Notes on R. Acerinum and R. Pseudoplatani. 185 



are developed from them in basipetal succession. The increasing 

 pressure due to the gro\^i:h of the conidiophore causes a rupture 

 in the wall, and the conidia escape through the aperture (fig. 3). 



4. Stroma. After the shedding of the conidia, the fungus layer 

 on the leaf thickens considerably by a very extensive gro^^'th 

 of the h\'phae and the inner and outer walls of the upper 

 epidermal cells are consequently pushed further apart. They are 

 however clearly ^*isible in this widely separated condition in 

 this and subsequent stages of the fungus (figs. 3 and 4). This 

 obser^'ation bears out the assertion that the main development 

 of the fimgus is actually \\ithin these epidermal cells. The h\*phae 

 immediately under the thickened outer walls of the epidermal 

 cells give rise, as has been noted, to a sclerotium, and below this 

 is a layer of thin-walled mycehum (fig. 4) . Those h^-phae which, 

 in this mycehum, are in contact \^ith the inner walls of the 

 upper epidermal cells frequently give rise to a shght develop- 

 ment of sclerotium (fig. 4) . The relative thickness of the fungus 

 laj-er and the leaf itself varies in the different forms studied. 

 In R. acerinum on Acer saccharinum the fungus layer may be 

 from three to four times as thick as the leaf, while in R. Pseudo- 

 platani on Acer Pseudoplatanus, the fungus layer is only sUghtlv 

 thicker than the leaf itself ^fig. 4) . 



5. Apothecia. The M'phae under the sclerotium which at first 

 appear as a more or less undifferentiated mycehum soon undergo 

 a change. Certain groups of vertically arranged hyphae, staining 

 much more readily than the rest, show the beginnings of the 

 h\Tnenium. These groups appear at intervals and around them 

 definite apothecial ca\'ities become marked off by a progressive 

 differentiation of the surrounding h}-phae. The h\Tneniimi itself 

 develops considerably and during the ^rinter months the dis- 

 tinction between asci and paraphyses is plainly ^•isible. ^liiller 

 states that this differentiation does not take place until March 

 but it has been seen that the ascus development depends greatly 

 on weather conditions, and while during the hard winters of 

 Wisconsin, U.S.A., there is Uttle differentiation during \\'inter, 

 in the British forms studied and in American forms collected 

 during a mild winter, asci and paraph vses were well differentiated 

 by January. The asci are uninucleate during the greater part 

 of the \rinter though they increase in size considerably. In the 

 spring when the ascospores are produced within the asci (fig. 4), 

 an interesting dehiscence mechanism has been observed which 

 brings about the sphtting of the sclerotium above the apothecium. 

 Small canals appear in the sclerotium and run longitudinally in 

 the ridges above the asci, in the median plane. In transverse 

 section these appear triangular. They contain a clear, somewhat 

 viscid liquid which is apparentlj^ produced bv the activity of 



