THE PHYSIOLOGICAL ROLE OF WATER IN PLANTS. 15 



period are able to resist drought to a considerable degree diiriug the 

 following ])eriods of their growth. The results of numerous horticul- 

 tural exi)eriiiieuts prove this statement. 



The different organs of the same plant vary in their requirements. 

 The optimum humidity for the individual, therefore, is not the same for 

 all stages of growth, nor for each organ of the plant. Not only has 

 each part of the plant certain characteristics peculiar to itself, but at 

 a given time different organs of the same plant are in different stages 

 of development. 



It is thus seen how extensive is the held of investigation in this line. 

 There is need of investigation of such (piestions as tlie following : With 

 a given species of plant in a given soil what amount of moisture must 

 be supplied during each stage of growth in order to obtain the best 

 result for each plant organ ■? For example, for sugar beets in a clay- 

 lime soil how much water must be given during the 3 months of growth 

 in order to secure the maximum i)roducti(m of foliage or in order to 

 obtain the maximum amount of sugar? 



The general formula which is given above may serve to suggest a 

 large number of practical experiments. To determine a (juestion of 

 this kind it is simply necessary to make comparative cultures in the 

 open soil and in pots. The effect of different moisture conditions is 

 noted — resorting to chemical analysis if it is a question of determining 

 the constituents of a plant, or simply w^eighing the products if only to 

 establish the gross returns. The following is an important question 

 which may thus be studied: What is the proportion of crude and 

 digestible fiber obtained by different methods of irrigation applied to 

 plants in natural meadows and to the same plants in artificial meadows? 

 This question is of vital importance to the rational feeding of animals, 

 since it permits a comparative valuation of forage plants grown under 

 known conditions. 



The proportion of the internal water infiueuces not only the dry 

 weight of the final product but also modifies completely the chemical 

 nature of certain of the elaborated materials. This influence should 

 be the subject of further study. If a plant is grown under different 

 moisture conditions, it will be found that the relative quantity of certain 

 intermediate products Avill also differ. The production of glucosids, 

 tannins, essential oils, fats, alkaloids, and coloring material is partic- 

 ularly influenced by drought and moisture.' The production of sugar 

 and organic acids in the fruit and in the sap also varies under the 

 same influence. The author's analyses have shown that the production 

 of chlorophyll is different upon dry and Avet soils, a fact which explains 

 many other variations in the elaborated materials of the plants. Illus- 

 trations of the above facts are found in the experiments of Lawe-s 

 and Gilbert, Deherain, and Lechartier on the comparative chemical 



' E. Gain, Sur la iniiti('ro colorante des organes souterraius, linl. Soc. Hot., France, 

 40(1893), p. 95. 



