180 ELEMENTS OF STRUCTURAL BOTANY. 



313. Transpiration. The openings in the epidermis, 

 called stomata, have already been described. Through 

 these the excess of water-vapour in the plant is exhaled, 

 it may often be observed, in hot, bright weather, that the 

 leaves of plants droop if exposed to the sun. This is 

 because the rate of evaporation through the stomata is 

 greater than the rate of supply at the roots. At night, 

 however, the stomata close and the balance being restored 

 the plant recovers. The water which is thus supplied to 

 the leaves appears to be conveyed through the stem by 

 means of the cell-walls of the wood-prosenchyma, since the 

 supply is not diminished if a ring of bark and the under- 

 lying bast and cambium be removed. 



314. But water is also supplied to the growing points, 

 and in a different way. It is well known that if two 

 liquids (or gases) of different density are separated by a 

 porous diaphragm they will tend to change places, the 

 fluid of less density passing through the diaphragm more 

 rapidly than the other. This is the principle of osmose, 

 and wherever in a plant a cell-wall separates cell-contents 

 of different density it is found to apply. Hence, water is 

 absorbed by freshly-formed cells, containing dense proto- 

 plasm, from neighboring cells which are a little older and 

 in which the protoplasm has been diluted. These absorb 

 from the older cells behind them, and so on. Such water 

 is transmitted, not-through the prosenchyma of the wood, 

 but through the parenchyma and the meristem. 



315. It is a matter of common observation that the 

 stems of many plants " bleed " if cut in the spring. This 

 is due to the upward pressure of the water with which the 

 roots are charged at that time, and it takes place in the 



