APICAL DOMINANCE 



621 



content of 20 per cent, after which it diminishes. An interpretation of the 

 physiological basis of these results is left to the student as a problem. 



TABLE 59 SHOOT-ROOT RATIOS OF CORN SEEDLINGS GROWN FOR l"] DAYS IN SAND AT VARIOUS 



WATER CONTENTS (dATA OF HARRIS, I9I4) 



Certain mineral elements, especially phosphorus, also have effects on the 

 shoot-root ratios of plants. It is also highly probable that the shoot-root 

 ratio is influenced by the movement of hormone-like substances from shoot to 

 root and vice versa (Chap. XXXII). 



The principle of multiple causation (Chap. XXXIII) is especially well 

 exemplified by the manner in which a similar end result, in terms of the shoot- 

 root ratio, is brought about by various environmental conditions. 



Apical Dominance. — In many herbaceous plants which produce aerial 

 stems growth takes place principally or entirely at the apex of the main axis 

 of the plant. Although a lateral bud is present in the axil of every leaf, side 

 branches do not often develop from these buds as long as the terminal bud 

 retains its vigor and continues to grow. If, however, the terminal bud is 

 destroyed or injured in any way, or is artificially removed, development of 

 one or more of the lateral buds usually starts at once. This inhibiting ef- 

 fect of a terminal bud upon lateral bud development is called apical dominance 

 and is much more pronounced in some species than in others. 



The phenomenon of apical dominance is usually also in evidence in all 

 woody plants which produce true terminal buds. The lateral buds on current 

 shoots usually do not develop unless the terminal bud is destroyed or injured. 

 Development of the lateral buds on older shoot segments is of more frequent 

 occurrence, indicating that the inhibitory effect of the apical bud diminishes 

 with greater distance of the lateral buds from the apex of the stem. In many 

 woody species most of the axillary buds regularly produce lateral branches the 

 next growing season after the one during which they were produced. 



The effect of the apical bud in apical dominance is apparently due to 

 its auxin content. When agar blocks containing either auxin b or hetero- 



