PHOTOSYNTHESIS AND RESPIRATION. 



101 



dioxide. In darkness, however, it can be shown that they 

 respire only, as is the case with roots and all other not-green 

 parts, in both light and darkness. 



Photosynthesis. 



A feeding process, associated 

 with building-up of plant's sub- 

 stance. 



Starch is (usually) formed. 



Plant gains in dry-weight. 



Only occurs in light. 



Only occurs in green plants and 

 in green parts of plants. 



Carbon dioxide passes in, oxy- 

 gen passes out. 



Respiration. 



A breathing process, associated 

 with breaking-down of substance. 



Carbon dioxide formed. 



Plant loses in dry-weight. 



Occurs in darkness as well as 

 in light. 



Occurs in all plants and all 

 parts. 



Oxygen passes in, carbon di- 

 oxide passes out. 



136. The "Balance of Nature." Animals and non-green plants 

 are all ultimately dependent upon green plants for their organic food. 

 In order to obtain a sufficient supply of energy, without which life is 

 impossible, the greater part of this food is consumed in respiration, 

 the carbon being re -oxidised into carbon dioxide, which is ultimately 

 again assimilated by green plants. 



The whole series of these interchanges between the self-supporting 

 (chlorophyllous plants and animals) arid the dependent members (non- 

 green plants and animals) of the organic world forms what is known 

 as the "Balance of Nature." If more organic food becomes available 

 animals and non-green plants tend to increase, and hence produce 

 larger quantities of carbon dioxide, which favours the development of 

 green plants. The balance maintained between the processes of 

 respiration (in all organisms) and photosynthesis (in green organisms) 

 keeps the amount of carbon dioxide in the air practically the same. 



QUESTIONS ON CHAPTER IV. 



1. What differences exist between the cotyledons and the foliage- 

 leaves of the Broad Bean ? How may these differences be accounted 

 for? 



2. How could you prove that a leaf has air-spaces inside it and air- 

 pores on its surface ? 



3. Describe the structure of a typical foliage-leaf. 



4. How could you prove that a green leaf makes food when exposed 

 to light ? 



5. Under what conditions is starch formed in the foliage-leaves of an 

 ordinary land plant ? Give experiments in proof of each statement. 



