242 THE AMPHIBIOUS QUILLWORTS. 



numerous as to almost obliterate the openings between 

 them, which thus resemble little pits. This appearance 

 gives the plant its specific name. The microspores are 

 dark brown, 22^ to 34^ long, and marked like the meg- 

 aspores. 



I so etes foveolata plenospora is a larger form, with longer 

 and more numerous leaves, pinkish in colour. The 

 megaspores are similar to those of Isoctes foveolata, but 

 with higher, thinner crests, especially in the lower part 

 of the spore, and thus have a great resemblance to the 

 spores of Isoctes Tnckeruiani. Both the species and 

 variety have been found in New Hampshire and Massa- 

 chusetts only, and there in but -few stations. 



Isoetes Gravesii. 



It is possible that geological conditions play an im- 

 portant part in the making of Isoctes species. It has 

 been noticed that a very slight change in the composi- 

 tion of the soil makes a change in the species inhabiting 

 it. A close student of the genus wrote recently : 



" You may always expect to find certain species in 

 certain soils. As soon as I see a pond in my region I 

 can usually tell what species of Isoetes you will find in it 

 if they occur at all." 



No experiments appear to have been made to dis- 

 cover whether the same species will take on different 

 forms when grown in different soils ; but, should this be 

 proved to be true, it would throw much light upon the 

 relationship of Isoetes riparia and Isoetes sac e liar at a, and 

 perhaps give a clue to the origin of the plants which 

 have been named Isoctes Gravesii. Thus far they have 

 been found in only a few stations in the State of Con- 



