CHANGES OF SHAPE OTHERWISE CAUSED. 181 



as to produce a corkscrew shape. This structure is ascrib- 

 able to differences of interstitial nutrition. Take a shoot 

 which is growing vertically. It is clear that if the molecules 

 are added with perfect equality on all sides, there will be no 

 tendency towards any kind of lateral deviation; and the 

 successively-produced parts will be perpendicularly over one 

 another. But any inequality in the rate of growth on the 

 different sides of the shoot, will destroy this straightness in 

 the lines of growth. If the greatest and least rates of mole- 

 cular increase happen to be on opposite sides, the shoot must 

 assume a curve of single curvature; but in every other case 

 of unequal molecular increase, a curve of double curvature 

 must result. Now it is a corollary from the instability of the 

 homogeneous, that the rates of growth on all sides of a shoot 

 can never be exactly alike; and it is also to be inferred from 

 the same general law, that the greatest and least rates of 

 growth will not occur on exactly opposite sides of the shoot, 

 at the same time that equal rates of growth are preserved by 

 the two other sides. Hence, there must almost inevitably 

 arise more or less of twist; and the appendages of the inter- 

 nodes will so be prevented from occurring perpendicularly 

 one over another. 



A deviation of this kind, necessarily initiated by physical 

 causes in conformity with the general laws of evolution, is 

 likely to be made regular and decided by natural selection. 

 For under ordinary circumstances, a plant profits by having 

 its axis so twisted as to bring the appended leaves into posi- 

 tions which prevent them from shading one another. And, 

 manifestly, modifications in the forms, sizes, and insertions of 

 the leaves, may, under the same agency, lead to adapted 

 modifications of the twist. We must therefore ascribe this 

 common characteristic of phasnogams, primarily to local differ- 

 ences of nutrition, and secondarily to survival of the fittest. 



its character. Variation of nutrition is unquestionably a "true cause" of 

 variation in plant-structure. We have here no imaginary action of a fictitious 

 agency ; but an ascertained action of a known agency. 



