PHYSIOLOGY 45 



continual circulation because it is in direct continuity 

 with the general atmosphere through the pores in the 

 epidermis. This air, circulating round the thin-walled 

 cells, tends to dry them, and thus removes the water 

 from them almost as fast as it reaches them through 

 the other tissues from the roots. Hence a stream of water 

 vapour is constantly being given off from the leaves. 

 The circulation of water from the soil through the 

 roots and stems and from the leaves once more into 

 the atmosphere is technically called the transpiration 

 current. When all goes well with a plant in this circu- 

 lation of water the roots supply as fast as the leaves 

 give off, and the cells are provided with all they want, 

 but in a drought, when the soil is parched, or if the con- 

 nection with the roots is severed, the leaves give off 

 more than they are receiving, and the plant wilts and 

 will ultimately die of lack of water. The amount 

 of water that is kept in circulation by a large tree is 

 enormous, as is brought home to one by the bleeding 

 of a trunk that has been cut off in the spring, when 

 the sap is flowing fast to supply the call of the young 

 leaves. 



One more relation to the atmosphere must not be 

 forgotten, and that is the breathing of plants. It is 

 a widespread error to imagine that plants do not breathe 

 at all, or else to confuse the carbon assimilation with 

 breathing. The process of breathing is really one for 

 the oxidation of the tissues, and in both plants and 

 animals oxygen is taken in for this purpose ; some of it 

 is used, and the waste product resulting is carbonic 

 acid gas. In the lungs of animals this process goes on 

 simply, but in the leaves of plants, where it also goes 

 on, it is masked by the other process of feeding, in which 

 carbonic acid gas is taken in as food and split up, and 



