The Water of Plants and its Movements. 57 



Section V. The Watee of Plants and its Move- 

 ments 



74. Plants Contain Large Amounts of Water. We have 

 seen that the cell-walls of living plants are constantly 

 saturated with water (63), 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 gener- 

 ally 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 constant in the same plants, but varies somewhat 

 with the water content of the soil and with meteorologi- 

 cal conditions. 



75. Transpiration ( trans-pi- ra'-tion). The water of 

 plants passes off more or less rapidly from parts exposed 

 to the air — usually as an invisible vapor. This invisi- 

 ble escape of water from plants is called transpiration. It 

 is mainly due to evaporation of water from the plant, 

 the same as takes place from other moist material. But 

 fluctuations 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 nourishment 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 surface, the nature of the epi- 



4 



