58 TEXT-BOOK OF FUNGI 



finally the denser portion was cut up into polygonal por- 

 tions, each of which a became a spore. 



Swingle has recently added very materially to our know- 

 ledge as to the exact mode of spore-formation in Rhizopus 

 nigricans and Phycomyces nitens. At first there is no 

 columella present in the sporangium, into which there 

 is a streaming of cytoplasm, nuclei, and vacuoles, forming 

 a dense layer containing many nuclei near the periphery, 

 and a less dense central portion containing fewer nuclei. 

 Next there is formed a layer of comparatively large, round 

 vacuoles near the inner zone of the denser protoplasm. 

 These vacuoles become flattened and fuse together, form- 

 ing a circular cleft in the dense protoplasm, thus delimitat- 

 ing the columella. The dense protoplasm with its 

 numerous nuclei, situated between the columella and the 

 sporangium wall, is broken up; in Rhizopus by furrows 

 advancing inwards from the sporangium wall, and out- 

 wards from the columella cleft, both systems branching, 

 curving, and cutting out numerous multinucleate portions 

 of protoplasm, which, after becoming rounded off and 

 provided with a cell-wall, constitute the spores, separated 

 by the secretion of an intersporal slime. Simultaneously 

 the columella forms a wall, and its contained nuclei 

 undergo partial disintegration. 



In Phycomyces the spore-plasma is cut up into multi- 

 nucleate portions homologous to those in Rhizopus^ by 

 angles forming in certain vacuoles, which extend as branch- 

 ing clefts, meeting others of a similar nature starting from 

 the columella. The furrows are partly filled with the con- 

 tents of the vacuoles that formed the cleavage lines. 



During the process of spore-formation the nuclei remain 

 in a resting condition. 



