30 SYMMETRY OF BRANCHING [CH. 



less light than they are adapted for. Even on single 

 trees in the open we see how the lower branches extend 

 their leaf-bearing tips further from the main axis, in 

 accordance with their greater age, and so avoid the over- 

 shadowing of the foliage by the leaves on the branches 

 above. 



But it is evident that an intense struggle for existence 

 goes on in both the above cases, resulting in the over- 

 shadowing of individual twigs, the starvation of their 

 leaves, and consequent death of the twigs themselves. 

 This suppression of branches brings us to another con- 

 sideration. 



If the plumule, as it developes into the primary shoot, 

 had a bud in the axil of every leaf, each bud in turn 

 growing out as a branch again with buds in its leaf-axils, 

 and so on, it is clear that we should have a shoot-system 

 of very symmetrical habit developed, just as in the typical 

 root-system we should obtain very symmetrical structures 

 if all the lateral rootlets developed equally and met with 

 no obstructions in the soil, differences of temperature, 

 moisture, marauding insects, fungi, &c. 



As matter of fact the typical shoot-system does begin 

 its life with remarkable regularity of structure ; but very 

 soon the action of the environment on the one hand, and 

 some not yet completely explained internal causes, come 

 into play to stop this formal symmetry. In illustration of 

 the statement that young shoots are very symmetrical the 

 student may examine any normally developed seedling of 

 a Maple, Cress, or Deadnettle. He will find that a pro- 

 jection of the primary stem and its leaves, seen end on 

 from above, gives a very symmetrical figure (compare 

 Fig. 11), more striking in the case of shoots with op- 

 posite leaves than in those with alternate ones, it is true, 

 but still very evident. Many well-grown Christmas-trees 



