142 GENERAL BOTANY 



TRANSPIRATION AND WATER ASCENT 



Transpiration is a term used to indicate the loss of water 

 from leaves and other exposed organs of the plant. It differs 

 from the evaporation of water from a free water surface in that it 

 is controlled by certain structural features of the epidermis 

 and bark, which greatly restrict the loss of water from these 

 organs. Thus, Sachs estimated that a given area of sunflower 

 leaves evaporated only about half as much water as a similar 

 area of free water surface. 



Transpiration, like evaporation, is also controlled by external 

 conditions of the atmosphere, such as temperature, humidity, and 

 air movements. Although the loss of water takes place from 

 all exposed parts of plants by evaporation, the term transpira- 

 tion is usually understood to apply to the loss of water from 

 leaves, where the greatest amount of evaporation takes place. 

 In the following discussion, therefore, leaf transpiration will be 

 mainly considered. 



Leaf transpiration. The same structural features which we 

 have* already noted as important in the gaseous exchanges con- 

 cerned with photosynthesis and respiration are also important in 

 transpiration. The delicate mesophyll cells of the leaf (Fig. 62, a) 

 are surrounded by a system of intercellular spaces which open 

 out into the external air through innumerable stomata. There- 

 fore the water which is supplied to these mesophyll cells from 

 the veins tends to evaporate from their cell Avails into the inter- 

 nal air within the intercellular spaces of the leaf, from which 

 it diffuses, like a gas, into the external air through the stomata. 

 If this evaporation of water is too great, the leaf wilts and the 

 plant is in danger. It is thus seen that the need for structural 

 adaptations in the leaf to facilitate gaseous exchange is often an 

 element of danger, since they may also lead to an undue loss of 

 water by transpiration. Therefore leaves often effect a compro- 

 mise in their structure between the need for gaseous exchange 

 and that of controlling water loss which might endanger the 

 life of the plant. Some of the important structural adaptations 

 designed to control transpiration are the following: 



