132 DISEASF. [\ I'l.AXTS. 



bitten or pierced the young tissue, or a fungus has 

 killed a minute area, or a drop of acid condensed 

 from fumes in the air is the lethal agent, and so 

 forth. And even on a much larger scale we see 

 the same kinds of agents at work. Wherever a 

 patch of cells is killed whilst those around go on 

 growing, there must result some deformation of 

 the resulting organ, since had the injury been 

 withheld the number and sizes of the cells now 

 fixed in death would have increased and covered 

 a larger area : they now serve to pull over to 

 their side the still living and growing cells. The 

 same results follow on any lateral wound : the 

 killed spot of tissue serves as a point round which 

 the continued growth of other parts of the organ 

 turns. Hence the malformation is in these cases 

 a secondary effect, and not, as in simple hyper- 

 trophy, a direct effect of the action of the cells 

 involved in the injury. 



There is another class of bodies secreted by 

 fungi, however, which act directly on cells, viz. 

 enzymes that is, soluble bodies which are able 

 to dissolve cellulose (cytases), starch (diastases), 

 proteids (proteolytic enzymes), and other sub- 

 stances, by peculiar alterations in their constitution. 

 It is by means of its cytase that Botrytis hyphae 

 pierce the cellulose walls of plants, and no doubt 

 in all cases where fungi pierce cell-walls it 

 is by the solvent action of such a cytase, 

 and similarly when haustoria penetrate into the 

 cells. It is also by means of these starch- 

 dissolving enzymes (diastases) and proteolytic 



