18 



which variation a reason may be found in the different conditions of growth to which 

 they are subjected in their several natural habitats. In the back of the leaf of the apple 

 tree, there are about twenty-four thousand stomates to the square inch. In the leaf of the 

 lilac there are a hundred and sixty thousand- of them to the square inch. Sixty thousand 

 have been reckoned in a square inch of the under surface of the white lily and three thousand 

 in a square inch of. the upper surface. In the leaves of the cherry -laurel there are none 

 on the upper surface of the leaf, but ninety thousand have been counted on the lower 

 surface of the leaf. In the true lilies they are so large that they may be seen with the 

 aid of a simple lens of an inch focus. In the water lilies and other plants having leaves 

 which float upon water, all the stomates are on the upper surface, where alone evapora- 

 tion can take place. Leaves of plants which grow entirely under water, where there can 

 be no evaporation, have none." 



The quantity of water drawn up from the soil by the roots is very great. It is not 

 well known how much passes back to the roots, or how much passes through the leaves 

 into the air, but all experiments show that very much more passes off through the leavea 

 into the air, than runs back through the roots into the ground again, as is shewn above. 

 The provision made for evaporation when nepessary, and for absorption when necessary, 

 in leaves is immense. In the leaf of the lilac for instance, as elsewhere noticed, there are 

 one hundred and sixty thousand openings for the purpose to the square inch. By these, 

 when the tree needs it, it throws off ; by them, when it needs, it obtains moisture from 

 the air again. It may be well here to say that, as regards the amount of water absorbed, 

 in case of need, by leaves, science has not as yet been able to give us such clear proof as 

 it has 'furnished concerning the amount given out by them. The last is certainly immense. 

 The former exists ; but is as yet unmeasured. In some trees the upward rush of mois- 

 ture from the roots is very powerful. The workmen in shipyards frequently find in the 

 centre of a teak log a core of sand fifty or sixty feet long, an inch in diameter, and 

 hardened to a marble-like consistency, which has been carried and deposited there by the 

 sap in its upward course. One main conclusion, we will, for our purpose at present, 

 notice — that the volume of moisture passing into the air from the leaves of trees must be 

 extremely large. 



How Moisture is Retained in Forests. 



The whole forest, in its natural state, forms a reservoir admirably fitted to receive- 

 large supplies of moisture, to hold it for a lengthened time, and to part with it at inter- 

 vals well calculated to benefit the vegetation of the surrounding country. The bed of the 

 forest is a widely spread surface, piled thick with leaves, twigs, pieces of fallen branches 

 and remnants of decayed logs, covering another layer of the same substances, in a state 

 of partial decomposition, overlying yet another strata completely decomposed, altogether 

 forming a deep porous hollow framework, penetrated with a myriad of pipes, tubes, and 

 aqueducts, and interpersed with millions of miniature cisterns. Then, every hollow on 

 the surface is obstructed by fallen and rotting logs, blocking and holding in positioa 

 the flow of water until the humus below fully absorb it, while the whole surface of the 

 earth is crossed, recrossed and crossed again by a chequer-work of partially elevated 

 roots, the box-like openings between wlij^oh perform the same function. If we go below- 



