It is shown, however, that the thickness of the cuticle of 

 wheat leaves affects their liability to rust. Out of several 

 experiments one may be selected, made upon an eminently 

 rust-resistant variety, Ward's Prolific, and an eminently 

 rust-liable wheat, Zimmerman. It was found that the cell 

 walls of the cuticle of the latter (Zimmerman), were less 

 than half as thick as those of the rust-resistant kind, Ward's 

 Prolific. This seemed to be a very general rule. Twelve 

 varieties of wheat were examined in this way, and Dr. Cobb 

 arrived at the conclusion that the structure of the leaf cuticle 

 in a variety of wheat has a most important influence in determin- 

 ing its liability to rust. 



The rationale of this is, according to Dr. Cobb, as follows : 

 " First, the cuticle might be so constructed as to prevent the 

 entrance of rust ; secondly, it might be so constructed that 

 although it permitted the rust to enter it would not allow it to 

 get out again ; in other words, to fructify so as to disseminate 

 spores. A thick tough cuticle would doubtless enable a wheat 

 to act in this second way, and we have no longer any doubt that 

 many rust-resistant wheats are such, because of a tough cuticle 

 acting in this manner." 



Not only had these rust-resistant wheats a thick cuticle, 

 but the leaves were tougher than the rust-liable sorts, as 

 was shown by the results of an elaborate series of ex- 

 periments made to test their relative tensile strength. This 

 toughness further tended to prevent the fungus from sending 

 out spores from within to form, in short, what are known 

 as teleutospores. Dr. Cobb concludes : " From these experi- 

 ments it will be seen that the wheats which we had selected as 

 most rust-liable, namely, Zimmerman, Steinweder, and King's 

 Jubilee, have a low tensile strength, while the resistant sorts have 

 the highest tensile strength." 



Dr. Cobb made an interesting observation on an hitherto un- 

 known function of the waxy covering, so characteristic of certain 

 wheats, especially when they are young. " This wax, or bloom," 

 he says, " is intended to protect the plant from the injurious effect 

 of water. We discovered, however, that it had no small influ- 

 ence in keeping out the promycelium of rust. This waxy bloom, 

 when it occurs in abundance on the sheaths of wheat almost 

 completely covers the surface of the cuticle, being interrupted 

 only at the stomata. When one examines such a sheath under 

 the microscope, it is some little time before the stomata are seen, 

 so perfectly are they hidden ; their position is indicated only by 

 a very narrow crack in the wax, a crack so narrow that the 

 promycelial threads fail to enter it. We made this observation 

 repeatedly on a wheat known as Ble carre de Sicile rouge, a 

 particularly glaucous w r heat, especially when young. We took 

 sheaths of this wheat and germinated on them large numbers of 

 the spores of Puccinia graminis, but we failed after a long search 

 to find a single promycelial thread that had gained an entrance. 

 These observations explain in a remarkably clear manner why 

 the sheath and straw of glaucous wheats often remain quite free 



