144 



BOTANY FOR BEGINNERS 



CHAP. 



(2) The drier the air the more rapid the transpiration ; this 

 is shown by noting how soon a plant withers on a very dry day, 

 and the fresh appearance of a plant on a damp, foggy day. 



(3) A windy day is favourable to transpiration. If a plant is 

 placed where there is a draught it fades more quickly than if 

 placed where the air is still. 



Why Plants Transpire. The effects of transpiration on 

 the economy of plants are very important and far-reaching. 

 These effects may be summarised as follows : 



(1) Transpiration is the principal way in which plants get rid 

 of the excess of water taken in by the roots. The solution 

 absorbed by the roots from the soil only contains a small 

 quantity of dissolved salts ; but since very large quantities of the 

 weak solution are absorbed, the parts of the plant where growth 

 is going on still obtain sufficient mineral matter. 



(2) Transpiration plays a very important part in the distribu- 

 tion of salts throughout the plant. The water vapour which is 

 given out through the stomata and lenticels causes a current 

 of sap to be set up from the roots to the leaves, and other parts 



of the plant. The salts are distri- 

 buted by the ascending current 

 to all parts of the plant where 

 they are required. 



(3) Transpiration aids the ab- 

 sorption of water and salts by 

 the roots. If a plant is growing 

 in a water culture solution, and 

 is covered by a bell-jar, the 

 transpiration is continuously 

 reduced, until at last it stops 

 altogether. If the bell-jar is 

 removed, transpiration increases 

 again, because the water vapour 

 can now escape. 



Liquid Water Given out 

 by Plants. If the leaves of 

 Grasses, Buttercup, Strawberry, Lady's Mantle, and most other 

 plants be examined on a summer morning, drops of glistening 

 water will be seen to hang from them. For a long time it was 

 thought that these drops had been deposited on the leaves from 



FIG. 159. Leaf of Nas- 

 turtium giving out 

 clops of water. (S.) 



