84 PLANT PHYSIOLOGY 
shown that solutions of certain salts poisonous to plants 
become innocuous upon the addition of certain other 
salts which of themselves may also be poisonous. This 
discovery has thrown doubt upon many of the con- 
clusions of earlier botanists as to the functions of salts 
that are supposed to be essential to plant life. 
122. So far we have merely considered what sub- 
stances are required by the plant and something of the 
form in which the plant takes them in. Before they can 
be used they must undergo various decompositions and 
recombinations; in other words after absorption there 
must be assimilative processes. Perhaps the most funda- 
mental of these processes is that by which the carbon 
compounds are built up by green plants, a process called 
photosynthesis. 
123. Photosynthesis. The green parts of all chloro- 
phyll-bearing plants absorb carbon dioxide from the 
surrounding water if aquatic plants, or from the air, which 
contains about three parts of it to ten thousand. This 
absorption goes on only when the plant is exposed to the 
light. At the same: time there is an increase in the 
amount of carbohydrates often manifesting itself to the 
eye by the formation of starch grains in the chloroplasts, 
but also demonstrable chemically by the increased 
amount of sugars (chiefly glucose CgH12O¢) in the cell 
sap. At the same time it can be demonstrated that 
oxygen is given off by the plant. It is this process, tlie 
manufacture of carbohydrates by green plants in the 
presence of light, that has received the name photo- 
synthesis (from the Greek meaning “putting together 
by light’’). 
.124. Careful experiments have shown that this 
process cannot occur in the absence of any one of the 
factors mentioned in the preceding paragraph. Thus a 
ey -ae 
7 — 
- ha arta oe oe a 
~ 
ge sent 
oe 
