THE SHAVES OF BRANCHES. 135 



of form result, are in many cases largely due to tlie inequali- 

 ties in the circimistances of the parts while in the bud (which 

 are however representative of inequalities in ancestral cir- 

 cimistances) ; yet these are clearly not the sole causes of the 

 unlikenesses that eventually arise. For the leaf-buds whence 

 the larger leaves in Fig. 205 were developed, instead of being 

 at first more favourably circumstanced than the others, were 

 less favourably circumstanced. So that this bilaterahiess 

 that results from the imequal sizes of the leaves, must be con- 

 sidered as wholly due to the differential actions that come into 

 play after the leaves have assimaed their typical structures. 



§ 225. How in the arrangement of their twigs and leaves, 

 branches tend to lapse from forms that are approximately 

 symmetrical to forms that are quite asymmetrical, need not 

 be demonstrated : it is sufficiently conspicuous. But it may 

 be weU. to point out how the tendency to do this further 

 enforces our argument. The comparatively regular budding- 

 out of secondary axes and tertiary axes, does not usually 

 produce an aggregate which maintains its regularity, for 

 the simple reason that many of the axes abort. Terminal 

 buds are some of them destroyed by birds ; others are bur- 

 rowed iato by insects ; others are nipped by frost ; others 

 are broken off or injured during gales of wind. The envi- 

 ronment of each branch and its branchlets is thus ever 

 beiag varied on all sides : here, space beiag left vacant by 

 the death of some shoot that would ordinarily have occxipied 

 it ; and there^ space being trenched on by the lateral growth 

 of some adjacent branch that has had its main axis broken. 

 Hence the asymmetry or heterogeneity of form which the 

 branch assimies, is. caused by the asymmetrical distribution 

 of incident forces — a result and a cause that go on ever com- 

 plicating. 



§ 226. One conspicuous trait in the shapes of branches 

 has still to be named. Their proximal or attached ends 

 differ from their distal or free ends, in the same way that 



