1922] 



PFEIFFER MONOGRAPH OF THE ISOETACEAE 95 



Such plants as this might be considered as grading readily into 

 the more distinctly terrestrial forms. Of these transitional forms, 

 some start growth in very moist habitats, fruit as the season 

 progresses, and die down except for the corm, during the very 

 dry intervals of late July and August. Such plants as these 

 could not have come into consideration when the name "Isoetes, 

 equal at all seasons" was applied. In our country, /. Orcuttii of 

 southern California appears "in level pastured meadowland, 

 growing scattered in grassy sod, especially in low places, but 

 never in places where water stands or has stood. The leaves so 

 nearly resemble surrrounding grass in size and color that the 

 plant can scarcely be found except by lifting sod and tearing it 

 apart." In some ways it seems comparable to the Algerian forms, 

 /. histrix and /. Duriaei, which grow in exposed places, the latter 

 with xerophytic grasses in crevices on granitic mountain slopes. 



Plant Associates. Data in regard to the plants occurring with 

 Isoetes are not available in many cases. The table shows what 

 has come to notice in this regard, in some American species. 

 Frequently it would seem that Isoetes has few competitors where 

 it succeeds in holding its own. On the the other hand, it may be 

 so thoroughly mixed with grasses, sedges, and other forms of a 

 like habit, that it is easily overlooked. 



Physiological and Structural Relations. In the matter of 

 structural characters, there is an unusual uniformity throughout 

 the group, regardless of environment. All display a corm of two or 

 three lobes, in which reserve material is deposited. In some cases, 

 the corm endures drought for one or more seasons and, with 

 return of moisture, again becomes active. The corm rarely 

 branches, and hence there is no vegetative spread by this method. 

 There is great diversity in number and size of leaves ; and though 

 this appears to have no invariable relation to the habitat, smaller 

 forms frequently occur in the terrestrial group. There is a gen- 

 eral plan of structure in the leaves which includes four lon- 

 gitudinal air spaces, separated by septa, and surrounded by a 

 peripheral wall of green tissue. There is a tendency for greater 

 development of the air-spaces, with relatively fewer layers (as 

 low as two) of green tissue and narrower septa in the submersed 

 forms. Species in dry situations show less surface development. 

 The leaves are usually narrower with small lacunae, walled by 



