NATURE OF PLANTS 21 



retarded by solutions containing copper or mercury which also 

 poison living matter. The distribution of foods made possible 

 by enzymes 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 

 understand 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 CO2 and escape of oxygen. The 

 intercellular spaces in the spongy mesophyll all lead to the 

 stomata, thus bringing about a quick distribution of the gases 

 and they also increase the area for the absorption of CO2 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 col- 

 lecting cells (Fig. 5, col) are closely applied to the palisade cells 

 and collect the sugars and other foods from them while the elon- 

 gated 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 CO2. Animals and 

 plants are constantly taking in oxygen and giving off CO2. This 

 interchange of gases is the reverse of that occurring in photo- 

 synthesis. Furthermore respiration goes on all the time when 

 conditions are suitable for life while photosynthesis only is pos- 

 sible in the light. Let us consider the significance of this inter- 

 change 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- 



