MICHIGAN ACADEMY OF SCIENCE. 



71 



An idea of the general habit of the parasite may be obtained from photo- 

 graphs I; II, III.* The under side of the leaves become "peppered" over 

 by a large number of the sori, each of which being one or two mm. in diam- 

 eter and elevated above the surface. (See photograph II.) The disease 

 is very much localized, that is, it is confined to a small area immediately in 

 connection with the sorus. This is especially true of the leaves, but on the 

 petioles and stems the sori may be much longer than broad and running 

 some distance up and down the petiole and stem from the point of infection 

 The sori on the petioles and stems are covered by the epidermal layer of the 

 host for a longer time than is the case with the sori on the leaves. (See photo- 

 graph III.) Upon the petiole the mycelium works under more readily and 

 more extensively, giving the sorus more strikingly the appearance of an 

 eruption. 



K 



H. Cells of mallow stem as seen in cross section, taken from cortical paren- 

 chyma showing the large mycelium and haustorium. 



K. Cells from same stem taken from a portion of tissue lying close in upon 

 the strands of collenclwma; mycelium much smaller than in '' H," 

 and apparently no haustoria. 



The teleutospore germinates very readily as soon as mature both on the 

 sori while on the leaves, and on a moist slide, if the spore be in the air and a 

 portion of the tissue of the host attached. The promycelium and the sporidia 

 very rarely develop while immersed in water, or when in a Van Tieghem cell. 

 The promycelium has a strong tendency to grow away from the teleutospore, 

 and in the same direction as the teleutospore developed. Consequently a 

 sorus of germinated spores presents the appearance of a gray or whitish mass 

 due to the promycelia extending out from, and nearly covering, the sorus. 



Examples of germinating teleutospores and of the formation of sporidia 

 may be seen in figure B. The s])oridia themselves send out germ tubes quite 

 readily in water. This is shown in figure C. Immediately outside of the 

 germ pore of the spore, the promycelium swells out almost to a spherical 

 form as shown in figure B. So far as the writer is aware this is peculiar to 

 the mallow rust. What its significance is can scarcely be conjectured. 



* Photographs were made by Professor Pettit. 



