsilia 

 and Pilularia, are widely distributed species occurring 

 in the warm and temperate regions of nearly the whole 

 earth. These plants are remarkable for the extraordinary 



2d Ser., Vol. IH. [ 1 ] April 19, 1892. 



