248 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



No evidence exists for the formulation of any theory of phyllo- 

 taxis to explain the origin of a normal asymmetrical system which 

 involves the conception of the application of some external pressure. 

 That the application of an external pressure on an empirically 

 constructed system will produce results somewhat analogous to 

 those seen in the plant, can never be an acceptable argument. 

 While the action of an internal pressure set up by the members 

 themselves may, it is true, subsequently alter the appearance 

 of the system, but it can have no relation to the mode of its 

 formation. 



III. The Influence of a Rigid Boundary. 



A boundary more or less resistant may be formed by older 

 members of a character dissimilar to that of the uniform system 

 previously considered. It has been previously pointed out that 

 the youngest cells of a plant tissue will grow against practically 

 any pressure that may be brought ,to bear on them in the living 

 plant, and the same should hold good for the youngest members. 

 This constitutes, in fact, the conception of youth. But such 

 vitality is not necessarily long continued ; this power of resistance 

 usually rapidly diminishes. There is thus always a point at which 

 cells or primordia begin to yield to surrounding pressures, and both 

 cells and primordia as they grow older begin to assume the form 

 adapted for least resistance to surrounding more rigid bodies, and 

 fill the space available to them. 



Such diminution of vitality is the more rapid in members 

 which attain no great specialisation; or, more correctly, the 



the vertical or shorter paths and have broken the approximately horizonial 

 ones. Further, it must be noted that a cylindrical system of spheres arranged 

 orthogonally would not pack by any pressure into a perfect hexagonally 

 arranged one, in the sense of the accurate packing of the "pile of shot." The 

 original contact-lines would necessarily be broken somewhere, and the 

 resultant contact-curves would not present the regular arrangement which 

 on the other hand, as normally obtains in the adult plant as it does m the 

 developing system. Nor, again, was there ever any reason to suppose that the 

 whole leaf-priniordia would slide over each other to such an extent when their 

 bases constitute the surface of the axis. 



