498 



of a specially made microscopic razor called a " microtome," and 

 one of these thin slices was put under a microscope, we would see 

 a section of the leaf showing on the outer edges one row of scales 

 of the cuticle, disposed somewhat like bricks or tiles, and in the 

 centre a mass of more tender living tissue, disposed somewhat like 

 a close-grained sponge, and through which the sap circulates. 



Now in the case of a diseased leaf, we see permeating- the 

 living tissue a meshwork of filaments which are the roots of 

 the fungus, and are called mycelial threads, then, emerging 

 through the breathing pores of the plant, slender thread-like stems 

 showing on the surface of the leaf, and on each of these thread-like 

 stems little branches carrying small sacks full of spores or of seeds 

 of the fungus. 



Thus we see that a parasitic fungus robs the plant, through 

 its mycelial threads, of the sap and the food which is pumped up 

 by the plant from its roots to the extremity of its branches for the 

 nourishment of its foliage and its fruit. In order to understand 

 the usefulness of spraying with fungicides, we must not lose sight 

 of these facts. The internal cells of the leaf are so delicate that 

 the application to them of a poisonous substance, even though in a 

 very diluted form, would prove injurious. But those internal cells 

 are covered over and protected by the outside layer of thick-walled 

 impermeable cells just mentioned, which would be able to bear the 

 effect of the poison in a stronger form. Of course, it will be under- 

 stood that a very strong poison would affect these outside cells also, 

 but there is a certain strength of the poison, slightly stronger than 

 would be injurious to delicate cells, that can be applied to them 

 with impunity. It so happens that the cells of fungi are as a 

 rule very delicate, and highly susceptible to the destructive 

 influence of poisons in solution ; and, often, such solutions as would 

 not be injurious to the outside thick-walled cells of leaves are 

 quite fatal to fungus cells. 



Tender leaves as a rule are, for reasons detailed in the fore- 

 going paragraph, less resistant than older leaves of the same plants ; 

 and, again, some sort of trees, such as the peach or the nectarine, 

 having a thinner and more delicate cuticle than the apple 

 or the pear, are sometimes injured by the application of poisonous 

 liquids that would not hurt the more resistant plants. 



Again, the facts brought to light here explain why, in a great 

 many cases, several sprayings are necessary for eradicating disease. 

 The application of a remedy will often only destroy that part of the 

 fungus parasite which shows outside, but fresh shoots will soon be 

 pushed out by the mycelial threads which permeate the more 

 delicate internal tissues of the leaf, and soon carry a fresh crop of 

 spores or fungus seeds, unless by timely treatment the parasite is 

 again destroyed. 



Many fruit-growers and farmers, however, who have no 

 conception of these facts, are not satisfied unless a remedy costing 



