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 the 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 



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