224 



COLLEGE BOTANY 



delicate and moist walls of the internal cells. Therefore, a 

 certain amount of oxygen is absorbed (see page 193) by the cell 

 sap and finally reaches the protoplasm, where it becomes a factor 

 in the catabolic activities of the cell. As a result of these ac- 

 tivities, a certain amount of carbon dioxide and water are elimi- 

 nated and kinetic energy released. A part of this energy is used 

 in plant growth and a part is set free as heat It is very 

 evident that respiration is greatest during the 

 period of most active growth and in those parts 

 of the plant where the growth is most active. 

 Germination is the period of greatest activity in 

 plant growth and therefore the period in which 

 respiration is the greatest Rapidly germinating 

 seeds may lose 3 per cent of dry weight per day 

 during the first ten days of germination. It is 

 also well known that respiration is greater in 

 wounded than in uninjured plants. 



Aeration. Since both the carbon dioxide and 

 FIG. 105. De- the oxygen of the atmosphere are necessary for 



vice for demon- 



ne r c a ti g brtw^n P lant growth and since the plant eliminates both 



the lenticels and < , i 11 , j 



the intercellular of these gases and also water, it is very evident 



spaces of the stem i_i i 



that the plant must have a suitable atmospheric 

 environment This is brought about by the wind movement of 

 the air, which is thus kept in constant circulation and is thor- 

 oughly mixed. In the case of young leaves 1 there may be some 

 exchange of gases through the walls of the outer cells, but in 

 the case of mature leaves the outer walls are cuticularized, 

 which prevents any such exchange. Therefore, the exchange 

 must be through the walls of the cells which line the inter- 

 cellular spaces and which are connected with the outside by 

 means of stomata. Here again the law of diffusion becomes 



