MEANS OF PREVENTION AND CONTROL I20; 



sit)' of their tissues in consequence of the shortage of available nitrogenous 

 substances in the compact soil, but also, and in a still greater degree, to the 

 acidity of their cell juices, owing to the incomplete combustion of the 

 carbohydrates chiefly contained in the roots, the air circulation being 

 difficult and sometimes interrupted in compact and uncultivated soil. 



Cultivation. — i) By cultivation man has rendered edible the organs of 

 wild plants which are to-day replaced by their cultivated varieties. 



2) Assiduous, intense and even forced cultivation results in induc- 

 ing an enlargement of the organs of the wild plant, and also renders them 

 succulent in consequence of the increase of volume in the anatomic elements 

 as compared with the cellular tissue, as well as by the continually greater 

 diminution of their walls, until the elements of the sclerenchyma are seen 

 almost to disappear. 



3) By progress in the improvement of a plant, its sensitiveness to ad- 

 verse causes is also increased. 



4) The development of a parasitic infection is always related to the 

 medium (environment in which it develops) which is offered by the tissues 

 of the plants attacked ; the result is that, all other conditions being equal, 

 the infection increases with the improvement gained by the plant. 



5) Conversely, for fruit plants, seed reproduction habitually produces 

 a retrogression in the improvement obtained ; the same retrogression is 

 brought about by the absence of cultivation ; the products, which in both 

 cases are marred in quality, certainly run less ri.sk of disease than similar 

 plants, but improved. 



Nitrogenous Maniive. — i) Manuring with suitable manure renders 

 the tissues more juicy and consequently more sensitive to bad weather; 

 they are also richer in sugar, and consequent^ more attractive to animal and 

 vegetable parasites. 



2) The richer a soil is in nitrogen, the shorter the period of time elaps- 

 ing between inoculation and infection with a vegetable parasite ; there is 

 also an increase in the receptivity of cultivated plants to parasites of every 

 kind. An abundant nitrogenous manure, in whatever form given, always 

 produces the same effects. 



3) Nitrate of soda, which calls forth greater productivity of the plant, 

 on the other hand renders it nuich more tender, and consequently more sen- 

 sitive to the bad effects of adverse factors. 



4) Excessive nitrogenous manure prolongs the growing period of 

 plants and retards lignification ; consequentl}' the foliage will be more sen- 

 sitive to winter cold, with a loss in the new production and risk of loss 

 of sap and gum. 



In the 2nd part of the work on plant resistance to disease, the writer 

 examines the following questions in succession : the resistance of the ])lant 

 organs; the chemistr}- of the tissues; the acidity of the sap and oxydases ; 

 the acidity of the sap and plant parasites; the acidity of the sap and animal 

 parasites ; tannic substances and parasites ; selection crossing and high 

 grafting ; manures. 



The conclusions are as follows : 



