335 



protrudes (Fig. 4). The size of pycnidium varies from 200-270 microns 

 iu the vertical diameter, by lSO-210 microns in the horizontal diameter. 

 Typically, there is a unilocular spore-bearing cavity and an ostiole; how- 

 ever, certain conditions of culture have developed pycnidia which lack 

 the ostiole (Fig. 5). This has been ol)served l»y Walker ("US)^ who 

 regarded the character as sufficient to call it a "new form." Whether or 

 iiot this is so to be regarded cannot be said, but the strain which pro- 

 duced pycnidia in our culture lacking the ostiole. originally possessed an 

 ostiole in nature. Isolations were made from unilocular pycnidia and 

 when mature fruit bodies had developed in culture, tliey were larger, 

 measuring 400-000 microns x 060-720 microns, and were nniltilocular. Just 

 bow we are to interpret these variations is yet a question, but it seems 



Fig. 4. Camera lucida drawing of a typical pycnidijm of iSpAaez-opsis malorum. 



that they are not to be taken too seriously when questions of taxonomy 

 are involved. If these characters A\ere constant they would be more 

 important, but since they are only variations, little importance should be 

 attached to the absence of an ostiole or to the number of conceptacles. 



Tlie pycnidial wall is thick, but not uniformly so. The reason for any 

 variation in thickness may be that less protection is needed at the base, 

 01 that its thickness, there, is determined by purely mechanical pressure 

 brought about by the resistance offered to the apex of the pycnidium by 

 the epidermal and cuticular layers of the host tissue. It is made up of 

 two distinct layers, the pseudo-cells of which are very thick-walled and 

 black, and an inner layer of thin-walled cells. 



Conidiophores arise from all points of the inner layer and extend 

 entad. They are variously shaped and each is terminated by a conidium 



>'08. Walker, L. B. A New Form of Sphaeropsis on Apples. Nebr. Agr. Exp. Sta. Rept 

 21:31-44. 1908. 



