1922] 



PFEIFFER MONOGRAPH OF THE ISOETACEAE 97 



is reported as fruiting throughout the summer. Most forms are 

 more restricted, as, for instance, /. melanopoda in May and early 

 June, or /. riparia in August and September. 



After spore production, there is a more or less rapid develop- 

 ment of the alternating generation, as shown by Engelmann in 

 his cultures. The sporeling stage then appears in a short inter- 

 val after spore production, a point demonstrated as early as 1828 

 by Delile in his careful observations on Isoetes setacea. Spore- 

 lings produce a small group of leaves which are sterile. Fruiting 

 structures appear only in the second or subsequent seasons. 



Geographic Distribution. It is the experience of collectors 

 that the distribution of Isoetes species is remarkably restricted, 

 though there are representatives in most parts of the world. 



In North America there is a striking exception to this in /. 

 Braunii Dur., a form found both on the east and west coasts, 

 and in much of the territory between these. Maine and Massa- 

 chusetts are perhaps richest in the number of stations reported, 

 but the form is common in eastern Canada, in the northeastern 

 United States, and is reported less frequently in Ohio, Indiana, 

 Michigan, Wisconsin, and Minnesota. There then appears a 

 3onsiderable gap in the continuity, with the western represen- 

 tatives appearing only as far as Utah, and then westward up the 

 coast. 



/. macrospora Dur. and 7. Engelmanni A. Br. possibly repre- 

 sent the forms next most widely distributed. The former is 

 northern, like /. Braunii, and is found in eastern Canada, and the 

 northern United States as far west as Minnesota. 7. Engelmanni 

 is of slightly more southerly occurrence, and is found from New 

 England, west to the Mississippi Valley, with Engelmann's own 

 station at St. Louis as the farthest west. 



More restricted in character are the following species of the 

 eastern border : 7. Tuckermani, I. riparia, I. saccharata, I. Eatoni, 

 and 7. foveolata. Farther south, in Georgia and Florida, occur 

 7. flaccida and its variety alata, while 7. melanospora is endemic 

 in one station, Georgia, and 7. lithophila in Texas. 



In the central states, besides 7. Engelmanni, the form 7. me- 

 lanopoda Gay & Dur. proves of rather extended range in Illinois, 

 Missouri, Nebraska, Iowa, Oklahoma, and Texas. Its companion 

 form, 7. Butleri, occurs in the western part of this range in even 

 drier localities. 



