NUTRITION AND SEASONAL LIFE OF PLANTS 123 



comparable to the living muscle and nerve cells in animals in their 

 need for a certain amount of oxygen for respiration, but there 

 is no definite circulating system in plants to carry this oxygen 

 to these cells from the external air. In plants the oxygen moves 

 through the intercellular system, which penetrates to all parts of 

 the plant body, by the slow process of gas diffusion. The oxygen 

 enters the plant through leaf stomata and through the lenticels 

 which we have already observed in the bark of twigs and stems. 

 The slow diffusion of oxygen suffices, however, for the produc- 

 tion of sufficient energy for the less active tissues of higher 

 green plants. In the case of more active plants, like bacteria and 

 yeasts, and in the active tissues of growing buds and flowers, 

 plant cells often equal or exceed animals in the energy of the 

 respiratory process. 



Comparison of respiration and photosynthesis. These two vital 

 processes of plants are often confused, owing to the fact that the 

 same gases are involved in both processes and that they may go 

 on at the same time in one organ, as, for example, the green leaf. 



Photosynthesis is a food-building process in which carbon di- 

 oxide is absorbed from the external air by cells containing green 

 chloroplasts and combined with water to make sugar and starch. 

 During this process the excess oxygen, contained in water and 

 carbon dioxide, which is not needed for making sugar, is liberated 

 as free oxygen into the intercellular system of the plant. This 

 absorption of carbon dioxide and the accompanying liberation 

 of oxygen can only go on in the daytime, when the chlorophyll 

 can absorb the sun's energy for the photosynthesis process. No 

 gaseous exchange, therefore, which is due to photosynthesis can 

 go on at night or in darkness. 



Respiration is exactly opposed to photosynthesis in its need 

 for and use of oxygen and carbon dioxide. In this process oxy- 

 gen is absorbed, carbon dioxide is liberated, and energy is formed 

 by all living cells of the plant body, regardless of whether they 

 contain chloroplasts or not. Furthermore, oxygen is absorbed 

 and carbon dioxide is liberated by plant cells at all times of day 

 and night as long as they live and need energy for maintaining 

 their vital functions. 



