ioo BOTANY OF THE LIVING PLANT 



impervious to water. The cuticle is, however, interrupted by the 

 very numerous stomata. Within the organs, and connected with the 

 stomata, we find a ramifying system of air-spaces, which ventilate the 

 internal tissues. They are thus brought indirectly into communication 

 with the atmosphere, making possible that exchange of carbon dioxide 

 and oxygen which the functions of photosynthesis and respiration 

 require. It is clearly possible that there will be loss of water by 

 evaporation from the tissues so ventilated, just as clothes hung out 

 in the open-air dry by evaporation. 



Experiment confirms that from the aerial organs of plants, especially 

 the leaves, there is an extensive evaporation of water, to which the 

 term Transpiration is applied. If leafy shoots are placed under a 

 bell-jar, a deposit of condensed water-vapour soon appears on its 

 inner surface. Further, if pieces of dry cobalt chloride paper be fixed 

 on the surface of a suitable leaf and protected from the atmosphere, 

 a rapid change of colour from blue to pink will indicate the liberation 

 of water-vapour from the leaf-tissues. The actual evaporation is 

 generally held to occur from the cell-walls of the mesophyll, which 

 contain imbibed water, into the atmosphere of the air-spaces : thence 

 the water-vapour diffuses through the stomata into the outer air. 

 The path can be traced in Figs. 48 and 49. There is also some trans- 

 piration through the cuticle of leaf and stem ; but that the greater 

 part is through the stomata is indicated by an experiment in which 

 pieces of cobalt chloride paper are placed on both surfaces of a 

 leaf showing stomata on the abaxial surface only. The change in 

 colour is found to occur considerably more rapidly in the lower 

 piece of test-paper than in the upper. Alternatively two similar 

 leaves may be taken and a thin film of vaseline smeared on the 

 adaxial surface of one, and on the abaxial surface of the other ; here 

 again leaves with stomata on the lower surface only are to be used. 

 It will be found by weighing that the first leaf loses water by trans- 

 piration much more quickly than the second, in which the stomata 

 are covered up. 



We thus realise that transpiration is for the most part an unavoid- 

 able result of aeration of the tissues. In times of water-shortage 

 transpiration may be a definite disadvantage to the plant, and it 

 has been remarked that more plants perish or are retarded in growth 

 through lack of water than from any other cause. Many plants, 

 especially those inhabiting dry situations (the so-called Xerophytes, 

 see Chapter XI.), display structural modifications tending to reduce 

 transpiration in times of drought. To a plant, however, that is 



