560 Cultivation of Plants. 
transpiration, especially when the atmosphere is very dry. To 
keep up a corresponding supply there must be free and rapid 
circulation, which is the case in the Vine, as everybody knows 
who has cut an active growing cane, and observed the copious 
discharge of sap. A transverse section of the Vine will show 
these vessels even without the aid of a lens. 
The chief substance useful to man furnished by the sap of 
plants is sugar, which is produced by such diverse plants as the 
Sugar-cane, Beet, Sugar Maple, and many Palms, etc. 
As we have previously observed, the leaves and other green 
parts of plants are also organs of absorption ; but their action 
is limited to the imbibition of aqueous vapour and the gases 
contained in the air, principally oxygen, carbonic acid, and 
ammoniacal gases. 
3. Respiration.—The respiration of plants is a very compli- 
cated function, having its seat in all the green parts, but more 
especially in the leaves. It results in the elaboration or assi- 
milation of the sap, which through this agency becomes avail- 
able for building up and nourishing the vegetable fabric. 
Looked upon from this point of view, then, the leaves are 
analogous to the lungs of animals, as those organs form the 
principal laboratory for the production and purification of the 
blood. We shall see that light exercises considerable influence 
over this important function. 
When the ascending sap reaches the leaves and fills their 
cells, it is rapidly concentrated by the evaporation or transpira- 
tion of its surplus water. This water, which is no longer 
separated from the outward air except by the thin cell-walls, 
penetrates them and is dispersed in the atmosphere in the 
form of invisible vapour, to which the stomates give issue. 
The drier the air, the greater the transpiration from the leaves, 
and the greater the consequent tax upon the roots to keep up 
an adequate supply by absorption. This is one of the most 
important points connected with economy of plant-life for the 
consideration of the practical gardener. The principal cause 
of loss by death of newly transplanted trees and shrubs, espe- 
cially if removed, as in the case of evergreens, in full fcliage, 
or shortly before deciduous species put forth their leaves, is 
due to the inability of the roots to maintain the necessary 
supply of water to compensate for that transpired, causing the 
tissues of the foliage to shrivel and ultimately become unable 
tu perform the operation, This will frequently happen when 
