JJ^-airr: effects of rusts upon transpiration 405 



Just as the experiments with the cereals were characterized by 

 a definite increase in transpiration of rust-infected plants, likewise 

 rusted cockleburs showed just as positive a falling oft of water 

 loss. As already mentioned, Reed and Cooley*^ obtained similar re- 

 sults with rusted apple leaves. They pointed out that the fungus 

 may work in one or more ways to diminish transpiration — either by 

 its intoxicating- action upon the host cells, or by causing hypertrophy, 

 which alters the normal activities of the leaves. It would seem that 

 the latter cause alone might be quite sufficient. The parts of the 

 apple leaves bearing the clustercups are three or four times the nor- 

 mal thickness. The hypertrophy, as Reynolds^ has pointed out, is 

 due to an enlargement of the sponge cells to twice or thrice their 

 normal diameter and the filling in of the intercellular spaces with 

 masses of mycelium. The stomata are scarce or lacking and the 

 sub-stomatal cavities are altogether lacking. Reed and Cooley con- 

 cluded "that the hypertrophy of the leaf and the subsequent oblitera- 

 tion of intercellular space diminishes the possibility of water elimi- 

 nation from the interior of the leaf. The thick cushions on which 

 the aecidia are produced have stimulated the leaf cells in that place 

 to enlarge, and obliterate the stomata." 



In the case of the rusted cockleburs changes even more profound 

 than those above noted occur. The normal leaf tissue is very loosely 

 formed. Very large air chambers permeate the spongy parenchyma 

 and the palisade is scattered. Reynolds working with Xanthium 

 canadense Mill. (?), parasitized by Piiccinia Xaiitliii Schw. gives 

 the following description : 



"This [normal leaf] tissue . . . is in many places almost com- 

 pletely replaced by the fungous mycelium. The cells which remain 

 have no protoplasm and are filled with oil globules. On both the 

 upper and the lower leaf surfaces the mycelium is abundant and the 

 telial spores are very numerous. Within the mixture of parenchyma 

 cells and mycelium, which replaces the normal tissue, there are 

 cystlike bodies which are composed of masses of mycelium. These 

 objects are hollow spheres, and from the inner surface arise telial 

 spores exactly similar to those borne in the normal way upon the 

 exterior of the leaf." 



The results with sunflower and carnation are by no means con- 

 clusive. It is apparent however, that the yellowish cushions upon 

 which the pycnidia of Puccinia Helianthi are produced, are caused 

 by hypertrophy of the underlying leaf tissue. No such change of 



