20 RESPIRATION OF PLANTS 



darker color and in the "best flour" it is excluded in the milling 

 process, leaving the flour very white. So the most nutritious 

 part of the wheat does not appear in white bread, but it should be 

 remembered that white bread is better than the much exploited 

 "whole wheat" products because the proteid material is diges- 

 tible to only a slight degree. These reserve foods are made 

 available to the plant in the same way as to the animal. They 

 are first put into solutions by ferments or enzymes. Ptyalin in 

 the saliva of animals and diastase in the plant are common ex- 

 amples of enzymes which change starch into sugar. So there are 

 enzymes which transform each reserve food and render it 

 capable of 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 

 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 



