654 Journal of Agricultural Research voi. xv. No. u 



cells apart and thus cause the swelling of the infected bark; in the latter 

 there is rarely an y marked aggregation of the hyphae, except in the case 

 of petiolar infection. 



(3) Haustoria may penetrate practically every cell in the infected 

 area in Pinus strobus. These haustoria are characteristic for Cronartium 

 rihicola and their presence in the bark cells of the pine definitely deter- 

 mines the identity of the parasite. A sheath develops around each 

 haustorium as it reaches maturity or old age. This sheath is apparently 

 unlike any other sheath so far described in cormection with the haustoria 

 of the rusts. 



(4) Haustoria are proportionafely much less frequent in Rihes spp. 

 than in Pinus strobus and much smaller in size. They are not enveloped 

 in sheaths as in the latter. 



(5) The morphology of the different sori is shown to be similar to 

 the morphology in other full-cycle rusts. 



(6) The structure of the spreading pycnial layer characteristic of 

 caVilicolous Peridermi'a is considered in detail. 



(7) The development of the deep-seated aecium and the formation of 

 its multilayered peridium is described. 



(8) The formation of urediniospores is shown to follow the general 

 plan in other uredinia where the spores are borne on stalks. The devel- 

 opment of the uredinial peridium and the bank of parench5mia-like cells 

 which surround the uredinium are described and figured for the first 

 time. 



(9) The telial column is shown to arise either from an old uredinium 

 or as a separate entity and to be indistinguishable from a young ure- 

 dinium in its very early stages. It is surrounded at the base with a 

 bank of parenchyma-like cells and is provided with a peridium similar 

 to the peridium of the uredinium. All of the spores of the telial column 

 may germinate in situ. The production of spoddia is described in 

 detail. 



(10) The destructive effect on the pine host as a result of the attack of 

 Cronartium ribicola varies. In young trees death may result quickly. 

 In older trees it is of the nature of a primary injury, which prepares the 

 way for the drying out of the infected bark and the entrance of secondary 

 fungi and insects which complete the destruction initiated by the para- 

 site. 



(11) The effect on Ribes varies with the species attacked. It may 

 result in early defoliation and a consequent poor crop, but in general 

 it is not serious on this host. 



(12) The cytological phenomena presented herein establish the 

 similarity of the nuclear processes in the ge|nus Cronartium and other 

 full-cycle rusts. 



(13) The resting nuclei in all cases, with the exception of the mature 

 fusion nucleus in the teliospore, have a deeply staining spot on the 



