GERMINATION OF SEEDS 87 
46. Respiration.—The escape of carbon dioxide, which 
follows the taking in of oxygen, is the superficial indication 
that the very important process called respiration is going 
on—a process that is essential not only to every living 
animal and plant, but also to every living cell. Just what 
happens in respiration is very uncertain; but it involves a 
series of changes in the living substance (protoplasm) itself— 
changes which are made possible by the presence of oxygen, 
and among whose results are the liberation of carbon dioxide 
as a waste product, and of energy for plant work, such as 
growth and movement. A plant, therefore, cannot work 
without respiration; and if it cannot work it ceases to live. 
The contrast between photosynthesis (§ 14) and res- 
piration should be kept distinctly in mind, as the former 
process so masks the latter in green plants exposed to light 
that the occurrence and the importance of respiration in 
them is not always fully appreciated. It was once custom- 
ary to contrast plants and animals by stating that the form- 
er take in carbon dioxide and give out oxygen (photosyn- 
thesis), and tne latter take in oxygen and give out carbon 
dioxide (respiration). It is evident that all living things, 
whether plants or animals, are dependent upon respiration; 
while green plants when exposed to light can also do the 
work of photosynthesis. The contrast between the two 
processes may be made still more evident by the following 
statement: photosynthesis occurs only in green cells, re- 
quires light, uses carbon dioxide, liberates oxygen, makes 
organic material, and accumulates energy; while respiration 
occurs in every living cell, does not require light, uses 
oxygen, liberates carbon dioxide, uses organic material, and 
liberates energy. 
47. Digestion.—Before any growth of the embryo can 
take place the reserve food must be changed. Most fre- 
quently in seeds the storage form is starch, but starch is 
insoluble and therefore cannot move out of the cells in 
7 
