Dec. 23. 1918 Parasitism of Cronartium ribicola 629 



quite thin, thickens and the sheath develops, the cytoplasm becomes 

 vacuolate (PI. 58, C, D, E), and the nucleus loses its normal structure, 

 condensing into a shrunken deeply staining mass. In such a state the 

 haustorium can not function efificiently as an absorbent organ. At any 

 rate the host cell and its nucleus do not shov/ much evidence that the 

 parasite seriously interferes with the normal cell growth. Even when 

 the cells are completely separated by mycelial stands, the host nuclei 

 may remain apparently normal in shape and structure. 



PYCNIA 



On any given area of infection the pycnia precede the aecia, by at least 

 one growing season. Succeeding generations of pycnia and aecia follow 

 a more or less definite schedule. The plan of the advance of the fungus 

 is illustrated in Plate 54, A. Immediately surrounding the point of in- 

 fection the bark may show pycnia and aecia at the same time (PI. 48, B), 

 but in such cases the pycnia have either passed maturity or the aecia have 

 developed abnormally early. After the infection has spread for some 

 distance, provided that the tree is not less than several inches in diam- 

 eter, the pycnial area is normally always in advance of the aecial area. 

 The sequence of the etiolated bark, pycnial area, and aecial area is evi- 

 dent in Plate 54, A. 



Very young pycnia are difficult to find. They seem to develop rapidly 

 when once started, for none have been examined which were mature in 

 the center and still young at the periphery, a condition which is com- 

 monly met in aecia and uredinia. The hyphae which contribute to forma- 

 tion of the pycnium force their way between the cells of the outer cortex 

 in a direction at right angles to the outer surface of the bark (PI. 49, B, a). 

 They are aggregated between the outermost cortex cells and the periderm, 

 forming a layer of pseudoparenchyma two to four cells thick (PI. 49, 

 B, 6). From this layer arises a series of short branching trunks (PI. 58, 

 A, c). Each of the ultimate branches from these trunks is a long sporo- 

 phore, on the tip of which a number of pycniospores are formed, one after 

 another (PI. 49, A, B; 58, A, a, b). Roughly speaking, the pseudo- 

 parenchyma makes up one-fourth, the short branching trunks one-fourth, 

 and the sporophores one-half of the vertical width of the sorus (PI. 49, B, 

 58, A). Pycniospores are produced in large numbers. Mixed with a 

 thin sweet gelatinous fluid, they collect between the sporophore tips and 

 the periderm layer (Pi. 49, A) forcing the latter up into the form of a 

 shallow blister. Finally a small break in the periderm layer allows the 

 spores to escape, together with the sweet fluid, in the form of a honey- 

 colored drop called the pycnial drop (PI. 48). The pycniospores are 

 typically pyriform (PI. 58, A, b), measuring when mature approximately 

 2.5 by 3.5 fx. Their mode of formation appears to agree closely with that 

 of the pycniospores in other rusts. At the rounded tip of the sporophore 

 a bud is formed which swells until it reaches the size of a mature spore 



