NUTRITION. 133 



by means of a piece of rubber tubing slipped over the end of the stem, 

 taking care not to expose the cut end to air. Fill glass tube with water 

 before fitting in plant ; erect the whole with lower end of tube dipping 

 in a cup of mercury. Set in light and note height of mercury in 1-48 

 hours. 



176. The loss of water. — Water is constantly evaporating 

 from the whole surface of the plant exposed to the air. Since 

 this loss is more or less modified by the vital activity of the 

 plant, it has received the special name, transpiration. 



EXERCISE XXVII. 



To show the loss of water by evaporation. 



Clean and dry the surface of a pot in which a thrifty single-stemmed 

 plant is growing ; close the hole in the bottom with a cork ; with a 

 brush paint the whole surface thickly and evenly with melted paraffin. 

 Cut out a piece of stiff paper which will fit around stem and just cover 

 the soil in pot. Using this as a pattern cut a cover for the soil from a 

 sheet of lead ; slit the cover from the central hole to circumference ; ad- 

 just it around plant and cement all cracks with grafting wax.* Weigh. 

 Weigh again at intervals of 24 hours, for 4 days. 



177. .Transpiration. — In the higher plants transpiration 

 from the surface is reduced by the viraterproofing of the epi- 

 dermis, so that most of it takes place from the surfaces of 

 internal cells into the intercellular spaces, wherever these 

 exist. Since the intercellular spaces are connected with each 

 other and also, through the stomata, with the outside air, 

 water vapor is constantly passing off by diffusion (see fig. 

 106). The leaves, affording the largest exposure, are espe- 

 cially organs of transpiration. After they have become fully 

 expanded no considerable amount of water is lost directly 

 from their surfaces. 



* Or the pot may be set in a tin or glass vessel which it fits ; this may 

 be covered by sheet rubber tied to the edge and about the stem ; or the 

 lead cover may be cemented on as above. 



