SUN RAYS—ABBOT 163 
that, if sunlight was exceedingly bright on a hot summer day, the 
evaporation of water could be so great that the chemical products 
left behind would exceed the requirements of the plant, and kill it 
by overfeeding. Against such a possibility perhaps the stomata 
might need to be partially closed. On the other hand, if the air was 
very free from clouds and moisture, and a strong cool breeze blow- 
ing, the plant might become chilled by excessive evaporation unless 
the stomata were partly closed. There are, at all events, automatic 
devices within the leaf mechanism which attend to this needful 
regulation of the stomata. Beside these regulating devices, the 
leaves themselves, under the influence of changing sunlight, turn 
face toward or edge toward the sun according to the plants’ 
requirements. 
If the penetration of gases through the stomata in such profusion 
was a great puzzle, the ascent of the sap is, perhaps, even more ex- 
traordinary. For imagine a forest of gigantic Eucalyptus trees, 
which sometimes reach heights of 500 feet, and conceive of the en- 
ergy demanded to lift in a single summer hundreds of thousands 
of tons of water on each square mile from the ground to the leafy 
tops. A common vacuum pump, it is well known, can not lft water 
above 33 feet, so that we dismiss at once the thought that the air 
pressure is working for the trees. What form of energy and appli- 
cation of force are these which the tree commands to do this lifting? 
The energy is the heat of sun rays, and the forces at work are 
the capillary attraction and surface tension of water. By means of 
the capillary tubelike network of cells, which run from the roots up 
through the trunk of the tree, there is formed a connection between 
the stomata of the leaves and the water of the ground. These enor- 
mously numerous capillary passages are filled with fluid, partly li- 
quid, partly gaseous. At their orifices, which are the stomata of the 
leaves and twigs, the sun’s heat produces a continuous evaporation 
of pure water, leaving behind in the tree the traces of chemicals 
which the soil furnishes with the water and which yield plant food. 
We seldom think of the forces of capillarity and surface tension 
which come into play, though they are the same that raise kerosene 
oil in a lamp wick, and that make drops of oil spread over a wet 
pavement. These forces are limited in their action to distances far 
less than the thickness of a single sheet of tissue paper, but are ex- 
tremely powerful in circumstances where they are at their maximum 
strength. For instance, a single drop of water introduced between 
two flat glasses slightly inclined to each other, will run rapidly to 
the narrowest spaces, and will draw the two plates together so 
strongly as even to bruise or crush the glass. Similarly, two blocks 
of ice placed loosely together, and so that the water which melts 
from them can drain away, will be drawn together by the remaining 
