ENGELMANN THE GENUS ISOETES IN N. AMERICA. 367 



requires a good deal of patience. Sometimes the application 

 of iodine will very distinctly show the stomata by coloring their 

 guard-cells blue when only these contain amylon, but of course 

 not when the other cells are also filled with that substance. A 

 magnifying power of 150 to 250 diameters is best adapted to well 

 exhibit the stomata. 



To find the bast-cells it is necessary to make the thinnest pos- 

 sible transverse sections of the leaf, boil them well, and, if they 

 do not then show under water as bundles of minute, thick-walled, 

 darkish cells close to the epidermis, very distinct from the much 

 larger epidermis cells, the application of a solution of caustic pot- 

 ash, to clear the preparation, will readily bring them out. The 

 same magnifying power which we use for the examination of 

 the stomata may be applied for the study of the bast-bundles. 



I would advise anyone who desires to study the structure of 

 Isoetes leaves to commence with well known species and good 

 (if possible fresh) specimens, and make himself familiar with the 

 manipulation and with the appearance of their parts under the 

 microscope before he proceeds to study unknown and difficult 

 specimens. 



The arrangement of the leaves in the species with two-lobed 

 trunks is at first distichous, and in /. melanospora it remains so 

 through life ; in all the others the leaves soon enter into a more 

 complicated phyllotactic order ; in the larger ones, with many 

 leaves the ^\ and even the %\ order is found. 



The number of leaves varies from 5 or 10 (/. fygmcea, I. me- 

 lanospord) to 100 or even 200 (/. Engelmanni, var. valida)^ 

 and their length from \ to i inch (in /. pygmcea) to 1-2 feet (in 

 some forms of I.Jlaccida and /. Etigelmanni) ; their color from 

 light and fresh yellowish-green (/. Engelmanni) to dark and 

 dull green /. lacustris) ; their rigidity is greatest in the terres- 

 trial species, and also in some submerged ones ; and least in most 

 amphibious species, which often float their leaves on the surface 

 of the receding water, or in some submerged ones, the leaves of 

 which, taken out of the water, collapse like the soft hair of a wet 

 pencil. The submerged species vegetate and retain their verdure 

 throughout the winter (whence, it is said, the name of the genus 

 is derived : Isoetes, equal at all seasons), but the others lose their 



