I208 MEANS OF PREVENTION AND CONTROL 



Resistance of the Organs. — i) The organs of wild plants have more com- 

 pact, i. e. less watery, tissues, and more acid, i. e. less sweet, juice than the 

 organs of the corresponding plants of cultivated species. 



2) More resistance is offered : [a) to rusts, by the varieties of wheat 

 with narrower and more waxy leaves (yellowish in colour), such as those 

 in the hottest and driest regions ; [h) to smuts, by those varieties of wheat 

 which are bearded, the loss of the beard being a sign of improvement ; (c) 

 to lodging, by those varieties of wheat which, like the Xoe, have the solid 

 part of the culm more fully developed ; {d) to diseases, by those varieties 

 of potato which have hard}^ and straight stalks, leaves small and down}-, and 

 the tubers covered with a coarse skin ; {e) to diseases, by those varieties of 

 olive tree which have smaller olives and a less developed mesocarp together 

 with a more compact wood : (/) to diseases, by citrus plants obtained 

 by sowing rather than those obtained by non-sexual reproduction ; (g) to 

 diseases in general, by those plants which present morphological characters 

 similar to those of hot and dry regions, non-fatty xerophytic plants. 



Chemiotaxy of the Tissues. — 1) Whilst the resistance is different in 

 the different races or varieties of the same species bred under like conditions, 

 no characteristic anatomical difference was found in their tissues by which 

 the different resistance to diseases could be explained. 



2) An}' variation in the environment or in the methods of culti- 

 vation produces a modification in the strength of resistance, so as to render 

 susceptible even those races which had previously been regarded as refrac- 

 tory. 



3) Both resistance and susceptibility are individual and inherent 

 characters rather than race characters. 



4) The substance endowed with positive chemiotropism is formed by 

 the sugars and starches, but more particularly by the former than by the 

 latter. 



Acidity of Sap and Oxydases. — i) in a given organ, other conditions 

 being equal, the receptivity to parasites increases with the quantity of 

 reducing sugars ; and on the other hand, resistance increases with the or- 

 ganic acids. 



2) In the cell sap the reducing sugars increase as the organic acids 

 diminish and vice- versa. 



3) Those organic acids which, during the night, serve to excite the 

 zymotic processes calculated to promote the migration into the leaves of 

 the organic materials produced photosyntheticalh^ are on the contrary 

 transformed during the da}-, partly at least, into food substances, i e. carbo- 

 hyrates. 



4) The organs (branches and fruits) which present a stronger resist- 

 ance to the attacks of parasites are the 3'oungest organs, above all when 

 in the period of growth and more acid. 



5) The organic acids help to provide growing organs with that 

 turgidity which is necessary in order to promote the zymotic processes 

 which, by mobilising the stored materials, are to supply sufficient nourish- 



