THE LEAF 53 
animals. The plants themselves manufacture these materials, 
while animals must obtain them, either directly or indirectly, from 
plants. These processes are, however, not a part of respiration. 
Respiration is equally essential in plants and animals, as it is 
the source of energy for vital activities. When respiration ceases, 
there is no further supply of energy for these activities, and death 
ensues. , 
Oxygen supply. Since respiration is a process of oxidation, a 
plentiful supply of oxygen is one of the prerequisites for normal 
respiration. If the supply of oxygen is inadequate, respiration 
will be checked, and this will limit the rate of vital activities, 
such as growth, which are dependent on the energy liberated 
by respiration. ) 
Plants will live for a time in an atmosphere that does not con- 
tain oxygen. In this case energy is obtained by a process known 
as anaérobic or intramolecular respiration. In anaérobic respira- 
tion, energy is liberated by the incomplete breaking down of 
carbohydrates. Carbon dioxide and, usually, alcohol are pro- 
duced. Anaérobic respiration is less efficient than normal res- 
piration. Much more material is broken down, and, as a rule, 
much less carbon dioxide is produced. 
Sufficient oxygen is available for the aérial parts of plants, 
but this is not always true of roots and seeds in the ground. 
The soil should be porous and should contain air in the spaces 
between the particles. If there is so much water in the soil that 
it interferes with the diffusion of sufficient oxygen to the roots, 
the growth of the latter will be retarded. For this reason soil 
should be well drained. The roots of plants may be killed if the 
ground is flooded for a considerable length of time. The same 
thing may result if the level of the ground around a tree is raised 
until the roots are buried so deeply that sufficient oxygen does 
not reach them. Seeds respire actively during germination, and 
so will not germinate without a sufficient supply of oxygen. 
Plants that are partially submerged in water usually contain 
special aérating devices to facilitate the diffusion of oxygen from 
the air to their submerged organs (Figs. 197, 516, 517). The 
