o0 A TEXTBOOK OF GENERAL BOTANY ‘ 
products of photosynthesis and not needed at the time may be 
conserved for use when it is required. 
Process. Respiration is not a particular function of any plant 
organ but is carried on by all living cells. In this process, oxy- 
gen is combined with compounds containing carbon and hydro- 
gen. This results in the breaking down of the compounds to 
form carbon dioxide and water, and in the liberation of energy. 
If sugar is the compound that is being oxidized, we may 
write the formula for respiration as follows: 
C.H,,0, + 6 0, —> 6 CO, +6 H,O. 
12° 6 
If we compare the above formula with that previously given 
for photosynthesis, we find that one is the reverse of the other, 
which is in keeping with the fact that photosynthesis stores ~ 
energy, while respiration releases it. 
Source of oxygen. In leaves that are carrying on photosyn- 
thesis, oxygen which is liberated by this process is used in res- 
piration. If photosynthesis is rapid, however, more oxygen is 
liberated than is used in respiration, and the excess diffuses 
from the cells into the intercellular spaces and thence into the 
external atmosphere. This is the case under ordinary conditions 
in bright light. 
Plant organs, such as petals and roots, which do not contain 
chlorophyll obtain oxygen from the air. The same is true of 
leaves when they are in the dark and so are not liberating oxygen 
by means of photosynthesis. In this case oxygen in the inter- 
cellular spaces goes into solution in the water held in the cell 
wall. It then diffuses in this. water until it enters the proto- 
plasm, where it can be used for respiration. 
Fate of products. The water produced by respiration mixes 
with the water of the cell. | 
In the case of organs without chlorophyll or of green organs in 
the dark the carbon dioxide diffuses out of the cell into the sur- 
rounding atmosphere. In green tissues which are actively carry- 
ing on photosynthesis most of the carbon dioxide will be used in 
photosynthesis without leaving the cell in which it is produced. 
