142 MORPHOLOGICAL DEVELOPMENT. 



standing alone, and in places where the winds do not injure 

 them nor adjacent things shade them, shrubs and trees develop 

 with tolerable evenness on all sides, is an obvious truth. 

 Equally obvious 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 



t98 V 1S>9 



inaccurately represents this difference, will serve to make it 

 manifest that while one of the peripheral trees can be cut 

 into something like two similar halves by a vertical plane 

 directed towards the centre of the wood — a plane on each side 

 of which the conditions are alike — it cannot be cut into simi- 

 lar halves by any other plane. A like divergence from an 

 indefinitely-radial symmetry towards an indefinitely-bilateral 

 symmetry, occurs in trees that have their conditions made 

 bilateral by growing 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 

 any other plane. Then, further, there is the bilateralness, 

 similar in general meaning though differently caused, often 

 seen in trees exposed to strong prevailing winds. Almost 



