On the Genus Isoetes. 



127 



a compact humus absolutely crowded with vegetation. More- 

 over, if the winter frosts should tend to " throw up " the 

 detached leaves, yet the spines or teeth, which are strong and 

 point upwards, would serve as anchors to keep it below the 

 surface. Now in the aquatic species there are no spines or 

 teeth, and here we at once see how the " wash " on the margin 

 of a shallow lake or pool would speedily disengage any mature 

 leaf bases, which might then by the agency of aquatic 

 birds, or even the larger water beetles, be dispersed far and 

 wide. But how does the dispersion take place in our terrestrial 

 plant ? 



And then again, which of the two forms most approaches 

 the original type ; the aquatic one, without spines, or the terres- 

 trial one, armed with stout anchoring teeth ? In other words, 

 is Isoetes hystrix the survivor of an almost extinct race of 

 spiny-bulbed, land-growing forms, from which the aquatic 

 species have sprung, and in the course of modification by 

 descent, have lost the no longer needful anchoring spines ? Or 

 on the other hand, does the spinous bulbed form represent the 

 more complex and highly specialised descendant of a simple 

 smooth-bulbed aquatic progenitor ? and lastly, of what use are 

 the spines at all, and what end do they serve ? 



