light at night, as well as by daylight. Lettuce thus “forced” was 
made ready for the market a couple of weeks earlier than the crop 
which had daylight alone. Quite a number of plants experi- 
mented with were, however, seen to be injured by the arc light. 
Probably this was due to the rays of ultra-violet light or else to 
particles thrown off from the naked arc light, since the plants did 
not seem to be so much injured if the arc was surrounded by a 
ground glass globe. But enough was found out in these experi- 
ments to justify the conclusion that there is no need for plants 
to rest at night, like animals; they can be made to work at the 
process of food-making by night as well as by day without 
apparent injury. 
The green leaves are thin and spread out so as to catch and 
absorb all the sunlight possible. The}' will absorb on a bright, 
warm, summer day as much as 40-70 per cent, of this light; or 
when the light is very dim and diffused, as in the depths of a 
forest, as much as 95 per cent. Of this absorbed light, only a 
very small amount is really used in the process of food-making; 
probably not more than an amount varying from 0.5 to 3 per cent. 
The sunlight, then, is the energy which starts and keeps 
going the machinery of this complex chemical laboratory. W hat 
happens sounds very simple when we learn that the carbon di- 
oxide of the air is being taken into the green leaf and there 
united with water to make a sugar-like substance, or, as others 
state it, starch. It is really a very complicated process, hcwe^ er, 
and probably takes place in a manner comparable to a sort of 
staircase, the successive steps becoming more and more compli- 
cated as we go up. We think that the steps as they go up probably 
include formic acid, formaldehyde, glucose (grape sugar) and 
finally starch. At any rate, the changes, whatever they are, go 
on very rapidly. If we put a green plant in the dark for a few 
days, then test it for the presence of starch in its leaves, we will 
not find any. Then if we bring the plant into the sunlight, and 
again try for starch with an iodin solution, we now find the leaf 
turning blue in an incredibly short time, showing that a few 
minutes only suffice for starch to be manufactured in considera- 
ble quantities in the leaf. This wonderful process of food- 
formation has been well named. It is called “photosynthesis” — 
“light building-up”. A chemist can imitate a few steps of the 
