VEGETABLE PHYSIOLOGY. 
387 
rally through the cellular tissue, and this with great rapidity, as will 
be apparent upon placing a branch in a coloured infusion, which will 
ascend and descend in the manner just stated ; and will also disperse 
itself laterally in all directions round the principal channels of its 
upward and downward route. 
“ With regard to the vessels through which this universal diffusion 
of the sap takes place, it has already been stated that its upward course 
is always through the woody fibre, and probably also through the ducts; 
and that it passes downwards through the woody fibre. But there can 
be no reasonable doubt that it is also dispersed through the whole 
system, by means of some permeable quality of the membranes of the 
cellular tissue, which is invisible to our eyes, even aided by the most 
powerful glasses. It has also been suggested that the sap finds its 
way upwards, downwards, and laterally through the intercellular pas¬ 
sages, which exist at the points of union of every individual elementary 
organ. That such a channel of communicating the sap is employed by 
nature to a certain extent I do not doubt, especially in those plants in 
which the intercellular passages are large ; but whether this be an 
universal law, or has only a partial operation, is quite unknown, and is 
not, perhaps, susceptible of absolute proof. 
“ The accumulation of sap in plants appears to be attended with 
very beneficial consequences, and to be deserving of the especial atten¬ 
tion of gardeners. It is well known how weak and imperfect is the 
inflorescence of the turnip tribe, forced to flower before their fleshy 
root is formed, and how vigorous it is after that reservoir of accumu¬ 
lated sap is completed. Mr. Knight, in a valuable paper on this sub¬ 
ject, remarks, that the fruit of melons, which sets upon the plant when 
very young, uniformly falls off; while, if not allowed to set until the 
stem is well formed, and much sap accumulated for its support, it 
swells rapidly, and ripens well. In like manner, if a tree is by any 
circumstance prevented bearing its crop one year, the sap that would 
have been expended accumulates, and powerfully contributes to the 
abundance and perfection of the fruit of the succeeding year. 
“ The course of the motion of the sap is a subject which has long 
excited great curiosity, and has given rise to numberless conjectures. 
It was for a long time believed that there was a sort of circulation of 
the sap of plants to and from a certain point, analogous to that of the 
blood of animals; but this was disproved by Hales, and is not now 
believed. This excellent observer thought that the motion of the sap 
(the rapidity of which he had found to be greatly influenced by the 
weather) depended upon the contraction and expansion of the air, 
