THE SAP OF TREES 
189 
movement of moisture in a tree, though there is unmistakable evidence of a 
movement downwards. One very striking fact in evidence may certainly be had 
by anybody who will take the trouble to bind a ligature so tightly round the stem 
of a young tree as to obstruct the flow of sap. If the movement of the fluid were 
upwards, from the roots, it would of course be natural to expect that the stem of 
the tree would swell out beneath the ligature. As a matter of fact, it will invari- 
ably bulge out above the obstruction. A few years ago the writer, who had no 
theory of his own on the subject, happened to twist a bit of wire round the stem 
of a young willow tree in his garden and left it there, and thought no more about 
it until a year or two afterwards, when his attention was drawn to the fact that 
the wire was embedded in the bark and the tree was being strangled. He set 
about to remove the girdle, and was much puzzled to observe this very pheno- 
menon — the swelling of the trunk above the ligature instead of underneath it. 
How could that possibly be, with the sap flowing upwards ? he asked himself ; 
but he found the riddle quite insoluble, and gave it up. Not a great while after- 
wards he met with a small book describing a great number of experiments with 
growing trees and plants designed to show which way the sap actually did move. 
Bands of various kinds had been placed tightly round trees and shrubs and sticks 
cf rhubarb, and so on, and with the invariable result that the writer had casually 
produced. The swelling was always on the upper side. Moreover, the author 
of the book, a Mr. A. P. Reeves, asserted that “ three remarkable effects are 
produced in a tree by the removal of a ring of bark from the trunk. First, the 
sap will exude from the upper lip of the wound, and not from the lower. Second, 
the roots and that part of the tree below the ring will dry and die first. Third, 
the part above the ring will thrive for a few years and then die.” Assuming this 
to be a fact, it is obvious that if the rootlets of a tree are the sources of its sap, 
and if the progress of this vital fluid is from the ground upwards, we have here a 
threefold riddle not at all easy to solve. 
The theory propounded by the author was that, as a matter of fact, sap is not 
produced by the roots of a plant, and it does not move upwards. It is elaborated 
by the foliage from the atmosphere and the rain and the dew, and instead of being 
sucked up or pumped up in some way unknown to science, it percolates down- 
wards by the simple action of gravitation. All the moisture requisite for the 
composition of a tree enters by the leaves, “ and by the common law of gravitation 
travels slowly downwards between the bark and the alburnum, where all the pure 
nutrient sap is found.” In the warmth of summer, according to this theory, this 
nutrient sap is fluid, and nourishes and expands the tree as it percolates down- 
wards, while in the autumn the sap is merely congealed, and does not fall at all. 
It remains hard and adhesive between the bark and the alburnum. The function 
of the roots, it is explained, is not to elaborate sap, but to absorb the gaseous 
products of decomposing matter, and moisture about the roots of a plant merely 
accelerates decomposition and fits the rootlets for the absorption of the gases 
which result from it, and which ascend through the vertical cells just as naturally 
as the aqueous fluid comes down. Whatever may be the truth of the theory, it 
certainly disposes of all the difficulties indicated by Professor Huxley, and it most 
satisfactorily explains the otherwise inexplicable effect of a tight ligature round a 
growing plant. The truth of a scientific hypothesis is shown by its explaining all 
the phenomena, and this unquestionably does explain some of them. It is much 
easier to understand how a tree may absorb the gases of decomposition at its roots 
and pass them upwards than it is to conceive of their being imbibed overhead and 
passed downwards, and it is similarly much easier to conceive of aqueous fluid 
being produced in the leaves above and transmitted downwards to the roots than 
it is to imagine the reverse. It is, at all events, a very interesting and a very 
seasonable subject for a little speculation and experiment for those who are that 
way inclined, and it is by no means wanting in practical importance either. For 
upon the supposed movements of the sap in trees and plants a good many impor- 
tant gardening operations are based, and any alteration of theory would necessarily 
involve corresponding alterations in practice. 
Mr. Millard’s comment is as follows : — 
“ I have had in my house for, I think, twenty years a plant 
of ivy trained on a sort of wire trellis over the fireplace. It 
