539 
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 farmer3, however, who have no con- 
ception of these facts, are not satisfied unless a remedy costing 
next to nothing is offered to them, which will in one application 
destroy the enemy for ever. The mode of attack of the parasite, 
explained above, will explain how unreasonable such an expecta- 
tion is ; as a remedy that would effect this end at one stroke would 
probably, at the same time, destroy the tissues of the leaves. 
“The application of fungicides does not effect a cure,” says 
Dr. Cobb, “it only prevents the fungus from fruiting and germin- 
ating. That part of the fungus inside the leaf can be killed only 
by killing the leaf itself.” Hence the advisability of repeated 
sprayings which weaken the fungus and also protect the plant 
for a time and until a change in the climatic conditions make a 
fresh outbreak of the fungus pest less likely. 
Many fungicides and insecticides either consist of particles of 
solid matter in the form of powder as sulphur, or in suspension in 
water as Paris green, or partly in suspension and partly in solu- 
tion as carbonate of copper in Bordeaux mixture. After a plant 
has been sprayed with one of these substances, therefore, it may be 
considered as being covered with an armour of solid poisonous sub- 
stance, which will destroy by contact any fungus shoot that shows 
on the leaf, or prevents its germination. 
All armours, however, have their weak points, and the armour 
in which we thus dress our plants is no exception. Let us examine, 
says Dr. Cobb, into the weak points of our system of spraying, and 
see whether they are capable of being remedied. It requires no very 
lengthened experience in spraying to learn that different crops 
receive a spray in very different ways. The leaves of some crops 
may wet easily, but to one crop of this sort there are many that do 
not do so. Often the spray will be seen to collect immediately in 
drops, in spite of its fineness when applied. This is due to the 
waxy covering with which many kinds of leaves and fruits are 
furnished. When the microscopic liquid particles, of which the 
fungicide is composed, come into contact with the bloom of the 
leaf, they refuse to adhere if the spraying is carried on too long, 
or they adhere so slightly that the attraction which adjacent par- 
ticles have for each other comes into play, and causes them to 
roll up into drops of visible size (Fig. B,) 
