The Erectness of Plants. 101 



factors are also acting. The same explanation may be applied to 

 the angles of stem-branches and of leaves. 



This extension of the theory was taken up by Miss Lynn and 

 the writer \ The angles, lengths and distances were measured in, 

 various plants. The results for root branches of the pea, and for 

 the stem branches of privet, cherry laurel, dock, yellow cress and 

 horse-tails were found to be in accordance with the theory, when 

 an allowance of 10° on either side of the theoretical angle was 

 made. Considering that many other as yet unanalysed factors 

 may act in the growth of such branches, a deviation of ten 

 degrees in either direction is quite a moderate "experimental error" 

 on the part of the geotropic orientating mechanism. The data 

 for leaves were not so satisfactory, and the regular action of other 

 factors was indicated. This was explained when Dr. Harold 

 Wager informed us that from some unknown cause^ young leaves 

 grow more on their under sides and old leaves grow more on 

 their upper sides. When these interfering effects are eliminated 

 by taking the angle as 90° less the angle of the leaf with the 

 vertical, the data for leaves are in as complete accord with the 

 theory of the angle of balance as are those for stem and root- 

 branches. 



Leaves and the branches of stems and roots may, therefore, 

 be said to be spread out primarily by the balancing action which 

 the leakage from the main stem has upon the effects of the action 

 currents in the lateral organs. 



How Leaves Originate. 



Up to the present there has been no detailed theory for the 

 origin of leaves but, with the new insight into what is occurring 

 in the stem apex, it should be possible to obtain some idea of how 

 leaves happen to develop at the apex of the stem. The following 



1 See the " New Phytologrist," Vol. XIX, p. 209. 1920. 



' This cau»e very probably is the effect of light on photosynthesis and tlie ctarboii 

 dioxide balance in upper and lower sides of the leaf. 



