STEMONITIS 159 



the surface the form of a distinct net which supports an evanescent 

 peridium. 



The genus is marked by its surface net supported at the tips of the 

 dichotomously branched divisions of the columella. Over the net is 

 spread, theoretically at least, the peridial film supported at times by 

 very short points projecting from the net— the peridial processes; 

 the peridium, however, is seldom seen; in some cases perhaps never 

 developed. Rostafinski first defined the genus as employed by 

 recent writers. Gleditsch simply renamed Micheli's Clathroidastrum; 

 all writers up to the time of Rostafinski included species of other 

 genera. 



The taxonomy of this genus is extremely difficult. Macroscopic, 

 defining characters are few, and even these sometimes uncertain. 

 Microscopic distinctions also tend to be illusive, variable in such fash- 

 ion that often at the critical point the most exact description fails. 

 All that may be done at present is to recognize two or three definite 

 types and then cautiously differentiate among these until more gen- 

 eral study of the group brings to service a wider range of observation. 



Since spore characters seem to be reasonably constant, they are 

 largely stressed in the accompanying key. The surface is in some 

 species beautifully and plainly reticulate, in others, the reticulations, 

 while present, are very delicate, and an oil immersion lens is required 

 to distinguish them. In some species, notably S.fusca, the reticulations 

 are combined with warts at the angles. These are referred to as 

 verrucose-reticulate. Some are rather coarsely papillate, others nearly 

 smooth even under oil. The capillitium is more variable but is often 

 distinctive. 



KEY TO THE SPECIES OF STEMONITIS 



a. Spores reticulate or verrucose-reticulate b 



a. Spores not reticulate; verrucose to smooth / 



b. Sporangia connate below 1. S. trechispora 



b. Sporangia distinct, appressed or scattered c 



c. Spore-mass black; net developed mainly at base; 3-6 mm. tall 2. S. nigrescens 



c. Spore-mass fuscous or paler <* 



d. Dark fuscous; surface net rather coarse; 18-25 mm. tall 3. S. dictyospora 

 d. Fuscous; surface net fine-meshed; usually 6-10 mm. tall 4. S.fusca 

 d. Paler, with vinaceous tints, in small clusters; usually 



under 6 mm. tall e 



e. Benzo-brown; spores banded-reticulate; surface net often 



incomplete ' 5. 5. virginiensis 



e. Lilaceous brown; spores faintly and irregularly reticulate; 



surface net developed below only 6. 5. hyperopia 



f. Sporangia connate with plate-like processes at junctions 



of capillitial branches of adjoining sporangia 7. S. confluens 



f. Sporangia separate S 



