ENGELMANN — THE GENUS ISOETES IN N. AMERICA. 375 



* * Velum complete. 



14. /. Nuttallii. 



II. Trunk trilObCd, numerous stomata and bast -bundles in the quadrangular 



leaves; velum partial. 



15. /. Cubana. 



§ .5. Geographical Distribution. 



Only a small part of the North American continent has been well ex- 

 plored for Isoetes, and there, from Massachusetts to the Chesapeake Bay, 

 they appear abundant enough; farther south, and in the whole interior 

 and western part of the continent, they have thus far been found only in 

 a few localities. Some species are quite local, as is the case also with 

 many species of the old world, while others are widely distributed. Our 

 two northern species are identical with, or closely allied to, European 

 forms, all the others are quite distinct from such, so that there is scarcely 

 more than a generic analogy between the species of our middle and 

 southern regions with the Mediterranean ones or those of other regions 

 of the globe. 



The old Linnean Isoetes lacustris is the only species which has been 

 found to extend from the Atlantic to the Pacific States, and it probably 

 occupies a northern belt of the northern hemisphere, though it seems 

 not to have been discovered as yet in Asia. The American forms allied 

 to I. eckiiiospora, the other North European species, are the most com- 

 mon in the belt of northern States as far west as Michigan, and have been 

 detected also on the western slope of the Rocky Mountains. Of the others, 

 /. Etigelmanni extends from Massachusetts to Georgia and westward to 

 Missouri, though thus far not found anywhere else west of the Alleghany 

 Mountains. J. Jiaccida is peculiar to Florida and /. Bolanderi to the 

 lakes of the western mountain chains, the Rocky Mountains as well as 

 the Sierra Nevada. /. melanofoda occupies parts of the Mississippi val- 

 ley from Central Illinois to Northeastern Texas, while /. Nuttallii is the 

 only species found in the valley of the Columbia river. All the other 

 species seem to be nearly or quite local, /. pygmcea, in the Californian 

 Sierra, but most of them on the Atlantic border ; thus/. Tuckermani oz- 

 curs only near Boston, /. saccharata on streams emptying into the Chesa- 

 peake Bay, and /. melanospora only on that peculiar and, botanically, so 

 interesting rock, the Stone Mountain of Georgia. Some species which 

 seemed local have lately assumed a little wider range, though yet quite 

 restricted ; among these I mention /. rifaria of the banks of the lower 

 Delaware river, which occurs also farther north, and T. Butleri, first 

 known only from the Indian Territory, now also found in Tennessee. 

 There can be no doubt but that some of the apparently local species will 

 yet be found in a more extended area, when botanists will include in their 

 researches these obscure and inconspicuous plants. 



From the warmer parts of North America we know only /. Cubana, 

 from Cuba ; none have yet turned up from Mexico. 



The following table will explain itself. 



