THE LEAF 209 



great deal of the water in the plant escapes as vapour. In a 

 lesser degree transpiration also takes place through the walls of 

 the cells o^ the outer skin, called epidermis. 



(c) Importance of Transpiration. — This process is of the great- 

 est importance to the life of the plant. It is generally known 

 that plants suck up water and mineral food dissolved in water 

 hy means of their roots. This liquid food is carried up through 

 the stem and the branches to the leaves, where, as we shall see 

 later on, certain materials, which the plant requires for its growth, 

 are formed. By a simple experiment we are able to find out 

 which way the water-current takes in. a leaf: we place a white 

 flower, e. g., a Eucharis Lily, with its stalk in water stained red 

 with eosin. In a short time the red liquid will be seen rising 

 in the white petals along the veins. 



All the water that comes up is not required for the growth 

 of the plant. When it has done its work as carrier of the mineral 

 food from the soil to the leaves, it is passed off as vapour, making 

 room for further supplies from the soil. The transpiration of 

 water from the leaves thus ads like a suction pump: it is always 

 draiuing tip fresh supplies of water and food from the roots. 



(d) Amount of Transpiration. — We can ascertain the quantity 

 of water evaporated by a certain plant in a given time, by means 

 of a simple experiment. We put the stem of a twig with leaves 

 into a tumbler of water, cover the surface of the water with a 

 coat of oil and place the whole on a balance. After a few hours 

 we shall notice a considerable loss of weight, due only to the 

 evaporation of water through the leaves of the plant. Thus, 

 for instance, it was found out that a Sunflower plant gives 

 off a quart of fluid in twenty-four hours. Some large trees give 

 off a thousand pounds of water in a single hot day. If the 

 height of such a tree be a hundred feet, the work done by that 

 tree amounts to the same as that of carrying some three hundred 

 ordinary buckets of water up a stair ten feet high. A piece 

 of land covered with trees brings an enormous amount of 

 water up from the depth of the earth to the atmosphere. From 

 this we may now easily understand, how important woods are 

 for the fertility of a country, and how disastrous it is to destroy 



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