9MI 
As the operation of the different physical agents 
upon the sap vessels of plants ceases, and the fluid 
becomes quiescent, the materials dissolved in it by 
heat are deposited in the cells of the alburnum ; 
and in consequence of this deposition, a nutritive 
matter is provided for the first wants of the plant 
in early spring, to assist the opening of the buds* 
and their expansion, when the motion from the 
want of leaves is as yet feeble. 
This beautiful principle in the vegetable economy 
was first pointed out by Dr. Darwin ; and Mr. 
Knight has given a number of experimental eluci- 
dations of it. 
Mr. Knight made numerous incisions into the 
alburnum of the sycamore and the birch, at dif- 
ferent heights ; and in examining the sap that 
flowed from them, he found it more sweet and 
mucilaginous in proportion as the aperture from 
which it flowed was elevated ; which he could 
ascribe to no other cause than to its having dis- 
solved sugar and mucilage, which had been stored 
up through the winter. 
He examined the alburnum in different poles of 
oak in the same forest; of which some had been 
felled in winter, and others in summer; and he 
always found most soluble matter in the wood felled 
in winter, and its specific gravity was likewise 
greater. 
In all perennial trees this circumstance takes 
place ; and likewise in grasses and shrubs. The 
joints of the perennial grasses contain more sac- 
charine and mucilaginous matter in winter than at 
any other season ; and this is the reason why the 
