^75 



normal vegetative factors and are disturbances in the equilibrium of the 

 interacting nutritive processes whereby certain ones are repressed while 

 others predominate. 



If we now speak of diseases due to a lack, or an excess of moisture and 

 nutritive substances we also involve in this the phenomena in which atrophies 

 and hypertrophies occur in various parts of the plant body. These need not 

 arise from an actual lack or excess of moisture and nutritive substances, but 

 are simply produced by the unfitness of the plant, from the combination of 

 the factors of growth, to nourish all its organs advantageously for the de- 

 velopment of the whole. The absolute phenomena due to lack and excess 

 are approximated on this account by the relative ones in the form of dis- 

 turbances of the local equilibrium. 



A. Lack of Moisture and Nutritive Substances. 



a. Lack of Moisture. 



Influence of the Various Plant Coverings. 



After having considered the physical processes leading to a lack of 

 moisture in the soil, and after having discussed a number of phenomena of 

 diseases arising therefrom, we must consider supplementarily the influence 

 which the covering of vegetation itself exercises on the water content of the 

 soil. On the same soil, with the same atmospheric conditions, a cultivated 

 plant will find a supply of moisture sutficient for its development on one 

 part of a field, and not on another part, if on the former some species has 

 been grown which makes a small demand on the water content. Therefore 

 the preceding crop is of significance for each planting. 



As Wollny^ has determined, the water content is less in the root region 

 of a planted field than in the corresponding layers of the naked soil. The 

 more luxuriant the plant growth and the thicker and longer lived, the more 

 v/ater is lost from the soil. Experiments have not determined any fixed 

 scale for the use of water, yet they indicate that, on an average, the ever- 

 green conifers require the greatest quantities while deciduous trees and 

 perennial fodder plants follow in a descending scale and the superficially 

 rooting field plants make less demand on the whole supply of the water in 

 the field. Of the latter group, the large, richly leaved, erect Papilionaceae, 

 such as the field and bush beans, seem to require the most water at the time 

 of their chief development, while the roots and tuberous plants cultivated in 

 wide rows should be named last. In summer the perennial fodder plants 

 use somewhat greater quantities than field plants and conifers. This is re- 

 versed in the spring and fall. In winter the requirements of the different 

 plants equalize, except the conifers, which in mild winter weather 

 constantly withdraw definite amounts of water from the soil. 



1 WoUny, E., Ueber den Einfluss der Pflanzendecken auf die Wasserfiihrung 

 der Fliisse. Vierteljahrsschr. d. Bayer, Landwirtschaftsrates 1900, p. 389. 



