10 INSECTICIDES, FUNGICIDES, AND WEED KILLEES. 



ness of the plant towards these ingredients must be known. Each 

 plant possesses its own particular sensitiveness towards substances 

 poisonous to parasites, and it is desirable to use these substances, in 

 each instance in an appropriate degree of concentration. AA'hen the 

 sensitiveness of the plant is gi'eater than that of the parasite, there is 

 reason to abstain from the use of such substances, or it is then necessary 

 to follow the spraying by washing with pure water, only giving them the 

 time required to act on the parasite. This latter precaution allows 

 the use of strong doses of toxic substances, doses w^hich would kill the 

 plants if the w^ashing did not intervene to prevent prolonged contact. 



Indispensable Properties of the Chemical Agents. — The 

 chemical agents should be of such a nature as to guarantee reaching 

 the parasites. Certain insects and their larvae are covered with hair 

 and down, or even with a coat of wax, which prevents aqueous solu- 

 tions from reaching them. The insecticides which should be 

 employed in such cases are alcoholic, ethereal, or oily solutions, 

 soaps and caustic alkalies having a solvent action on the organs of 

 protection, and capable of moistening them, so as to let the toxic sub- 

 stances penetrate as far as the sensitive organs of the insect. The 

 treatment should often be curative and preservative, and it is then 

 necessary that the substances used should persist for the longest time 

 possible on the surface of the plant. This problem would be easily 

 realized if rain did not remove in a short time the deposit of substances 

 created by spraying. Attempts have been made to protect plants 

 from the effects of the natural washing by the use of substances of 

 poor solubility in water, and with a perfect adherence to the organs 

 of the surface treated. The agents only slightly soluble in water 

 spread on the surface of the plant, m the form of bouillies, form 

 deposits which the rain cannot remove owing to their own adherence, 

 or to an adherence acquired artificially, by the incorporation of gluey 

 substances, insoluble in water (silicate of soda, saccharates, soaps, 

 gelatine, rosin). But cai-e must be taken not to use too insoluble 

 substances and in too great quantity, for there is a risk of covering the 

 whole of the respiratory surface of the leaves wath a layer rendering 

 the exchange of gases impossible, which, if it does not cause asphyxia, 

 produces at least an annoying disturbance in the growth (evolution). 

 The insoluble, or the only slightly soluble products are, in general, of 

 greater service than the soluble products. In addition to their less 

 injurious action on the plant, they persist longer on the surface of the 

 vulnerable organs and their action is of longer duration. 



The insoluble products are intended to poison insects through the 

 stomach. A slight layer of arseniate of lead, or of arsenite of copper 

 (Paris green), 1 on the leaves penetrates into the stomach at the same 

 time as the leaf and kills the insect. In the case of cryptogamic 

 parasites, slightly soluble substances are used, which in contact with 

 the dew causes it to become toxic, owing to the traces of poison which 

 it has dissolved, and to kill the spores, which require its aid for their 

 evolution. Briefly, it is necessary in each particular case to choose 



1 Note by Tra7islator. — Arsenite of copper is not Paris green but Scheele's green. 

 Paris green is our emerald green, the aceto-arsenite of copper. 



