54 



THE SOIL AND THE SAP 



is an unusually large space. Now water that is coming up in the 

 vessels is conveyed along the vems or ribs of the leaves, which 

 gradually get smaller and smaller and finer and finer. Ulti- 

 mately by osmosis the water passes into the spongy tissue 

 of the leaf and then into the walls lining the spaces between the 

 cells of the spongy tissue. These spaces thus become extremely 

 humid and damp, and if the surrounding atmosphere is not 

 saturated the watery vapour passes out by diffusion through 

 the mouths of the stomata into the surrounding air. This 

 process of getting rid of the water which was in the first case 

 taken up by the root-hairs is known 

 as trmispiration. 



The number of stomata is astound- 

 ing. They are for the most part placed 

 on the under surface of the leaf, except 

 in floating leaves such as the water- 

 lilies, where they are found only on the 

 upper surface. They are extremely 

 minute, the sioindle-shaped opening 

 being in the field spurry, Spergula 

 arvensis, 0-007 mm. in diameter, 

 whereas in some kinds of lilies, Ama- 

 ryllis, they are as wide as 0-02 mm. 



A typical sunflower-leaf may have fig. 16. Three stomata ^dth 

 as many as 18,000,000 of these Sto- surrounding epidermic cells 

 mata and the amount of fluid Avhich (E); G, G, guard cells of a 



4-1, u 4-1 • stoma. 



passes through them is very con- 

 siderable. It has been mentioned above that an elm tree 

 may have 7,000,000 leaves whose surface areas when added 

 together may amount to some five acres. A single birch 

 tree with some 200,000 leaves mil pour into the atmosphere 

 15 J gallons of water on an ordinary day, and on a very 

 hot, dry day as much as 85 gallons. A sunflower with a 

 leaf- area of 5616 square inches loses a pint and a half in 12 

 hours, and it has been estimated that a beech forest evapor- 

 ates about 14,000 tons of water per acre during the summer 

 months. An average acre of wheat will from start to finish 

 give off during the life-time of the plant some 1000 tons of 

 water. Certain Danish investigators have calculated that the 



