164 



Mycologja 



agrees with Van Tieghem that the spore plasm is divided at once 

 into polyhedric granular spores separated by a non-granular sub- 

 stance. Moreau, 1 studying spore formation in Mucor mucedo, 

 came to the conclusion that the spore plasm divides into irregular 

 multinucleated fragments which subsequently become the spores. 



I have studied the method of the columella and spore formation 

 in M. mucedo. I find that the columella does not originate as a 

 plane wall, which is subsequently arched up into the sporangium, 

 as is so often depicted in textbooks on botany, but that as in the 

 Zygomycetes studied by Harper (24) and Swingle (50) the colu- 

 mella is from the first dome-shaped as I show in PI. 16, fig. 19; 

 a dome-shaped series of vacuoles appear, these flatten, fuse end to 

 end, and thus delimit the spore plasm from the columella plasm. 



The spore plasm is first cut up into comparatively large proto- 

 plasmic blocks. During this stage considerable contraction occurs 

 for the blocks are not in close apposition (PI. 16, fig. 20). These 

 blocks are now subdivided by cleavage into roughly polyhedral 

 spore initials. This subdivision is attended by still further con- 

 traction, followed by an expansion stage in which the spore initials 

 become polygonal, as figured by Leger ( (34), plate 8, fig. 35; my 

 figure, plate 16, fig. 21). These spore initials eventually contract 

 and form the ovate definitive spores. I have not been able to 

 determine with certainty the number of nuclei in the ripe spores. 



Rhizopus nigricans and Pilobolus crystallinus 

 The process of spore formation in Sporodinia grandis is much 

 abbreviated, the spore plasm being cut up only into relatively large 

 multinucleate blocks (text fig. C, 1-5), which quickly round up to 

 form the definitive spores. In Rhizopus nigricans (text fig. E, 

 1-5) the spore plasm is cut up, progressively, into numerous much 

 smaller multinucleate, angular to ovate spores, but never reaches 

 the uninucleate stage. The relative extent of the cleavage is well 

 illustrated by comparing the size of the spores of Rhizopus nigri- 

 cans with that of those of Sporodinia grandis. In Sporodinia 

 grandis the spores measure, on an average, 20-30 x 17-24 fi, in 

 Rhizopus nigricans 9-12 x 7.5-8 ^. In Pilobolus crystallinus 

 (text fig. F, 1-6) the process of spore formation is still further 



1 Bull. Soc. Mycol. Fr. 31: 71-72. 1915- 



