HARD WICKE ' S S CIE NCE-GO SSI P. 



229 



for the production of sporules. The first sign of this 

 is seen in the appearance on the upper surface of the 

 leaf of small round black dots, which arise from a 

 brownish discoloration of the cuticle. Beneath each 

 of these an hymenial layer is formed by the up- 

 turning of the ends of the mycelial hyphas. These 

 upturned ends assume the function of basidia, that 

 is, each produces a little bud from its extremity, 

 which enlarges and takes the form of a slightly 

 flexuous cylindrical rod. When complete, this rod is 



yellow border. But though the surface of the spot 

 may appear to the eye uniformly black, yet on hold- 

 ing the leaf up to the light, the places where the 

 sporules are being produced will be easily distinguished 

 by their greater opacity. 



A section of the leaf in this stage has the appear- 

 ance shown in Fig. 149, where a is the hyphasma 

 and b the blackened epidermis. A portion of the 

 hymenium is shown on a larger scale at Fig. 150, 

 a being the upper chlorophyll cells of the leaf, £j.he 



Fig. 150. — Alelasmia accrina; section ot 

 the hymenium of a spermagone, show- 

 ing the origin of the spermatia. X 1000. 



Fig. 151. — Rhytisma acerinum. 

 View of a mature pustule. 

 X 3 - 



Fig. 152. — Two Asci, one immature, from the disk of Rhytisma 

 acerinum, with two separate sporidia. X 1000. 



abstricted and falls off, and the formation of another 

 immediately begins. By this process, the space above 

 the hymenial layer becomes crowded with an enor- 

 mous number of these sporules, and the epidermis is 

 raised in a small blister-like pustule, which opens at 

 the apex to permit the escape of the sporules. 



Simultaneously the blackening of the leaf above 

 these places progresses more and more, and finally 

 extends over the whole of the yellow spot, except for 

 a small space round its margin. Thus the final result 

 is a roundish black spot surrounded by a narrow 



mycelial layer, c the basidia, and d the abstricted 

 sporules. These latter are perfectly colourless, and 

 measure about 7/x long by i/j. broad. They were 

 formerly called spermatia, from the belief that they 

 were a kind of male organs, having some fertilising 

 power from the exercise of which (in a way unknown) 

 the further development of the fungus arose. The 

 pustule beneath which the spermatia originated was 

 called a spermagone. This view may still be a true 

 one, but I believe nothing is known to support it in 

 this particular case, although in other cases, such as 



