Dec. 23, i9i8 Parasitism of Cronartium ribicola 637 



late. The germ tube passes through the exospore without the aid of a 

 germ pore, assuming at first the shape of a more or less swollen vesicle 

 (Pi- 59. Q)- This lengthens rapidly into the young germ tube (PI. 59, 

 R), into which pass the contents of the spore. The endochrome and 

 protoplasm are densest at the tips of the tube and in its branches. Duff 

 {12) has shown that change of temperature stimulates the urediniospores 

 to germinate and that light, especially ultra-violet light, may completely 

 inhibit germination. This action of light may have a direct bearing on 

 the apparent failure of the spores to cause infection at long distances 

 from their source. All the evidence at hand points to the conclusion 

 that the germ tube causes infection by passing through the stomata on the 

 lower surface of the leaves. However, the tube may extend some dis- 

 tance over the leaf surface before actually entering a stoma, in such cases 

 passing directly over stomata in its path. Urediniospores may retain 

 their vialUility for some weeks. The uredinium is the repeating sorus in 

 the life cycle of Cronartium ribicola (fig. i), and the spores in succeeding 

 generations infect leaves of Rihes spp. until late summer. 



TELIA 



Telial columns arise either from old uredinia or as entirely new and 

 separate entities. They appear later than the uredinia and are more 

 common in late summer. In the fall they are usually the predominant 

 spore generation present. In the greenhouse they are produced through- 

 out the year. On a given infected area the columns may occupy the cen- 

 tral part where the uredinia were first produced, surrounded by a narrow 

 peripheral region bearing the most recently formed uredinia. The 

 development of the teUal column is the same whether it is from an old 

 uredinium or in a new sorus and, as the latter is probably the most 

 common occurrence, it will be described. The massing of the hyphae, 

 formation of the peridium, and the parenchyma-like cells which surround 

 the spore-bearing part of the sorus, and the orientation of the basal cells 

 proceed exactly as in the uredinium. It is impossible to tell very young 

 uredinia and telia apart. The binucleate basal cells undergo division 

 (PI. 59, S, T, U, V, W) in the same manner as in the case of the uredinium, 

 but the cells cut off do not divide as in the other sori described. Instead 

 they lengthen out and become teUospores. Each basal cell cuts off a 

 vertical row of spores (PI. 52, A, B), the central cells of the sorus pro- 

 ducing spores slightly ahead of the cells at the periphery, as in the 

 aecium and uredinium. As the spore columns lengthen, the peridium 

 is pushed up into a dome (PI. 52, A), which later ruptures (Fl. 52, B) 

 and goes to pieces. The spores lengthen and soon reach full size, at 

 about which time they become provided with a substantial wall slightly 

 thickened at the upper end (PI. 57, A, B, C). The first spores cut off — 

 those at the tip of the column — are more or less polyhedral (PI. 52, B; 

 57, A); the other spores are typically broad spindle-shaped (PI. 52, D; 



