388 



BOTANY. 



spores, while the interior ones contain microspores. Both macrospcres 

 and microspores are produced in great numbers iu the sporangia. 



Tlie Quillworts are for^tlie most part aquatic plants ; ihey are found 

 chiefly in the north temperate and warm regions. The species, of 



Pig. 280. — flerraination of the microspores of Tsoeies laeustris. ^, a microspore, 

 side view. B, the same, ventral view ; the ppore contents have divided into a few 

 cells, of which v in each figure lepresenta the rudimentary prothallium ; j3^ /3 are the 

 ventral, and (! (! the dorsal cells. C, a side view of microspore ; the four cells, ^g^ ^g^ 

 6 6 have disappeared, and spermatozoids have formed. D, ventral view of C; a to 

 /, development of spermatozoids. e and/ X 700, the others X 580.— After Millardet. 



which there are from forty to fifty or more, all bo/ong lo the single 

 genus Isoetes; we have representations of about fourteen within the 

 United States. 

 Two species of Isoetes occur as fossils in the Tertiary (Miocene). 



