DISEASES OF PLANTS. 153 



that i>r()hnl)l.v a fourth should be iiK-Uided. I'cronospora wdiidh. thus iuchidintr 

 all tho Peronosporaceiv known to occur on grasses. 



Note on the infection and histology of two wheats immune to the attacks 

 of yellow rust, Dorothka V. E. Markyat (Jour. Agr. Sci.. 2 {1901), No. 2, pp. 

 12!i-l,l'^. pi. 1). — Infection and histological experiments are reported in which u 

 variety of wheat (American Club) and Einkorn. a half-wild form of Triticinn 

 iiioiiococciim i'iilf/are, were studied to determine the infection by the yellow rust 

 (PKCcinia f/1i(i)ioniin). For purposes of comparison a very susceptible variety 

 (Michigan Bronze) was used. Seedlings of these three varieties of wheat were 

 grown and when the young plants were well developed all were infected by 

 placing spore.s on the leaves. These were observed from time to time and the 

 infected material subjected to histological examination. In the normal devel- 

 opment of the fungus the germ tubes pass through the stomata into the tissues 

 beneath where haustoria are developed with considerable rapidity. Ordinarily 

 numerous nuclei are present in the liyi)hie, hirge numbers of haustoria are 

 developed, and a healthy condition of the host cells is maintained about the 

 developing fungus. After about 10 days the small hypha^ begin to mass them- 

 selves beneath the epidermis and later pustules develop. 



In a case of the very resistant Einkorn, the germ tubes enter as usual through 

 the stomata, but almost in the beginning the contents of the hyphse look watery 

 and show very few nuclei. They appear to be too feeble to send out any 

 haustoria, the host cells in the vicinity of the fungus are shrunken, and in later 

 stages the leaf tissue apppears a dead, shriveled mass. 



It appears that in this innnune wheat, although the fungus makes its entry and 

 produces abundant hyph:e, yet sooner or later it is starved to death by the break- 

 ing down and death of the host tissue in its vicinity, and as a consequence the 

 fungus is unable to make further progress. The host plant, on the other hand, 

 having checked the parasite, continues to flourish except for the small dead 

 areas mentioned above. With the third species of wheat, which appears to be 

 intermediate between the susceptible Michigan Bronze and the resistant Einkorn, 

 the entry takes place through the stomata. The hypha^ appear for a time 

 I)erfectly healthy and some develop normal haustoria, but before long the progress 

 of the parasite is checked and only in occasional cases are spores produced. In 

 the ease of immune varieties, it appears that while the fungus succeeds in 

 making good its entry and producing hypha', further progress is completely 

 checked by the breaking down and death of the host tissue accompanied by the 

 starvation and death of the parasite, as in the Einkorn, or else a more protracted 

 struggle takes place, as in the American Club. In the latter case the develop- 

 ment proceeds to a farther point, but is retarded as compared with a normul 

 case such as described from the Michigan Bronze. 



The reason for this innnunity is unknown, but for the present the author 

 suggests that it is probably due to the in-oduction of certain toxins and antitoxins 

 by host or parasite, or both, which are mutually destructive. 



A leaf-curl disease of cassava, A. Zimmermann {P/laiizer, 2 {1906), A'o. 10. 

 p. lJ,o: ahx. in Centhl. Bali, [e/c], 2. AM., 18 {1907). No. 10-12, pp. 366, 367).— 

 A description is given of a disease of cassava in which the leaves become curled 

 and variously distorted. So far a large number of causes have been assigned to 

 this disease, but after investigating the subject the author reports that he is 

 unable to find any fungus or animal i)arasite in the affected leaves, and he 

 believes that it is very likely due to causes similar to those i)roducing the mosaic 

 disease of tobacco, the infectious chlorosis of mallows, etc. Some varieties of 

 cassava seem to lie especially subject to it, and in making plantings those that 

 are known to be liable to it should be rejected. 



