128 
ized as follows: The mechanism of plant 
processes not at present explainable on a 
physico-chemical basis would be termed by 
the vitalistie school as ‘vital,’ by the phys- 
ico-chemical school ‘unknown.’’’ ‘This 
well expresses the current thought of ex- 
perimental biologists. 
When we have once recognized our ignor- 
rance and have found a means of learning, 
the situation looks more hopeful. Having 
arrived at this physico-chemical conception 
of the organization and activities of the liv- 
ing plant, the energy point of view fur- 
nishes the best means now available of ma- 
king the experimental attack on the nature 
of the processes involved in plant metabol- 
ism. Change in the physical system must 
always be conditioned by the energy avail- 
able. Since we look upon the living plant 
as a physical system we must be concerned 
with its energy transformations. We have 
until very recently considered the activi- 
ties of the plant largely from the view point 
of the materials received by it from the 
outer world and the products formed from 
these materials. Now we are turning our 
attention toward the question of what 
forms of energy the plant may be able to 
“use in these various transformations, or 
rather, we are broadening our vision to in- 
elude both the materials and the energy. 
In our consideration of the energy rela- 
tions of plants we are largely concerned 
with four questions: (1) From what 
sources does the plant receive its energy ? 
(2) What work does it accomplish by 
means of the energy received? (3) How 
effectively does it use this energy? (4) In 
what classes of plant problems does the 
energy point of view suggest a hopeful 
means of attack in research. 
Under certain conditions the plant may 
absorb heat from the surrounding air. It 
also commonly takes in from the soil solu- 
tion and perhaps even from the air certain 
SCIENCE 
[N. S. Vou. XLVIII. No. 1232 
energy containing compounds. Still the 
plant receives much of its energy directly 
from the sun in the form of light rays. 
What we know of its reception and use by 
the plant is largely confined to that portion 
lying within the visible spectrum, though 
some scientific excursions have been made 
into the realms of the ultra-violet and the 
infra-red rays. 
The leaf is the most useful portion of the 
plant for receiving and utilizing this radi- 
ant energy of the sun. Being by nature a 
surface-exposing organ, the leaf brings its 
green cells into such a situation that they 
advantageously receive light energy from 
the sun’s rays. 
It is in the green cells of the leaf that 
those transformations of energy take place, 
which are most significant to the plant in 
its own problems of existence as well as to 
man in his. 
The radiant energy falling upon the leaf 
of the plant may be disposed of in several 
ways. Some of it is reflected from the leaf 
surface and is then lost so far as its imme- 
diate effect upon the plant is concerned. 
This is a very small amount, but still it 
can not be entirely neglected. 
Some of the sun’s energy passes entirely 
through the leaf and is then also lost. Di- 
rect measurements of both the intensity and 
the wave-length of the light thus passing 
through the leaf have been made. Some of 
the energy retained by the leaf is used in 
evaporating water from the surface of the 
leaf. This is a much larger amount than 
both of the preceding combined. The 
amount of water thus evaporated from the 
aerial portion of a plant is large relative 
to the weight of the plant itself. Grass 
plants often give off in the form of vapor 
in every twenty-four hours of dry hot sum- 
mer weather, a quantity of water equal to 
their own weight. The grass of an ordi- 
nary city lot 50 x 125 feet would give off 
