1922] 



PFEIFFER MONOGRAPH OF THE ISOETACEAE 101 



appear, leaving a smooth surface. As illustration of intergrading 

 between closely related forms, the /. velata complex of the Med- 

 iterranean region serves well. Here one must use all the avail- 

 able distinctions to hold the forms apart; yet, were these not 

 used, the result would be simply a complex, incapable of treat- 

 ment as a unit. Examples of pairs of species in which the same 

 difficulty arises are /. riparia and /. saccharata, I. macrospora 

 and /. Tuckermani, I. echinospora and /. Braunii. In all these 

 cases, absolute separation on spore characters alone would be a 

 difficult task ; other features, even geographical range, may prove 

 invaluable as supplementary points. 



In all instances, it has been the aim of the writer to so treat 

 the forms as to avoid points of an insignificant nature and to 

 emphasize what seemed more important features. Only fairly 

 definite varieties have been maintained. In some cases, it has 

 been necessary to reduce long-recognized varieties and forms, 

 in others those of more recent standing. Possibly this has oc- 

 curred more frequently than usual in a work of this sort. The 

 most obvious explanation for this is related to the relatively 

 rare occurrence of the genus in the experience of most workers. 

 The result of this is the seeming distinctiveness of any single 

 collection made. One other possible result of inexperience is 

 description based on immature material. Many collections of 

 under-ripe plants are made without realization of the absolute 

 necessity of mature characteristics for positive determination. 



It is furthermore not always easy to determine from collections, 

 sometimes scant in amount and accompanied by few or no 

 habitat notes, to exactly what extent ecological factors have re- 

 sulted in variations. The writer is inclined to believe that such 

 variations have at times been the basis for described varieties 

 and has attempted in this work to avoid as far as possible clas- 

 sification of varieties and forms resulting from such factors as 

 the relative dryness of the season. 



SPECIES NOT EXAMINED 



Isoetes as a genus is distributed widely over the earth in very 

 local fashion, frequently in remote and inaccessible stations. In 

 such a group, it is not surprising that some forms have been 

 collected only once, nor is it unexpected that representation 



