44 PLANTS 



oxygen is liberated than is needed by the plant and the excess 

 escapes. But during the night, or when for any reason no 

 oxygen is set free in the tissues by the synthesis of starch, the 

 plant absorbs oxygen directly from the atmosphere. This is 

 at all times true of plants which contain no chlorophyll. The 

 process of absorbing oxygen of whatever source, by vegetable 

 tissues, is called respiration and is identical with respiration 

 in animals. 



Translocation of Food Substances 



92. The midribs and veins of the leaf are continuations of 

 the vascular bundles of the stem. Besides giving support to 

 the softer tissues they also bring the leaf into communication 

 with the rest of the plant through the vascular system, per- 

 mitting the passage of liquids and gases between the leaf and 

 the stem. 



93. Since starch and, consequently, sugar, are formed only 

 in cells containing chlorophyll, all other cells must be dependent 

 for their food upon those which contain chlorophyll. Conse- 

 quently, in the larger number of plants, the leaves must elabo- 

 rate all the food for the stem and root. Starch is frequently 

 found in parts containing no chlorophyll. In such cases it 

 has been formed from sugar by the action of colorless corpuscles, 

 called amyloplasts, which differ from chloroplasts only in the 

 absence of chlorophyll. The course of the sugar through the 

 stem is chiefly along sieve vessels and the surrounding paren- 

 chyma. In passing from cell to cell it is frequently converted 

 into starch and then reconverted into sugar preparatory to the 

 next osmotic transfer. 



Other Food Substances 



94. Besides the carbohydrates, starch and sugar, there are 

 several other kinds of food substances elaborated in the leaf. 



