172 ASCENT OF SAP. AMOUNT OF FLUID EXHALED. 



governed by that of the leaves, the former organs ceasing to 

 absorb the fluid, when it is not drawn off by the latter. This 

 connexion is not only shown by the experiment just mentioned, 

 but by a still more remarkable one, which explains in great 

 degree the cause of the ascent of the sap in the spring, after it 

 has been nearly stationary during the winter. If a Vine be 

 growing on the outside of a hot-house, and a single shoot be 

 trained within, in the midst of winter, the warmth to which the 

 latter is exposed, will cause its buds to swell and unfold them- 

 selves ; whilst those on the outside are quite inactive. A de- 

 mand for fluid will thus be occasioned along this particular 

 branch ; and this will be supplied by that existing in the vessels 

 below. When these are emptied, they will be again supplied 

 from the parts .below them ; and thus the motion will be propa- 

 gated to that division of the roots, whose fibres are connected 

 with those of the vegetating branch ; these will absorb fluid for 

 its support, whilst all the rest are completely at rest. In the 

 spring of the year, when the cheerful rays of the sun call the 

 whole of the buds into activity, the whole of the roots are 

 similarly affected ; and that the sap begins to move in the 

 upper branches, before it commences ascending in the trunk, has 

 been shown by experiment, notches having been cut at inter- 

 vals, by which the period of its flow could be ascertained in 

 each part. 



261. Various experiments have been made at different times, 

 to ascertain the quantity of fluid thus exhaled by plants ; and 

 the results of many of them are very interesting. Those made 

 by Dr. Woodward, 150 years ago, have been already noticed 

 (. 100), as indicating the large quantity of water absorbed. 

 There is no great difficulty in ascertaining the amount upon a 

 small scale ; for if a plant be supplied with a known weight of 

 water, and the weight it has gained during a certain time be 

 deducted from thi?, allowance being also made for the evapo- 

 ration from the surface of the water, in which its roots are im- 

 mersed, the quantity of which may be easily estimated, the 

 difference must be the proportion exhaled. This differs much 

 in different plants, chiefly according to the rapidity of their 

 growth. 



