MUCORALES 253 



cycle. It is essential, therefore, that the sporangium be dis- 

 charged toward the Hght rather than into the crevices of the 

 substratum. The subsporangial vesicle functions as a lens in 

 much the same way as does a flask filled with water. If the rays 

 of the sun strike on one side of the vesicle they are refracted 

 through it and converge on the opposite side forming a spot of 

 light. The protoplasm at that point receiving thus a heliotropic 

 stimulus, growth and elongation occur on that side of the sporan- 

 giophore. In consequence the sporangium is turned toward the 

 light until the rays strike it head on. A condition of physio- 

 logical equilibrium is then reached and the turning movement 

 ends. 



2. Pilaira van Tieghem (1875: 51). 



Sporangiophore erect, unbranched, lacking a subsporangial 

 vesicle; sporangium terminal, not shot away, when young globose; 

 the membrane in the upper hemisphere black and thickened as in 

 Pilobolus, below thin, hyaline, at maturity greatly distended, and 

 finally disappearing; columella flat to globose, not falling away; 

 zygospore formed as in Pilobolus, the conjugating branches erect 

 and tending to twine about each other. 



At least four species, all of them from dung, have been described 

 in this genus. The possibility that they were based on abnormal 

 material of Pilobolus, leads the writer to regard the genus as 

 somewhat doubtful. 



Thamnidiaceae 



Sporangia and sporangiola formed together, usually on the 

 same sporangiophore; sporangium large, multispored, thin- 

 walled, and provided with a columella; sporangiolum much 

 smaller, lacking a columella, provided with a thicker persistent 

 wall, and faUing away at maturity; zygospore not enclosed in a 

 hyphal envelope. 



Key to Genera of Thamnidiaceae 



I. Spores in the sporangium differing in size and shape from those in the 

 sporangiola; sporangiolum clasped at the base by several sharp- 

 pointed claw-like branches. 



1. Dicranophora, p. 254 

 II. Spores in sporangium like those in sporangiola; branch bearing sporan- 

 giolum lacking terminal claw-like projections. 



2. Thamnidium, p. 25(5 



