50 PRINCIPLES OF PLANT CULTURE 



THE WATER OF PLANTS AND ITS MOVEMENTS 



73. Water-content of plants. — Plants contain large 

 amounts of water. We have seen that the cell-walls of 

 living plants are constantly saturated with water (62), 

 and that the cells of the growing parts are always more 

 or less distended with it. The proportion of water con- 

 tained 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 constant in the 

 same plants, but varies somewhat with the water content 

 of the soil and with meteorological conditions. 



74. Transpiration. — The water of plants passes off 

 more or less rapidly from parts exposed to the air — 

 usually as an invisible vapor. This invisible escape of 

 water from plants is called transpiration. It is due mainly 

 to evaporation of the water from the surface of the in- 

 terior cells of the plant and its diffusion as water vapor 

 through the stomata (65). The fact that the stomata 

 control the rate of escape of this water vapor from the 

 interior of the leaf explains why fluctuations occur in the 

 amount of transpiration from living plants that do not 

 occur in the evaporation from dead organic material under 

 similar conditions. For example, transpiration is more 

 rapid in light than in darkness, because the stomata 

 (65) are open in the light and thus facilitate the escape of 

 water from the intercellular spaces. Plants poorly sup- 

 plied 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 epidermis and 



