499 



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." 



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 

 particles have for each other comes into play, and causes them to 

 roll up into drops of visible size (Fig. B). 



The illustration shows two patches of wheat- leaves sprayed 

 with the fungicide known as ammonio-carbonate of copper --that 

 is, carbonate of copper dissolved in dilute ammonia. After a plant 

 has been sprayed with this solution, the diluted ammonia evapor- 

 ates and leaves the carbonate of copper in the shape of an exceed- 

 ingly fine powder. Consequently, when we examine the surface 

 of the sprayed plant with a microscope, we instantly notice this 

 deposit distributed in patches, as shown in the adjacent illustration. 

 Each patch of deposit is a dried-up droplet of the spray ; about 

 twenty such patches are shown in one illustration. Between the 

 patches is seen that part of the wheat-leaf which was not hit by 

 the spray. As long as the patches of poison remain they consti- 

 tute a protection against the infection by means of spores, which 

 may fall upon a sprayed leaf. A spore falling upon the poison 

 will be killed as soon as it attempts to germinate. Even should it 



