CHAPTER XIX 

 TRANSPIRATION 



If we measure the amount of water absorbed by a green plant 

 grown by the aid of a culture-solution, we see that it soon 

 exceeds the volume of the whole plant. This proves that not 

 only does the plant absorb water by means of its roots, but 

 that it also gives some water back to the air by the aid of its 

 shoot. This fact is also shown by the familiar experience that 

 a plant, or a cut shoot, withers if it be not supplied with water. 

 In these cases the water passes off in the form of an invisible 

 vapour: it is evaporated from the shoot of the plant. The 

 evolution of water in the form of a vapour from those parts 

 of living plants which are in contact with the air is termed 

 transpiration. It is important to note especially that transpir- 

 ation refers only to water given off in the form of a gas, and 

 that it does not include water which exudes in the form of 

 drops. As we shall see later, the leaves of some plants, in 

 addition to transpiring, excrete liquid water. 



How to measure transpiration. — {i.) Method of weighing. — 

 We take a potted plant, cover the earth in the pot around the 

 base of the plant's stem with a piece of tinfoil, and now weigh 

 the plant, together with the pot and its contents. We weigh a 

 second time after the lapse of an hour or two. The weight 

 has decreased because the plant has lost water by transpiration. 

 The loss of weight does not exactly represent the weight of 

 water transpired, because the plant has become slightly heavier 

 by reason of the carbon which it has absorbed from the atmos- 

 phere. But the gain in weight due to the absorbed carbon is 

 so excessively small, compared with the weight of water tran- 

 spired, that we may neglect it and regard the loss of weight as 

 measuring the amount of water transpired. The tinfoil is 

 placed over the soil in the pot in order to prevent water from 

 evaporating from the surface of the soil. [This experiment is 

 best performed on a fine day or in a dry room : see later.] 



209 o 



