2i 4 THE ISOETACE^E. 



lar in cross-section, and have a smooth papillose or spiny 

 surface mimicking the megaspores. The size of the 

 spores is usually given in thousandths of a millimeter, 

 the Greek letter /" standing for a single thousandth. 

 Three hundred and fifty thousandths of a millimeter 

 would be written 350". 



The prothallia of the quillworts lack 

 chlorophyll, and, like the prothallia of 

 Selaginella, do not separate from the 

 spores. At maturity the spore simply 

 opens along the three commissural 



(Greatly enlarged.) . , A . . . . . 



ridges, exposing the archegoma, and in 

 this position the eggs are fertilized by the* sperms. 

 Some of the aquatic forms have the power of producing 

 buds in place of sporangia, and these finally float away 

 and become new plants. 



Of the fifty or more species of Isoetes a large number 

 dwell constantly on the bottom in lakes, ponds, and 

 slow-moving streams, at depths of from one to ten feet 

 beneath the surface of the water. Others, however, 

 occur in such situations that the receding water in sum- 

 mer exposes them to the air, while a few are rarely if 

 ever submerged, though they always grow in wet places. 

 Those that are exposed to the air have stomata or 

 breathing-pores, but those always beneath the surface 

 have none. It is interesting to observe that in those 

 species that are only occasionally exposed to the air, the 

 stomata are found only on the tips of the leaves, in the 

 position where they would be most useful to the plant 

 should the water leave any part of it exposed. 



The species of Isoetes are so much alike externally that 

 for exact identification a compound microscope is neces- 

 sary. An examination of the ripe megaspores alone is 



