650 Journal of Agricultural Research voi. xv, no. 12 



haustoria continue to remain alive and apparently active in spite of 

 haustoria. As has been pointed out above, these haustoria become 

 invested with a sheath which may render them very ineffective absorb- 

 ing organs, in which case they might become simple mechanical irritants 

 in the cell content, no more detrimental to cell activity than crystals. 

 The denting of the host nucleus by the tips of the haustoria does not 

 seem to injure the nucleus except to alter its shape. There is no evi- 

 dence of increased cell division reported by Reynolds {48) for many 

 plants as a direct result of a parasite's action, or of nuclear migrations 

 like those figured by Schiirhoff (52). Storage starch is usually present 

 in excess, but this phenomenon has been shown by Halsted (77) to be 

 a general condition in and around areas infected with fungus parasites. 

 This starch is not completely used up by the fungus, for many grains re- 

 main in the old dead cells after the cells are completely dried out. The 

 cells just beneath the pycnial layer and in the region of the young secium 

 (PI. 52, B) contain normal grains as well as decomposition products. 

 Excess starch production is probably due to a la,ck of balance in the 

 physiological processes in the host cells, and the fungus may, of course, 

 contribute to the unbalancing; but other environmental factors which 

 are little understood result in excess starch production in trees which 

 appear to be perfectly normal in other respects. 



The wood laid down in the annual rings under infected bark is much 

 less than in healthy trees, but the tracheids are apparently normal in 

 everything except number. The presence of the hyphae in the tracheids 

 has no appreciable influence on their form; neither are the characters 

 of the ray cells perceptibly changed. 



Cronartium ribicola may be the primary cause of the death of a 

 young tree. HowcA^er, the swelling of the bark is not in itself a seri- 

 ous hindrance to conduction in the phloem. The actual severe 

 injury occurs when the aecia form and burst through the outer bark; 

 for the gecial cracks thus fonned allow the inner bark cells to dry out 

 and die. This results in the breaking of resin canals and the consequent 

 exudation of resin in large quantities. The girdling of the tree is due 

 to these two causes working together — namely, the cracking and drying 

 of the bark, and the impregnation of the whole cortex and phloem in 

 the cracked area with resin. Complete stoppage of the conducting 

 elements of the phloem results. Seedlings and young trees may suc- 

 cumb to the attack of the fungus almost as soon as the first secia appear; 

 but wnth older trees death is sometimes delayed for a number of years. 

 It depends, of course, on how complete a girdle has been effected. The 

 part played by secondary fungi acting in conjunction with C. ribicola 

 to make a girdle complete is very important. It will be recognized at 

 once that the cracking of the bark at the time the aecia are formed is a 

 source of danger to the trees not only by exposing the inner bark cells 

 to the air, but also by providing an avenue of entrance for secondary 



