NATURE OF DISEASE. 131 



poison may alter the physiological activity of 

 living cells, inducing pathological phenomena, 

 while larger doses kill them. 



Now we know at least one parasitic fungus 

 which poisons the cells of its host, and kills them, 

 with similar symptoms to those resulting from 

 excessive doses of the above-named toxic agents. 

 Botrytis hyphs, living in the cell-walls of plants, 

 but not entering the cells, excretes a poison 

 which kills the protoplasm, and the fungus then 

 feeds on the debris. Numerous other fungi form 

 powerful poisons, but we do not know whether 

 or how they employ them -e.g. Ergot. 



It is obvious that if all the young cells of a root- 

 tip or of the apex of a shoot, or those of a young 

 leaf, are growing and dividing regularly, the killing 

 of one or a few cells at one point on the side of 

 the organ must result in irregularities in malfor- 

 mation of the adult organ. This has been proved 

 experimentally by destroying a few cells with a 

 needle. It can also be done by planting a minute 

 mycelium of Botrytis laterally on a young organ 

 e.g. a very young lily-bud. The fungus adheres 

 to the surface, kills a few epidermis cells, and 

 forms a foxy-red spot, which becomes concave as 

 the dead cells lose water and dry. Since the rest 

 of the bud goes on growing, however, while this 

 dead point remains stationary, the latter gradually 

 becomes the centre of a concavity, the growing 

 tissues having grown round it : the bud is de- 

 formed. Numerous cases of malformed organs 

 are explained in this way ; a minute insect has 



