56 BOTANY 



ments. True, at first sight they seem a little antagonistic 

 to other needs. The evaporation of the water and the 

 illumination of the chloroplasts demand a large and 

 increasing shoot-body, but its increase in size brings 

 with it a distinct danger to the stability which we have 

 seen is one of the first necessities of the plant as a whole. 

 The reconciling of these demands must add to the 

 interest with which we study the form and distribution 

 of the members of the shoot system. 



We have seen that the axis of the latter is very much 

 subdivided, the ultimate divisions, the branches, taper- 

 ing to points, in some cases extremely gradually, in 

 others more abruptly. These cylindrical or conical 

 divisions bear a number of flattened organs, the leaves, 

 which are usually attached to the axis by flexible stalks 

 or particles. We can now see the reason for this sub- 

 divided conformation. It secures strength by the 

 cylindrical form of the twigs, surface by the flattened 

 form of the leaves. The winds can blow freely through 

 the mass of twigs, while the long leaf stalks allow of 

 sufficient displacement of the flattened parts when the 

 pressure of the wind is brought to bear upon them. 

 Moreover, the parts concerned are all extremely flexible 

 and elastic, so that they can yield to pressure and 

 regain their positions as soon as it is removed. 



The form of the shoot system of a plant will depend 

 upon the manner of its branching, and the number, size, 

 and arrangement of the leaves its branches bear. 



The branching will be affected by two main factors : 

 firstly, the number of branches produced at a node ; 

 secondly, the relative degree of growth of each main 

 branch and those to which it gives rise. 



The first of these does not show as much variety as 

 might be expected. Usually one branch, not infre- 

 quently two, appear at a node, but seldom more. 



The second factor, however, plays a much more 



