20 PLANTING. 



vary from , part of the atmosphere, being "-renter as tlie weather 



is cfry and liot, at which time it is most useful to the growth and health of 

 plants, being absorbed by the leaves *. It is clear that water constitutes 

 immeasurably the largest portion of what is taken up by the roots and 

 furnished to the plant by the soil ; and when it is considered that water 

 is composed of oxygen and hydrogen, it cannot be supposed to act merely 

 as a vehicle of the food of the tree; it contributes, probably, to the in- 

 crease of the solid parts of the living structure by decomposition into its 

 elements, through the agency of the vital powers. 



Such are the general facts disclosed by chemical examinations of the 

 soil and atmospherical air, with respect to the substances supplied by them 

 to plants as food. An analysis of the sap itself immediately after its 

 absorption by the spongeols of the rootlets, and before it enters the 

 ascending vessels of the alburnum, t would probably leave nothing more to 

 be desired on this important subject, that might apply to the operations of 

 the practical planter. The sap hitherto examined chemically, has been taken 

 from the alburnum of the tree, and consequently after it had undergone a 

 change in its original constitution, or that which characterised it at the mo- 

 ment of its entering the spongeols of the rootlets immediately from the soil. 

 That the sap undergoes a change in the ascending vessels of the alburnum 

 before it is acted upon by the leaves, has been proved by Knight and others. 

 In these instances, the sap extracted from the lower part of the tree, con- 

 tained much less saccharine matter, than that taken from a more elevated 

 part of the stem. According to Vauqueliri, water, extract, mucilage, sugar, 

 and acetic acid, combined with potash or lime, are found in sap taken from 

 the alburnum or ascending sap vessels of the birch, elm, and beech; but 

 these vary in the sap of different species of trees. Saccharine matter is most 

 abundant in the birch and sugar maple. These results, however, alibrd but 

 little light in the investigation of the question, as we know that the same 

 sap which produces the acid, astringent crab, produces also the saccharine, 

 aromatic pippin. By the action of heat, light, air, and the peculiar 

 organic structure in different species of trees, under the influence of the 

 vital power, are those substances which are soluble in water, or saccharine 

 and mucilaginous fluids converted into insoluble or resinous and oily 

 substances. 



* The valiu- of valour iu uir to the health of plants, is well known ami appreciated hy 



. ,1 cultivator of tropical plants in an artificial atmosphere, as \\ell as hy the 

 UCCettful forcing IVuit and flower rardeneX in the hothouse. Plants are enahle'l \>v 



ir in the air to withstand the effect! of extreme heat and drought, which othei 

 would d.-troy the organi/.ation of the leaves. "We ourselves have found the leaves of the 

 province rose, when in an artificial atmosphere, at an early season (and when its vital 

 powers could not he so strongly exerted, as when under the circumstances of its natural 

 ;i of trowth and exposure) to unfold and increase in healthy growth when subjected 

 for a certain time, eacli day for the space of a fortnight, to hot air strongly charged with 

 'tr, while leaves of the same species did not uni'old, or when unfolded previously to 

 fhe application, shrivelled up and perished under the application of a dry current of air, of 

 the like temperature, and though all other circumstances were eipul. 



f Th' 1 rapid communication which exists between the spongeols of the rootlets and the 



. emity of the tree, as evidenced by the sudden eliects produced on the latter 



hy the application of water t.> the roots of a tree whose !ea\es have become flaccid or 



: ing from the want of it, warrants the idea that the ascent of fluids from the roots to 



the ! ct than our knowledge of the structure of the vessels will allow, or 



thut a principle e\i.-,ts in the vegetable structure analogous to that of the irritability of tin; 



iiniiiri'i fibre. The well known experiments of Ila .tin the force with which the 



sap of treei ascend:,, shew that the sap of a vine branch four or five years old rises with a 



''.erably superior to atmospherical pressure. Plants having the leaves firm and 



'.nbite. 1 proportionally less force in their ascending sap. Vide / '<-yf-Uiblc Stali< *, 



vol. 1., p. 114. 



