92 A Monograph of the Erysiphaceae 



depressed, or pezfzoid in age. This statement rests on a curious 

 misconception. On looking at a leaf on which there are mature 

 examples of U. aceris we nearly always find that some perithecia, 

 as seen from above, appear very concave, or even cup shaped ; 

 a closer examination, however, shows, that these are perithecia 

 which have become turned over, so that the upper half, with the 

 appendages which it bears, is pressed to the surface of the leaf. 

 The base of the perithecium, thus exposed, soon loses all traces 

 of mycelial hyphae, and becomes concave, simulating the truly 

 "pezizoid" perithecia of such species as Erysiphe taurica. 

 Whether this turning over of the perithecium is natural, and 

 due to some growth of the mycelium or action of the appendages, 

 or whether it is accidental, I am not able to say. The appen- 

 dages, which through the turning over of the perithecium are 

 pressed against the leaf, sometimes seem to become slightly 

 attached to it, and it is noticeable that the apices of these 

 appendages show, under the microscope', signs of having become 

 slighdy disorganized ; they may possibly, therefore, adhere to the 

 leaf through some mucilaginous degeneration. Observations 

 living plants are necessary before the question of how this curious 

 reversal (already in 1861 observed by Tulasne (370)) of the peri- 

 thecia takes place can be answered. 



Acer campestre and A. pseiido-platanus are the usual host- 

 plants of the present species ; it is only very rarely indeed that U. 

 aceris occurs on A. platanoidcs, being usually replaced on this host 

 by the var. Tulasiui* and it is not common on the other species 

 of Acer mentioned {A. monspessnlaman, A. nibrum, etc.). The 

 Japanese examples occurred on A. pictum and .4. spicatum, and are 

 very interesting geographically. 



As a rule, the simple appendages are very few in number, and 

 more or less completely hidden by the very numerous forked 

 ones ; on Acer tartaricwn, however, the simple appendages are 

 numerous, and this form certainly shows an approach, in this re- 

 spect, to the var. Tulasnd ; the mycelial characters, however, are 

 those of the type. 



_yj_^^i^ is not known to occur in America ; the only record 



' " 'M . " - - ■ __^ * 



* Eriksson, however, states that he has frequently ob.served botl7these plants grow- 

 mg together on Acer p^atanoides in nurseries at vStockholm. 



on 



