THE SAP OF TREES. 
HE Rev. F. M. Millard, of Otham Parsonage, Maidstone, 
sends us an article on this subject from a spring 
number of the Evening Standard , 'with some diffi- 
culties of his own. To make the matter clear, we 
must quote the article in extenso, only premising that whilst 
the difficulties of the subject are fairly stated in the first of 
its three paragraphs, the constructive part of the essay involves 
many confusions of terms, misstatements of facts, and consequent 
wholly erroneous conclusions. The article is as follows : — 
This is the time of year when, according to a theory very generally accepted, 
though never very clearly explained, the sap of irees begins to rise. Rough winds 
shake the trees about, sunbeams warm their trunks, and heavy showers drench 
them with moisture. The moisture and the movement and the warmth all 
combine with the mysterious throbbing of spring life, and the sap begins to flow 
upwards until the autumn, when, gardeners are accustomed to say, it goes down, 
and then they may set about their transplanting and their pruning. On the face 
of it, the theory looks to be a very simple one, but when it comes to be examined 
it is not very easy to understand. “ That some sort of circulation of fluids must 
take place in the body of a plant appears to be certain,” says Professor Huxley, 
“ but the details of the process are by no means clear.” He proceeds to state 
the usual explanations of the phenomena of the upward movements of sap. 
Evaporation from the surfaces of the plant and leaves, he says, pulls up the fluid. 
The absorptive action of rootlets pushes up from below, while a certain portion of 
the sap is pumped up and exudes laterally from the side walls of the plant. Root- 
pressure, capillary attraction, “ endosmose,” “a kind of suction,” “imbibition,” 
“transpiration,” these are some of the terms employed to explain the movement 
that sap is supposed to make from the extremities of the roots to the topmost 
twigs and leaves of a plant or tree. As Professor Huxley has said, however, the 
details of the process are by no means clear, and when we come closely to 
scrutinise the terms employed and the phenomena implied in them, the explana- 
tion they' are intended to afford is not at all satisfactory. We have to account 
for the rising of a fluid, in the case of some trees, to a height of three or four 
hundred feet. It is suggested that this may be done by capillary attraction. But 
if we come practically to test the power of capillary attraction to raise a fluid 
even through such a substance as sponge or blotting-paper, we find that power 
limited to a few inches. Take, again, the assertion that sap is “pumped” up 
from the rootlets, or sucked up by evaporation. What can this pumping or 
sucking be? An atmospheric pump, we know, cannot possibly be made to suck 
up fluid to a height of more than thirty or thirty-two feet, while if we conceive 
it possible that the fibrous roots of a tree act as a force-pump to drive up the 
fluid, it is quite certain, having regard to well-known hydrostatic laws, that in 
order to drive the tiniest tube of sap three or four hundred feet, or even a 
hundred feet upwards, the force would have to be something very considerable. 
Many writers on the subject have recognised the inadequacy of any one of the 
suggested forces to perform the work of circulating sap in tall trees, but have 
supposed that it might be accounted for by the combination of all of them. But 
this will clearly not do, for the simple reason that the condition which would be 
necessary for the operation of one force would be fatal to another. A pumping 
process, for instance, would necessitate some sort of valve. As a matter of fact, 
there are no valves ; but, if there were, capillary attraction requires a perfectly 
free passage. They [the forces] cannot all combine to raise a current of sap 
hundreds of feet into the air, and there is no one of the suggested forces that can 
do it alone. 
The rather startling idea has been propounded that the notion of the rising 
of the sap in the spring and its going down in the autumn is all a delusion. It 
has been plausibly urged that there is no evidence whatever of any upward 
