The Water Economy and the Hydrature of Plants

Abstract

that the plant as such is a unit and that these parts of economy are interrelated and dependent on each other. They can scarcely be separated in a plant growing under natural conditions. The attempt shall therefore be made to understand the economy of a plant as a whole, and to discuss the variations shown by the separate groups of plants distinguished by the complexity of their organisation. The importance of economy appears in the fact that most of the weight of the fresh plant, particularly the protoplast, is represented by water. A proper understanding of the economy of plants is only achieved, however, if it is taken into consideration that not only is the quantity of the important but, above all, the of the in the living substance; its water potential. We may demonstrate this condition by the use of four examples: (a) Let us assume that in one cubic meter of air there are 12 gm. of vapour, and that the temperature in one case is 15°C. and in another case 20°C., respectively. Supposing that in each case a piece of bread is introduced into this air. Mould will grow on it at 15°C. but not at a temperature of 20°C. A temperature of 20°C. does not prevent the growth of mould, and the same amount of is at the disposal of the mould in either case, but the con­ dition of the water, as shown by the relative humidity, differs. The latter at 15°C. is 94 per cent but at 20°C. only 70 per cent. (b) Bean seedlings are to be transferred into Knop's culture solution, each of the containers have a capacity of 11. of water. In one container the culture solution is left unchanged. In the other, 171 gm. of saccharose are added, so that the osmotic pressure is increased considerably. Though the