MEANS OF PREVENTION AND CONTROL I209 



nient to the growing organs; they contribute to this b}' absorbing and re- 

 taining the water in circulation. 



6) In alkaHne soils, such as limestone soils, and those irrigated with 

 alkaline water, cultivated plants present a lesser quantity of free acid in 

 their "lissues and are more sensitive to adverse factors. 



7) Oxydases are more abundant in the more juicj- ti.ssues and in 

 those diseased organs which are richest in nitrogenous substances; they are 

 most active in the hottest regions, where also the organic acids contained in 

 the plants burn up; this combustion produces COg. which is eliminated, thus 

 depri\4ng the plant of the acidity required to render it resistant. For this 

 reason, in hotter regions, other conditions being equal, the plant is more 

 sensitive than in less hot regions. In proportion as the acidit}' of the juices in 

 the organs diminishes, their sweet taste will become more evident, and 

 the parasites will find them a more agreeable and more abundant food for 

 their growth. 



Acidity of Sap and Plant Parasites. — i) Organic acids are poisonous 

 to micromycetes. 



2) The greater acidity of the sap produces a greater resistance in 

 plants as follows : {a) wheat to rusts; {h) wine stocks to oidium, mildew and 

 "tuberculosis" . (c) the olive to Cy do conium and Stictis Panizzei; {d) the 

 the pink, potato and hyacinth to bacterial infection ; {e) fruit plants in 

 general to root rot and gummosis. 



3) Pruning, by rejuvenating the branches and foliage of fruit trees, 

 increases the resistance to adverse agencies. 



Acidity of Sap and Animal Parasites. — i) As the increase of the quant- 

 ity of reducing sugars (very much sought after by insects) in an organ is 

 attended by a reduction in the organic acids and vice versa, it follows that 

 acidity is the weapon most used by the plant against animal parasites likewise. 

 Among acids, however, the most poisonous appears to be malic acid. 



2) The greater acidity of the vegetable juices presents a defence in 

 the following cases : {a) the young branches of the white nuilberry* from 

 Diaspis pentagon a ; (b) the young citrus plants from Pseiidococcus citri ; 

 (c) citrus plants from Crysomphalns dictyospermi; (d) improved young fig trees 

 and those obtained from seed, from Ceroplastes rusci ; {e) young olive trees 

 from Saissetia oleae and Philippia oleae; (/) ordinary- and citrus fruits, from 

 attack by scale insects ; {g) olive trees, from the " fly "{Dacus oleae) ; 

 {h) the grape, from Albinia Wockiana ; (i) vines, from phylloxera. 



Tannic Substances and Parasites. — i) Tannins, which are acid sub- 

 stances, behave like organic acids, and that is wh}' they abound in growing 

 organs, defending the latter against the attack of parasites, especially 

 vegetable parasites. 



2) In artificial nutrient media, the presence of tannin added in differ- 

 ent proportions prevents the germination of the spores or else it stops the 

 growth of the mycelium. In this case, tannin clearly causes a more effective 

 antifungal action of the wall of the cell itself. 



3) Antocyanine, as a substance derived from bodies (tannins) en- 

 dowed with a relatively high osmotic power, contributed, like tannic sub- 



