24 PEACH LEAF CURL: ITS NATURE AND TREATMENT. 



closed, the cell walls bounding the intercelkilur passages and the air 

 in the passages themselves are thoroughly saturated with water and 

 aqueous vapor, respectively, and the movements of gases must be 

 retarded accordingly; turgescence is promoted, and the water contents 

 accumulate to a maximum, owing to the disturbance of equilibrium 

 between the amounts absorbed by the active roots in the relatively 

 warm soil and those passing off into the cold, damp air; much more 

 water is absorbed by the roots in the relatively warm soil than passes 

 oft' as vapor in equal periods of time." Further than this, Ward states 

 that "the low temperature, feeble light, and partially blocked ven- 

 tilation system have for a consequence a depression of respiratory 

 activity and the absorption of oxygen generally." This must give a 

 lowered v itality and an accumulation of organic acids. The reduced 

 light also leads to a decided reduction in the assimilative power of the 

 leaves. "The turgid condition of the cells, and the diminished inten 

 sity of the light," Ward says, "will favor growth." If this takes 

 place, "the tendency will be for the very watery cell walls to become 

 relatively thinner than usual, as well as watery, because the ill-nourished 

 protoplasm does not add to the substance of the walls in proportion. 

 This being so, we have the case of thinner, more watery cell walls 

 acting as the only mechanical protection between a possible fungus and 

 the cell contents." 



It is generally known that the conditions of moisture and shade, 

 which are above shown as making the tissues of a host plant more 

 tender and watery (more subject to fungous attacks), are also the 

 conditions most favorable to the development of fungi. This holds 

 equally as good for Exoascus deformans as for other forms. In speak- 

 ing of these conditions in relation to a fungus known as Botrytis^ Pro- 

 fessor Ward gives some generalizations equally applicable to Exoascus 

 deformans in its relation to curl. He says that just those external 

 climatic conditions which are disturbing the well-being of the green 

 host plant are either favorable to the fungi considered or, at any rate, 

 not in the least inimical to their development. "Thus," he says, "-the 

 oxygen respiration of the fungus goes on at all temperatures from 0° C. 

 to 30^ C. and higher, and although we still want information as to 

 details, experiments have shown that the mycelia flourish at tempera- 

 tures considerably below the optimum for higher plants. Moreover, 

 light, so indispensable for the carbon assimilation of the green host, 

 is absolutely unnecessary for the development of the fungus. ^ Then, 

 again, the dull, damp weather and saturated atmosphere, so injurious 

 to higher vegetation, if prolonged, because they entail interference 

 with the normal performance of various correlated functions, as we 

 have seen, and render the i^lant tender in all respects, are distinctly 

 favorable to the development of these fungi; consequently the very 

 set of external circumstances which make the host plant least able to 



