In the following arrangement I have based the key on charac- 

 ters which will most easily allow the determination of species with- 

 out recourse to the compound microscope. Though not perfect, I 

 hope it will be helpful to the general student, 



SYSTEMATIC ARRANGEMENT. 

 Group i. 



Plants habitually under water in the driest seasons, growing in 

 sand or gravel. Bast absent, and stomata few or none. 



Submersae. 



Leaves 1.5 to 2mm. in diameter, stiffly erect, stomata none. 



1015 cm. long, spores (averaging under 600 //) covered with 

 short crests, rarely a little reticulate below. i. lacustris. 



5-8 cm. long, spores larger (averaging over 600 n, at times 

 over 1000 /*), of various shapes, more densely cristate, 

 crests anastamosing but not reticulated. 2. heterospora . 



Leaves spiral or recurved, stomata few or none. 



Leaves very slender, under i mm. in diameter, reddish when 

 young, becoming olive-green, 8-15 cm. long, stomata in 

 single series over air cavities, or none, macrospores wavy 

 crested above, more or less reticulate below, 600 /j. or less 

 in diameter. 3. Tuckerniani. 



Leaves dull green, stouter, about imm., not spiral, slightly 

 recurved, stomata not seen, spores larger, averaging above 

 650 /*. 33. Tuckermani borealis. 



Leaves stouter, over i mm. in diameter, shorter, 8-iocm., 

 rigidly recurved, spores less than 600 fj., loosely covered 

 with vermiform wrinkles. 4. hieroglyphica. 



Leaves very stout, 2.5-3mm., short, 5-6cm., purple bronze in 

 color, rigid, recurved, spores as in No. 3. 5. Harveyi. 



Group 2. 



Plants growing usually in mud on borders of ponds or rivers, 

 inundated most of the year, but fruiting as the water recedes. Sto- 

 mata abundant but peripheral bast-bundles none. Never far from 

 water, and always in very damp soil. Amphibiae. 



Spores reticulated below, jagged cristate above, much as in 

 No, 3, but leaves stouter and erect. 6. riparia. 



Spores smaller, averaging 440 /*, covered with very small 

 pits. 7- foveolata. 



Spores covered with spinules, stomata absent. 



8. echinospora, not American. 



