TRANSPIRATION 193 



The rate at which transpiration is carried on is influenced by 

 the character of the external cell-walls of the various parts of the 

 plants. 



From cells with suberised and cutinised walls the loss of water 

 is small, hence, from the stems and leaves of cactuses and house- 

 leek, from many fruits such as apples and pears, with a well- 

 developed cuticle, and also from stems and tubers covered with 

 cork-tissue and bark, the amount of transpiration is comparatively 

 slight : vegetable marrows, potatoes, and many kinds of apples 

 containing a large proportion of water, retain a large amount of 

 it for many weeks and even months. 



The presence of a covering of woolly hairs upon the leaves 

 and other parts of plants aids in the prevention of excessive 

 transpiration, and the excretion of a waxy ' bloom ' on the exterior 

 of the epidermis of many leaves such as those of the cabbage, 

 swede and onion, and upon fruits such as plums and grapes, acts 

 in a similar protective manner. Experiments show that when 

 the ' bloom ' is rubbed from leaves and fruits a greater loss of 

 water takes place than from similar parts untouched. 



The amount of what may be termed cuticular transpiration, or 

 loss through the external cell-walls of leaves, stems and parts 

 normally exposed to the air, is slight in all cases, except in the 

 youngest members whose epidermal cells have not yet become 

 fully cutinised. 



The chief escape of water is by diasiomatic transpiration, that 

 is by loss through the openings of the stomata, and as these are 

 always met with in greatest abundance upon the leaves of plants, 

 the latter may be considered as the chief organs of transpiration. 



The cells of the spongy parenchyma of the leaf (^, Fig. 75) 

 possess uncutinised walls which freely allow the passage of 

 water-vapour into the intercellular spaces, and it is mainly from 

 these spaces that the vapour escapes by way of the stomata 



(X). 



Generally there are more stomata on the lower surfaces of 



N 



