634 Journal of Agricultural Research voi. xv, No. 12 



in the ice box, were perfectly capable of producing infection on species of 

 Ribes. These results confirm those of Klebahn (25), who found that 

 seciospores which would not germinate in water did germinate very 

 rapidly on leaves of Ribes spp. and only less rapidly but still abundantly 

 on a gelatinous decoction of leaves of Ribes spp. These experiments sug- 

 gest the probability that some direct chemotactic stimulus is exerted 

 by the leaves of Ribes spp. on the seciospores. Klebahn has pointed 

 out that there is considerable difference between the faculty for germina- 

 tion, as determined by artificial cultures, and the faculty for infection. 

 Spaulding and his assistants have further determined that seciospores 

 frequently germinate — -even then only a relatively low percentage of 

 fresh spores do so — more readily after cooling in the ice box than at room 

 temperature, and that sometimes they have to remain in the ice box to 

 secure germination. Too much water is often as inimical to germination 

 as too little. A single spore may produce one to several germ tubes 

 (PI. 59, A), which attain considerable growth in artificial cultures. 

 Where the germ tube passes through the heavy exospore it is constricted 

 as shown in Plate 59, B. The tubes branch freely. The protoplasm is 

 densest at the advancing tips of the hyphge. 



INFECTION OF RIBES SPP. AND MYCElylUM IN THE LEAF 



Whether the germ tubes have the power to pierce the upper epidermis 

 of the leaf of Ribes spp. or must always come to rest in a favorable posi- 

 tion on the lower epidermis in order to cause infection is not definitely 

 known. All the evidence gathered from the examination of artificially 

 inoculated leaves points to the conclusion that infection occurs normally 

 as a result of the germination of the seciospore on the lower surface of the 

 leaf and the subsequent passage of the germ tube through a stoma. No 

 evidence either of any break in the upper or lower epidermal cells, or of 

 the remnants of any liypha passing through them, has been discovered. 

 Furthermore, the mycelium is always abundant in the air chambers 

 adjacent to the stomata, even in very young infections, and occasional 

 remnants of spores and hyphge near and in the stomata point to the 

 stomata as the avenue of infection. 



The first indication of infection in the leaf of species of Ribes is often 

 indicated by the paling of the infected areas. Sections of such areas 

 show that the mycelium has spread in the intercellular spaces and air 

 chambers of the mesophyll. Haustoria (PI. 59, D) enter all types of the 

 leaf cells, with the possible exception of the xylem elements of the bundles, 

 although they are comparatively rare in the epidermal cells. The cells 

 of the mycelium and the haustoria are binucleate (PI. 59, C, D). Gener- 

 ally there is a much smaller relative amount of mycelium in the leaf 

 tissue than among the same number of host cells of the pine. The loose 

 structure of the mesophyll allows plenty of room for the hyphae to grow 

 without severe crowding of the host cells. In fact, the hyphse are aggre- 

 gated only at the time of production of uredinia or telia. 



