August 9, 1918] 
under these conditions about 125 gallons of 
water in every twenty-four hours. If this 
is raised an average of 1 foot, it means the 
expenditure of 1,100 foot pounds of energy 
per day through the medium of the grass 
on the city lot. 
A birch tree standing in the open has 
been found to give off over 800 pounds of 
water per day. A man equipped with two 
ordinary water pails would have to make 
thirty-two trips in order to carry this 
amount of water. If he had steps up to the 
top of the tree and could make a round 
trip very ten minutes he would work over 
five hours per day to carry this amount of 
water. 
This evaporation is a large factor in rais- 
ing water to the tops of plants. Recent in- 
vestigation indicates that this molecular 
diffusion which we call transpiration exerts 
suction throughout the whole vascular sys- 
tem of the plant—leaves, branches, stem 
and roots. The contained water seems to 
be under tension even to the tips of the 
roots. 
Transpiration operates through osmosis 
in leaf cells and through the tensile strength 
of the water column in the conductive tubes 
of the plant to accomplish this. While the 
actual vaporization of this water is de- 
pendent solely upon the evaporation power 
of the air (i. e., temperature and relative 
humidity of the air actually in contact with 
the leaf cells) it is very probable that other 
forces arising from the presence of energy 
containing compounds in the plant are 
largely responsible for placing this water 
on the surface of the cells where the evapo- 
rating power of the air can act. 
I have now mentioned two ways in which 
the plant allows radiant energy to escape 
without making use of it, and one way in 
which it uses it. The fourth possible fate 
for this energy is in food synthesis by the 
plant. A very large amount of the radiant 
SCIENCE 
129 
energy received by the leaf is used in ma- 
king compounds of high energy content 
from compounds of low energy content. 
The basic process in this group of synthesis 
is the manufacture of sugar and starch 
from carbon dioxide and water. This proc- 
ess depends upon the eatalytie action of 
chlorophyll, the green coloring matter of 
plants. 
This is a reduction process resulting in 
the storing of energy in such a form that it 
may be released by oxidations, such as oceur 
in respiration, for the use of the plant. If 
not thus freed for the direct use of the 
plant this energy may remain stored in the 
form of coal or of hydrocarbon oils, and 
even ages after its capture by the plant 
may furnish heat for our homes or power 
to drive our railway trains or autos. More 
immediately it furnishes us through our 
food, all of which comes directly or indi- 
rectly from plants, the energy to perform 
work with our hands and our brains. The 
activities of the world are based on photo- 
synthesis—the reduction of carbon dioxide 
by the plant resulting in the formation of 
carbohydrates. 
This series of syntheses started in the for- 
mation of carbohydrates by the plant is im- 
portant in the more direct production of 
mechanical energy, as well as in the mat- 
ters just discussed. It is perhaps looking 
far into the future to consider a time when 
the world’s supply of coal and of hydro- 
carbon oils may be exhausted, but such a 
time may come. If we keep up our pres- 
ent rate of stripping the earth of its cloth- 
ing of forest trees, the supply of wood will 
not last forever and a constantly increas- 
ing portion of it will be necessary for struc- 
tural purposes. The situation tends to turn 
our attention toward those sources of 
energy, which are not destroyed in the 
using and those that are capable of re- 
newal within a brief time by growing plants. 
