58 Principles of Plant Culture. 



plants is generally very large. In the root of the tur- 

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

 plants passes off more or less rapidly from parts ex- 

 posed to the air— usually as an invisible vapor. This 

 invisible escape of water from plants is called trans- 

 piration. 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 stoma ta (65) are open in the light 

 and thus facilitate the escape of water from the inter- 

 cellular 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 surface, the nature of the epidermis and cuticle 

 (64), the number of stomata (65), etc. Some plants, 

 as purslane, the sedums, cacti, etc., have special water- 

 storing tissue, from which transpiration is extremely 

 slow. 



Experiments indicate that the transpiration from 

 most leaves is between one-third and one-sixth as much 

 as the evaporation from an equal area of water. When 



