96 



[Vol. 9 



AKDEN 



of compact tissue and separated by as many 



five to seven layers in the partitions. 



In terrestrial forms, the bases of the leaves often prove per- 

 sistent. Sometimes they remain as simple brown papery scales, 

 but in their most striking forms they become horny, with ex- 

 tended spines, marginal and central, as in /. histrix. 



The Leaf shows variation in two other directions, in the dis- 

 tribution of groups of mechanical cells in the peripheral region 

 and in presence of stomata. The former may be entirely lacking, 

 as usually occurs in submersed forms, and sometimes in am- 

 phibious. Wii 



i 



development of 



ps of supporting cells, often to the number of four, sometimes 



chief aggregations, which are accompanied by smaller ones 



on occasion. The degree of development, of such accessory 



groups varies greatly, 

 shows more of th 



more exposed 



In the ease of stomata, there is no uniformity within the sub- 

 mersed group. Isoetes echinospora and a close ally, I. Braunii 

 of America, both submersed, show striking dissimilarity in this 

 regard. The former has no stomata, the latter always some. 

 Other species may show even more. In the amphibious and 

 terrestrial forms, it is obvious that there will always be more or 

 less development in the regions exposed to air. 



A third leaf feature, the velum, which covers the basal spor- 

 angium, has attracted more or less attention. It is included here 

 for that reason, rather than because of any definite correlation 

 with ecological features. The range from very narrow to com- 

 plete veils seems to be run in each group. 



Very little attention has been paid to root systems, which are 



fibrous, as a result of 



Seasonal R 



r 



of 



with a longer or shorter growing season. Submersed form 



fruit late, as Au 



lie, and usu- 

 ibious forms 



fruit in the spring, and then die down during the summer, some- 

 times to become green again in fine fall weather. Engelmann 

 traced through the seasons the activity of Isoetes Engelmanni, 

 a vigorous amphibious form, which continues active throughout 

 the summer and fall season, producing spores for a relatively long 

 period. /. M al inv ernian a of the aqueducts of Piedmont, Italy, 



