Dec.23, i9i8 Parasitism of Cronartium ribicola 633 



area is more or less indefinite in extent at the basal end of the spore. 

 Verrucose and smooth areas grade one into the other along the irregular 

 line which separates them. The smooth area is fissured near its edge 

 into blocks which become smaller and smaller until they approach the 

 size of the tubercles of the verrucose area (PI. 58, Z). 



No completely satisfactory explanation of the manner in which this type 

 of spore sculpturing arises has come to the writer's attention. The follow- 

 ing theory is reservedly offered. The two walls of the spore are present 

 when the spore is quite young, and both continue to grow and thicken 

 up to a certain point or until the spore has nearly reached its full size. 

 The outer wall hardens more rapidly than the inner one and in conse- 

 quence becomes fissured irregularly as the still elastic inner wall con- 

 tinues to expand under the pressure of the growing spore content. The 

 longer growth in size continues the more complete will be the fracturing 

 process and the larger the verrucose area in proportion to the smooth 

 area. Experimentally, the smooth area can be converted into verrucose 

 area by soaking the spores in water. After several hours, if germina- 

 tion does not take place in the meantime, the spore absorbs water enough 

 to cause it to increase appreciably in size. The tubercles of the verru- 

 cose area become free from their attachment to the inner wall and float 

 around in the water. The smooth area, under the expansion pressure 

 exerted as the inner wall swells, cracks and fissures until it becomes 

 irregularly verrucose, approaching the conditions found on the normal 

 verrucose area of the mature spore. The process of Assuring can be 

 watched quite easily. The experiment at least suggests the manner in 

 which a type of spore sculpture so oddly irregular could arise. 



The spore wall on the smooth area has been heretofore considered to 

 be thicker in section than the verrucose area. Examination of complete 

 sections of the spores shows that this is not always the case. In fact, 

 it is only in occasional instances that it holds true. As a rule, there is 

 no appreciable difference in the thickness from the inner edge of the wall 

 to the outer tip of any given tubercle and the thickness from the inner 

 edge of the wall to the outer edge of the smooth area. When dealing 

 with whole spores, refraction phenomena increase the difficulties in 

 measuring the true thickness of a curving wall of the type presented in 

 the smooth area of the aeciospore of Cronartium ribicola and may account 

 in part for the misinterpretation of the actual condition. 



GERMINATION OP ^CIOSPORES 



Germination of the aeciospores may take place rapidly under favorable 

 conditions, but as Maire (32) aptly puts it "la germination des ecidio- 

 spores * * * est par fois ires capricieuse." The work of Spaulding and 

 his assistants has shown that spores which would not germinate at all in 

 hanging-drop cultures, on a water film, or on moist filter paper, either at 

 room temperature or in the ice box, or at room temperature after cooling 



