On Evil in Small Doses 



So also with the thickening of the epidermis walls 

 when a plant is exposed for the first time to desert con- 

 ditions ; particle after particle of protoplasm is, so to 

 speak, '' hurried to the front," and there secretes itself 

 into cellulose until the outer wall, the first line of defence, 

 is so thick that no harm can arise to the live protoplasm 

 within. But it is upon the method of, and resistances 

 to fungus attacks, that we have the most valuable of 

 recent researches. 



The first effect is to stimulate the living protoplasm 

 of the leaf cells. It has been found by direct testing that 

 the amount of enzymes is greatly increased. In diseased 

 mulberry leaves there are more diastases, invertases, and 

 oxydases than under ordinary circumstances.^^ One 

 can realise what is going on in such cases from the vivid 

 account given by Miss Gibson of one of her experiments. 



A rust-spore (Uredo chrysanthemi) was placed upon 

 the leaf of a chrysanthemum. The spore put out its 

 tiny but deadly germ-tube, which felt its way to a stoma 

 and then grew down, through the opening, into the 

 leaf. Then the battle between the secretions of the 

 fungus and the counter-secretions of the leaf cells con- 

 tinued for four days. But the leaf conquered, for the 

 fungus-tube withered and died. Yet it was not won 

 without some loss, for a circle of dead leaf cells marked 

 the batdefield.2i 



Mr. Salmon actually interfered in a struggle of this 

 kind, and with disastrous results, so far as the plant was 

 concerned. There are certain mildews (Erysiphe) 

 which attack the leaves of Brome-grasses. He found by 

 experiment that one of these mildews was unable to 

 attack a particular Bromus. Then he began to help 

 the fungus. He sliced a very tiny piece of the surface 

 away from the under side of a leaf. This depressed its 

 vitality and it succumbed to the attack. Even a slight 



333 



