INTRODUCTION. 7 



undoubtedly the most highly specialized of the mildews and represents 

 the culmination of the developmental tendencies found in the group. 

 It is one of our most widely distributed mildews and has been repeat- 

 edly the object of investigation from the standpoint of its general struc- 

 ture and habits of life. Recently Salmon (82, p. 224) has investigated 

 very fully the published species of the genus, and has concluded that 

 we have but one polymorphic and cosmopolitan type. He adopts the 

 name Phyllactinia corylea (Pers) Karst. 



Salmon (83, pp. 201-205) has also made very extensive and inter- 

 esting comparative studies of the form of the peculiar penicillate cells, 

 and concludes that the most extreme types of these cells are connected 

 by a series presenting all possible intermediate variations in form. Palla 

 (76) and Smith (86) discovered the interesting fact that Phyllactinia, 

 unlike other mildews, is not strictly epiphytic, but sends branches 

 through the stomata into the intercellular spaces of the mesophyl, pro- 

 ducing haustoria ultimately in the cells at the base of the palisade 

 parenchyma, and especially where nutrition is most abundant. Neger 

 (68) has also corrected the erroneous view that the conidia are borne 

 singly, instead of in chains as in other mildews, and has given very 

 interesting data as to the operation of the spine-like appendages, which, 

 by their hygroscopic motions, loosen and lift up the perithecia from the 

 leaf of the host when they are ripe. 



Phyllactinia occurs on a considerable series of host plants. I have 

 sectioned and studied especially material from the white ash {Praxinus 

 americana) , the hazel (Corylus americana) , the bittersweet (Celastrus 

 scandens), and the yellow birch {Betula lutea). The material was 

 fixed, immediately upon being gathered in the field, and its subsequent 

 treatment was essentially the same as I have previously described. 



