o4 A TEXTBOOK OF GENERAL BOTANY 
petiole of a water lily contains conspicuous air spaces (Fig. 40), 
through which oxygen diffuses from the leat blade toward the 
parts of the plant in the soil. 7 
Temperature. Under ordinary conditions the rate of respira- 
tion increases with the temperature. The rate is very low at 
0°C. and high at 30°C. This is one of the reasons why fruits 

Fria. 40. Cross section of a petiole of a water lily (Vymphaea stellata), 
showing large air passages. (x 30) 
and vegetables keep better in cold storage than at high temper- 
atures. The low temperature inhibits not only the respiration 
of fruits in storage but also the respiration and growth of decay- 
producing organisms. : 
TRANSPIRATION 
Definition. Transpiration is the evaporation of water from 
plants, the water passing in the form of water vapor into the 
surrounding atmosphere. 
Process. We have seen that water permeates the walls of the 
chlorenchyma cells, so that the surfaces of the walls surround- 
ing the intercellular spaces are wet, and that this condition is 
