CHAPTER XXI 

 INTERRELATIONS OF THE PARTS OF A PLANT 



One part of a plant may be dependent upon processes that occur in 

 another part. Roots and leaves are mutually dependent. Consequently 

 the effects of an environmental factor upon the processes in one part of 

 a plant may influence the development of other parts. We now have a 

 suflBcient background of facts to understand some of these interrelations; 

 others will be postponed to later chapters. 



Interrelations involving movement of materials. Several of these inter- 

 relations are known to be dependent upon the movement of materials 

 from one part of the plant to another. Many of them depend upon the 

 movement of sugar from the green to the non-green parts of a plant. The 

 growth of seeds, roots, and underground stems depends upon sugar 

 made primarily in the leaves. Even green fruits and green stems, except 

 in such plants as cacti, are dependent in part upon sugar from the 

 leaves. The influence of any factor upon the making of sugar in the 

 leaves may be reflected in the growth of all these organs. The processes 

 in the leaves, in turn, are dependent upon the movement of water and 

 inorganic salts from the roots. The relative growth of the aerial stems, 

 leaves, and fruits also depends in part upon which ones get the major 

 supply of these substances from the roots, especially when the supply is 

 low or inadequate. 



We may begin the analysis of interrelations by citing a number of 

 growth phenomena which are dependent in large part (1) upon the 

 manufacture of sugar in the chlorenchyma and its movement to other 

 parts of the plant, and ( 2 ) upon the manufacture of proteins from this 

 sugar. The correct interpretation of some of these examples may be evi- 

 dent as rapidly as they are read; others will be correctly interpreted 

 only if the reader pauses long enough to analyze the problems in con- 

 siderable detail. 



Specific examples of interrelations. Under good growing conditions the 

 largest Jonathan apples are not obtained if there are fewer than 30 

 leaves per apple. The largest Delicious apples are not obtained if there 



188 



