CAUSES OF THE ASCENT OF THE SAP. 
Gentlemen intending to take part in the Congress should apply to Dr. Maxwell Masters* 
the Honorary Secretary, at the Office of the Exhibition, 1, William Street, Lowndes 
Square, S.W. 
CAUSES OF THE ASCENT OF THE SAP. 
The causes of the ascent of the sap have long been a subject of controversy, and it 
can by no means be stated that we have even now arrived at any perfectly satisfactory 
conclusions in the matter. Endosmose, capillarity, and the profuse evaporation which 
takes place from the leaves, have all been considered as potent agents in producing the 
upward flow, but even those who have the most insisted on the power of these agencies 
have admitted that they were not sufficient to explain all the phenomena. 
Professor Unger and M. Boehm, in a paper in the Transactions of the Imperial 
Academy of Vienna, conclude that the rise of the sap is brought about by atmospheric 
pressure on the elastic walls of the cells. Air enters into the plant, and is distributed 
throughout its structure by means of the vessels, the air so introduced compressing the 
cells, and thus pushing their liquid contents upwards. 
Quite recently, too, Mr. Herbert Spencer has brought before the Linnean Society his 
views on the ascent of the sap and the formation of wood in plants. Mr. Spencer’s 
paper excited so much attention, and gave rise to so animated a discussion, that we feel 
pleased to lay before our readers a brief abstract of the inferences drawn from his ob¬ 
servations and experiments, and which has been kindly furnished to us by Mr. Spencer 
himself:— 
“ The leading idea in my paper is that the oscillating movements of the stems, 
branches, twigs, and petioles of plants are largely, if not indeed mainly instrumental, 
both in producing circulation, and in causing the deposit of woody matter. TV hen any 
part of a plant is bent by the wind, the tissues on its convex surface are subject to 
longitudinal tension, and these extended outer layers compress the layers beneath them. 
Such of the vessels or canals in these subjacent layers as contain sap, must have some 
of this sap expelled. Part of it will be squeezed through the more or less porous walls 
of the canals into the surrounding tissue, thus supplying it with assimilable materials; 
while part of it, and probably the larger part, will be thrust along the canals longi¬ 
tudinally upwards and downwards. When the branch, or twig, or leaf-stalk recoils, 
these vessels, relieved from pressure, expand to their original diameters. As they expand, 
the sap rushes back into them from above and below. In whichever of these directions 
least has been expelled by the compression, from that direction most must return during 
the dilatation; seeing that the force which more efficiently resisted the thrusting back 
of the sap is the same force which urges it into the expanded vessels again, when they 
are relieved from pressure. At the next bend of the part a further portion of^ sap will 
be squeezed out, and a further portion thrust forwards along the vessels. This rude 
pumping process thus serves for propelling the sap to heights which it could not reach 
by capillary action; at the same time that it incidentally serves to feed the parts in 
which it takes place. It strengthens them, too, just in proportion to the stress to be 
borne; since the more severe and the more repeated the strains, the greater must be the 
exudation of sap from the vessels or ducts into the surrounding tissue, and the greater 
the thickening of this tissue by secondary deposits. A further part of my argument is 
that by this same action the movement of the sap is determined either upwards or dovvn- 
wards according to the conditions. While the leaves are active and evaporation is going 
on from them, these oscillations of the branches and petioles urge forward the sap into 
them ; because so long as the vessels of the leaves are being emptied, the sap in the com¬ 
pressed vessels of the oscillating parts will meet with less resistance in the direction of 
the leaves than in the opposite direction. But when evaporation ceases at night, this 
will no longer be the case. The sap drawn to the oscillating parts to supply the place 
of the exuded sap, must come from the directions of least resistance. A slight breeze 
will bring it back from the leaves into the gently-swaying twigs, a stronger breeze into 
the bending branches, a gale into the strained stem and roots—roots in which longi¬ 
tudinal tension produces, in another way, the same effects that transverse tension does 
in the branches.” 
