72 NUTRITION IN VEGETABLES. 
matter which has been absorbed transformed into muci 
and sugar, and a large proportion of water. 
253. The sap is found in the woody part of the stem 
through which it begins to ascend towards the branches 
early in spring. At this period it may be easily collected by 
piercing the woody parts in the Vine, Birch, or Maple, when 
it will flow out in great abundance. About a pint of sap 
may be obtained in the course of twenty-four hours 
from a divided branch of the vine. It rises or falls’ 
' the vessels according to the temperature of the atm 
phere, heat distending the vessels, and promoting the 
and vigorous circulation of the sap. By promoting 
flow of the sap, heat is favourable to the growth of plants, 
and their vegetation is often accelerated by the application 
of artificial heat, wliich is called forcing. The greater size, 
and quicker end more vigorous growth of vegetables in tro- 
pical climates, is owing partly to this cause: and ai 
the influence of light, to which they are more exposed in 
countries. Ifa branch of a vine, or any tree be introdu 
intoa hot-house, while the root and the remainder of th 
tree are outside, soon the branch within will shew signs ° 
far more vigorous vegetation than the parts without. 
254, When the buds are developed and have put forth 
their leaves, they cause the sap to flow more equally 
_ steadily, by constantly consuming what is presented to them 
and if the wood be now pierced, little or no effusion of sap 
_ takes place, the leaves drawing it towards them as it is re 
quired, and removing large quantities of the watery we by 
evaporation. 2 
255. The cause of the ascent of the sap has not been W 
ascertained. It has been referred to the action of heat, t 
capillary attraction, to a vital contraction in the cells or i 
vessels, to Endosmose, and to Electric influence. 
‘abs. Grew considered the ascent of the sap to depend on 
‘the play of the cells, which, when full, by their pressure 
> vessel caused the sap to ascend, but he does not 
2 age Naer slot cca a 
