17 



■quantity of water, containing in solution the nutriment for the tree. Arrived at the 

 leaves, a process takes place which separates, by means of contact with the air, most of 

 the water the roots had taken in, from the valuable nutriment, and throws off, in vapour, 

 the surplus water into the air. At this same time certain constituent portions of the air 

 are utilized and mingled with the nourishment retained. This all, now a small por- 

 tion in comparison with what had arisen from the roots, yet retaining enough water to 

 serve as its vehicle back, is returned towards the roots, depositing in its way, in leaf, bark 

 and root, what is needed there for the growth of the tree. In these they undergo, 

 especially in the bark, further fitting and digesting processes, before they assimilate with 

 the substance of the tree. The water which was retained to carry them down, being now 

 needed no longer, passes out at the roots. 



If the reader choose to peruse the three following paragraphs, he will find from the 

 pen of the learned Mr. Brown, a more scientific and exact description of this process : — - 



" The water thus absorbed by the several cells composing the spongioles of the root 

 is by a similar process absorbed- from them by cells behind them ; and by continuous 

 repetition of it by those beyond the moisture absorbed from the soil is passed on and on, 

 from the extremities of the rootlet to the extremities of the smallest twigs, and to the 

 furthest and the loftiest extremities of the branches of the trunk. There, through the 

 leaves, a part, and that a large portion of it, is given off into the atmosphere, while a 

 part, comparatively a small portion, is returned by the same duplex process of exosmose 

 and endosmose by the same cells, and others, their progeny, towards the root. By the 

 way is deposited, by exosmose, nutriment for the tree, the leaf, the flower and the fruit ; 

 and the residuum is in part deposited by the same process in the leaves, the bark, 

 ■or the root, and passed ofi" into the soil by the exosmosic action of the cells composing the 

 spongioles of the root. " 



" In the back of the leaf are numerous stomates, or mouths. The structure of these 

 •difiers in different plants, but what may be considered the typical structure is two elon- 

 gated cells, resembling a microscopic black pudding or thick sausage, so built into the 

 ■structure of the skin of the leaf, that this will not admit of their being further elongated ; 

 each of these is, along one side, attached to that skin, but on the sides along which they 

 are in contact they are free. When moisture is in excess, they become distended, but 

 the structure of the skin of the leaves is such that they cannot be elongated, and they 

 bulge away from each other, leaving a wide opening between them through which the 

 vapour with which the air surrounding the cells in the interior of the leaf is charged, 

 finds an open exit. When the pressure is relieved, they, having lost some of the moisture 

 or water with which they were filled and distended, collapse to such an extent as to dimin- 

 ish the opening ; and in this way, exactly to the degree required, they vary and regulate 

 that aperture — varying it, it may be, I shall not say twenty times in the day, but, if 

 necessary, twenty times in the minute ; and if drought become such as to render it desira- 

 ble that every drop of moisture in the plant should be preserved, under the influence of 

 that drought they become flaccid and completely close the aperture." 



" Of the extent of the provision made for this evaporation some idea may be formed 

 from a consideration of the number of the stomata or stomates to be found in the leaves 

 -of plants, often symmetrically disposed. The number varies in different plants, for 

 2 



