20 RESPIRATION OF PLANTS 



transport and incorporation into the substance of the plant body. 

 The distribution of the foods has been compared to the currency 

 of a country; the storage foods are the bank reserve while the 

 sugars and other solutions are the circulating currency. 



11. Significance of the Leaf Structure. We now begin to 

 undersand the significance of the structure and form of the leaf. 

 The broad blade is a device to catch the energy of the sunlight. 

 The stomata afford access of CO 2 and escape of oxygen. The 

 intercellular spaces in the spongy mesophyll increase the area 

 for the absorption of CO 2 and excretion of oxygen to such an 

 extent that the internal surface of the leaf exceeds the external 

 many times. It will be noticed that the cells of the spongy 

 mesophyll vary in character. This is because of the different 

 functions that they perform. The collecting cells (Fig. 5, col) 

 are closely applied to the palisade cells and collect the sugars 

 and other foods from them while the elongated conducting cells 

 transport this food to the cells of the vascular bundles. 



12. The Second Function of the Leaf. Respiration or breath- 

 ing is a second function of leaves. While work of this kind is 

 performed by all living cells it may be considered at this point 

 because in the leaves the nature of the work is very well illus- 

 trated. The work of respiration is usually associated with the 

 absorption of oxygen and the giving off of CO 2 . Animals and 

 plants are constantly inhaling oxygen and exhaling CO 2 . 

 This interchange of gases is the reverse of that occurring in 

 photosynthesis. Furthermore respiration goes on all the time 

 when conditions are suitable for life while photosynthesis only 

 is possible in the light. Let us consider the significance of this 

 interchange of gases. Why do animals and plants breathe? 

 The construction of the foods to which attention has been called 

 may well be termed the storing up of the energy of the sunlight. 

 When the chlorophyll absorbs certain rays of light it does not 

 destroy any of the energy in those rays. The energy is changed 

 to another form and it still exists in the sugars and other com- 

 pounds that are built up through its power. The energy is 

 locked up, so to speak, in the food compounds. If these com- 

 pounds are decomposed into their simple elements again the 



