THE GENERAL SHAPES OF PLANTS. 



127 



standing alone, and in positions where tlie winds do not injure 

 (liem or adjacent objects shade tliem, shi^ubs and trees develop 

 witli tolerable evenness on all sides, is an obvious truth. Equal- 

 ly ob\ious is the truth that, when growing together in a wood, 

 and mutually interfered with on all sides, trees still show 

 obscurely radial distributions of parts ; though, under such 

 conditions, they have tall taper stems with, branches directed 

 upwards — a difference of shape clearly due to the different 

 incidence of forces. And almost equally obvious is the truth, 

 that a tree of this same kind growing at the edge of the wood, 

 has its outer branches well-developed*and its inner branches 

 comparatively ill-developed. Fig. 197, which very inaccur- 



ately represents this difference, will serve to make it manifest 

 that w^hile one of the peripheral trees can be cut into some- 

 thing like two similar halves by a vertical plane directed to- 

 wards the centre of the wood — a plane on each side of which 

 the conditions are alike — it cannot be cut into similar halves 

 by any other plane. A like divergence from an indefinitely- 

 radial sjTnmetry towards an indefiiiitely-bilateral sAonmetry, 

 occurs in trees that have their conditions made bilateral by 

 gro"«^g on inclined surfaces. Two of the common forms 

 observable in such cases are given in Fig. 198. Here there 

 is divisibility into parts that are tolerably similar, by a vertical 

 plane running directly down the hill ; but not by am^ other 

 plane. Then, further, there is the bilateralness, similar in 

 general meaning though differently caused, which we see 

 in trees exposed to strong prevailing winds. Almost every 

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