[Vol. 9 

 96 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



several layers of compact tissue and separated by as many as 

 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 I. 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. With exposure to air, there is development of these 

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

 six, chief aggregations, which are accompanied by smaller ones 

 on occasion. The degree of development of such accessory 

 groups varies greatly. Frequently the upper, more exposed region 

 shows more of these groups of cells. 



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

 mersed group. Isoetes echinospora and a close ally, /. 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 

 in all cases fibrous, as a result of dichotomous branching. 



Seasonal Relations. All species of Isoetes appear perennial, 

 with a longer or shorter growing season. Submersed forms are 

 reputed to be green during a longer period, as a rule, and usu- 

 ally fruit late, as August to October. A few amphibious 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. 7. Malinverniana of the aqueducts of Piedmont, Italy, 



