COPRINUS ATRAMENTARIUS 275 



in fungi; they are usually 0-12 to 0-17 mm. long and 0-02 

 to 0-03 mm. wide. The largest I have seen were 0-21 mm. 

 long and 0-03 mm. wide. They attain their maximum diameter 

 at a very early stage in the development of the hymenium — long 

 before the basidia begin to produce sterigmata — and then they grow 

 in length for some hours. I have observed that in Coprinus niveus 

 the cystidia elongate by means of apical growth and, doubtless, the 

 cystidia of Coprinus atramentaritis elongate in the same manner. 

 A cystidium, when very young, is short and provided with a large 

 amount of protoplasm ; but, as it grows in length, its living sub- 

 stance gradually becomes used up. A full-grown cystidium contains 

 a slight accumulation of protoplasm at both its apical and basal 

 ends, a very thin protoplasmic "layer pressed against the wall of its 

 cylindrical shaft, and a large central vacuole filled with clear and 

 colourless cell-sap. The cell- wall is very thin and quite smooth. 

 A cystidium collapses as soon as it has been killed. It is therefore 

 evident that the rigidity which it exhibits when connecting two 

 gills in a living pileus is due to its turgidity. 



The distribution of the cystidia over the surface of the gills is 

 fairly uniform, as is shown in Fig. 117. It was estimated, after 

 camera-lucida drawings had been made, that from 30 to 60 cystidia 

 are present on each square millimetre of the hymenium. The cystidia 

 are sufficiently numerous to make it impossible for the gills to sag 

 laterally and thus to touch one another with their hymenial surfaces. 



As shown in Fig. 114 (p. 271), there are no cystidia projecting 

 radially into the interlamellar spaces where the gills join with the 

 pileus-flesh, and having free ends which point towards the stipe. 

 In view of what I believe to be the function of the cystidia this 

 absence is significant. 



When one considers the shape, size, structure, position, and 

 number of the cystidia in the interlamellar spaces of a young pileus 

 and also the extraordinary thinness, breadth, and mechanical 

 weakness of the gills when detached from the cystidia, one is justified 

 in coming to the following conclusion : the cystidia are unicellular 

 organs which function by holding the gills apart and thus maintaining 

 interlamellar spaces in which the spores can develop without mechanical 

 hindrance. 



