152 Contribution to Knowledge of Silver -Leaf Disease 



In the production of abnormalities in the protoplast various other 

 factors have been found to play a certain part. The important literature 

 on this subject is contained in the work of Reynolds (13). 



In the cases cited various modifications in the protoplast were 

 attained artificially, viz. by the introduction of a definite external 

 agent. When such a factor induces marked degeneration — as e.g. in 

 the electrical radiation experiments of Klemm — then we can look upon 

 it as a strong stimulus in the life of the cell. The pathological factors 

 (vapours of chloroform, electrical radiation, etc.) acted directly from the 

 immediate vicinity of altered, cells. If we notice in the Prunus leaves 

 changes of a like magnitude in the cells, then we can judge in an analo- 

 gous way of a strong stimulus which probably also is acting somewhere 

 in the immediate vicinity. We may judge of it even more inasmuch 

 as the changes observed in the mesophyll cells of Prunus remind us 

 strikingly of those which take place in the various galls, so that it is 

 permissible to compare a leaf attacked by Silver-leaf disease with a 

 gall, even according to the definition of Kiister (7, p. 2). 



Within the last few years cytological investigations have been carried 

 out on the pathological reactions of the diseased plant. The important 

 literature of this subject is to be found in Kiister (7, pp. 198-205) and 

 in the recent work of Reynolds (13). The attention of Reynolds was 

 directed toward the reactions of leaf tissue to fungal invasion in the 

 various phanerogamic plants. Zea Mays parasitised by Ustilago Maydis, 

 Pirus malus by Gymnosjporangium sp. and Viola cucidlata by Puccinia 

 Violae are among the cases examined. There are also included some 

 host plants (Panicum, Smilax) upon which the cause of the disease is not 

 clear but is certainly of fungal nature. It cannot be doubted that 

 many changes in the Prunus leaves attacked by Silver-leaf disease 

 remind us of those described by Reynolds. This author also states 

 that the nuclei in the parasitised leaf tissue are, as a rule, more or less 

 deformed and enlarged, varying from globular to pear- or even crescent- 

 shaped ; that the chloroplasts may be affected, at least in shape and 

 size (Viola parasitised by Puccinia) and the chlorophyll may disappear 

 (e.g. Panicum, Potentilla by rust, etc) ; that the cells before collapse 

 are filled with a granular yellowish substance (Gaylussacia baccata, Zea 

 in epidermis) and that the pathological tissue is sometimes entirely 

 destroyed and killed (Smilax, Castanea). All these modifications brought 

 about in the cells by influence of a parasite from close proximity are 

 quite comparable to the changes of the cytological elements in the case of 

 Silver-leaf disease. The affected plants which formed the subject 



