Host and Parasite m certain Diseases of Plants. 



427 



Fig. 10. Portion of a transverse section through an infection-spot in the tissues of 

 snowdrop (such as that at a in fig. 11), showing the swollen cell- walls with 

 hyphse of Botrytis in them. The cell-contents also show changes ; the proto- 

 plasm contracts, dies, and turns brown, and stains less and less readily the 

 further the changes proceed ; the cell-sap escapes and suffuses the cell- walls ; 

 the nucleus is the last to succumb. The above changes are exhibited by cells 

 some distance away from the hyphse, and are the less pronounced the further 

 away the cells are. The colour reactions are of course not reproduced. (Very 

 highly magnified.) 



cellulose substance,* the hyphse also excrete a soluble substance which 

 kills the protoplasm (with which they are not in contact) of the cells 

 in the immediate neighbourhood : whether this substance is a 

 separate zymase, or whether it is the same soluble ferment as that 

 which swells the cellulose, is not clear, or whether the protoplasm 

 simply dies after excessive plasmolysis due to water passing into 

 the swollen walls, but it is clear that some such, poisonous action is 

 exerted at a little distance from the tip of the hypha, and therefore 

 by means of a soluble poison or z}auase of some kind. It is difficult 

 to decide what this poison is, and the following questions arise : — 

 first, Is the poison the same zymase as that which causes the 

 swelling and solution of the cellulose ? This must be denied pro- 

 visionally, at any rate, because if sections of the tissues are put into 

 solutions containing extracts of the mycelium which have been pre- 

 viously boiled for a minute or two, the protoplasm contracts and dies 

 much as before, though the cellulose walls no longer swell as before 

 because the zymase has been killed by the boiling. This experi- 

 ment is not quite conclusive, because che contraction of the proto- 

 plasm may be due to the action of bodies in the boiled extract which 

 did not exist in the freshly expressed liquid, f 



* See 'Annals of Botany,' vol. 2, p. 356 and figs. 55 and 56. 



f De Bary inclined to the belief in a special ferment in the case of his Peziza 

 (Joe. eit., pp. 418 — 420), but admitted that he had not proved the point either way. 



2 i 2 



