CURRENT LITERATURE 



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^ NOTES FOR STUDENTS 



Phenomena of parasitism. — Two further contributions to a series of 

 studies begun by Brown 1 on the parasitism of Botrytis cinerea have appeared. 

 In the first of these, Blackman and Welsford 2 describe the microscopical 

 details of the process of penetration of the cuticle by the germ tubes; in the 

 second, Brown 3 deals more specifically than in his former paper with the 

 action on the cuticle of extracts and exudates of the germ tubes. 



Blackman and Welsford observed in the earliest stages of penetration 

 a slight indentation of the outer epidermal wall as a result of the action of the 

 germ tube, which is held fast to the cuticle by a mucilaginous sheath whose 

 presence was made evident by means of a suspension of silver particles. The 

 actual penetration of the cuticle is accomplished by a narrow peglike out- 

 growth from the tip of the germ tube. No swelling of the cuticle or of the sub- 

 cuticular layers previous to penetration was observed, and in no case was 

 an injury to the epidermal cells or subepidermal cells apparent before the 

 breaking of the cuticle. Soon after the penetration of the epidermis, the cells 

 of the palisade layer begin to disintegrate, and with the advance of the hypha 

 the cells of the spongy parenchyma also are killed. The toxic action of the 

 fungus extends considerably beyond the region actually invaded. After a 

 portion of the leaf tissue had been killed, other hyphae were observed to pene- 

 trate through the stomata, probably as a result of the diffusion of food sub- 

 stances from the dead cells, for primary infection though a stomate was 

 never seen. 



From their observations the authors conclude that the cuticle is ruptured 

 by mechanical pressure exerted by the germ tube and not by the solvent action 

 of any substance secreted by it. They believe that the germ tube is enabled 

 to exert the pressure necessary for the indentation of the cell wall and pene- 

 tration of the cuticle by virtue of the gelatinuous sheath which holds the germ 

 tube in place. It is not clear, however, how the germ tube is thus enabled to 

 bring about an indentation of the cell wall over an area more extensive than * 



that covered by the tip of the tube itself, as shown in some cases (notably 



1 Rev. Bot. Gaz. 61:79. 1916. 



2 Blackman, V. H., and Welsford, E. J., Studies in the physiology of parasitism. 

 II. Infection by Botrytis cinerea. Ann. Botany 30:389-398. pi. 10. figs. 2. 1916. 



* Brown, Wm., Studies in the physiology of parasitism. III. On the relation 



between the infection drop and the underlying host tissue. Ann. Botany 30:399-406- 



1916. 



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