The Water of Plants and Its Movements. 55 
Section V. THe WaTER oF PLANTS AND Its MovEMENTS 
74. Plants Contain Large Amounts of Water. We 
have seen (63) that the cell-walls of living plants are con- 
stantly saturated with water, and that the cells of the grow- 
ing parts are always more or less distended with it. The 
proportion of water contained in living plants is generally 
very large. In the root of the turnip, and in some fruits, it 
may exceed ninety per cent. of the whole weight. It is 
greatest in young plants, and in the younger and growing 
parts of older plants. The proportion of water is not con- 
stant in the same plants, but varies within certain limits, 
with the water content of the soil, and with meteorological 
conditions. 
73. Transpiration (trans-pi-ra’-tion), The water passes 
off more or less rapidly from parts of plants exposed to the 
air,— usually as an invisible vapor. This invisible escape 
of water from plants is called transpiration. It is mainly 
due to evaporation of water from the plant into the air, the 
same as takes place from other moist material. But fluctu- 
ations occur in the amount of transpiration from living plants 
that do not occur in dead organic material under similar 
conditions. For example, transpiration is more rapid in 
light than in darkness, because the stomata (66) are open 
in the light and thus facilitate the escape of water from the 
intercellular spaces. Plants poorly supplied with nourish- 
ment transpire more freely under the same conditions than 
those well supplied. The amount of transpiration varies 
greatly in different plants, and depends upon the leaf sur- 
face, the nature of the epidermis and cuticle (65), the num- 
ber of stomata (66), etc. Some plants, as purslane, the 
