42 THE STORY OF PLANT LIFE 



Loose layers of cells occur in the neighbourhood of 

 the stomata, and these are found where corky tissue 

 replaces the epidermis, and communicate with the 

 intercellular spaces. Gases can pass in and out of 

 these cells. 



The lenticels play a great part in the entrance of 

 air in respiration, both in the leaves and stem and 

 roots. This explains how respiration can go on at 

 night, for then the stomata are closed, as a rule, and 

 air must enter the lenticels and carbon dioxide be 

 given off. This is why it is unhealthy for plants to 

 be placed in a bedroom. During the day oxygen is 

 given off in carbon assimilation and carbon dioxide 

 utilised, and the latter given off in respiration, which 

 goes on at all times. This gas is used up in the plant at 

 once in carbon assimilation. Little carbon assimila- 

 tion is carried on through the lenticels, and transpira- 

 tion also is not carried on through them to any 

 extent. 



Heat given off in respiration is more than used up 

 in transpiration, and a plant cools quickly. 



When air is devoid of oxygen plants use up the 

 oxygen in the protoplasts and still give off carbon 

 dioxide, but soon die. This is called intra-molecular 

 respiration. The carbon dioxide given off during the 

 day is diffused amongst the cells with chloroplasts 

 and there directly synthetised. Where no chloro- 

 plasts occur there is no evolution of oxygen for 

 respiration, but the carbon dioxide being given off by 

 respiration flows to the cells capable of utilising 



