164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
water so closely that the two ice blocks become united into one by 
cohering together. ‘This process is called regelation. On the con- 
trary, the fluid, mercury, which does not wet glass, will, on account 
of surface tension escape from between two glasses, even if by doing 
so it must create a vacuum gehind. 
The rise of water and other liquids 1 in very fine tubes is a conse- 
quence of surface tension, which in this connection is often called 
capillary force. ‘The heights to which a column of liquid will rise in 
a tube which it wets is inversely proportional to the diameter of 
the tube. The extreme fineness of the porous structure of the trunks, 
twigs, and leaves of trees, therefore, is adapted to convey the liquids 
imbibed by the roots to very great heights. The evaporation of 
water from the stomata of the leaves and twigs makes place for the 
continuous renewal of liquid by capillary action from below. This 
upward current is conveyed by the interior part of the tree stem. 
As it has been observed to reach great heights in dead trees, we must 
adopt some such physical explanation as has been given, and not 
invent a mysterious “life force” for the purpose, as older botanists 
were prone to do. 
The soft, live, outer part of the plant, just within the bark, has 
another function. It is to carry downwards to the extremities of 
the roots the chemical products built up im the leaves and green 
parts under the action of sunlight. 
Thus, in a live plant, as in a live animal, there is a fluid circulation. 
The manner of it, to be sure, is exceedingly different. Instead of the 
force pump, which we call the heart, there is substituted in the 
plant the force of capillary action, lifting the watery fluid to the 
tops of majestic trees. It brings, dissolved, the chemical plant foods 
from the ground, and so feeds the trees. ‘The return current, much 
less in volume, is probably maintained by still another modification 
of surface tension which we call osmotic pressure. This is a force, 
which often greatly exceeds atmospheric pressure. It is always ex- 
isting between watery solutions of chemical substances in different 
concentrations, tending to drive the more concentrated solution into 
the one less concentrated. Thus, the force of osmotic pressure tends 
to produce a uniform mixture. In a tree, it takes the more concen- 
trated products of solar chemistry from the laboratory of the leaves, 
and conveys them downward through the living layers under the 
bark to the roots, to nourish these, and to be laid up, beyond the in- 
fluence of wintry frost, for the renewal of the leaves in the spring. 
The reader should not conclude from what has been said that the 
life processes of a tree are wholly understood. On the contrary, 
the best informed plant physiologists admit that they are confronted 
by a maze of mysteries which become more bewildering with addi- 
tional research. They are becoming convinced that the simple proto- 
