226 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



Although uniform growth may be postulated for the main 

 shoot, or at any rate in some part, however small, of the First 

 Zone of Growth in which the new impulses are being initiated, 

 it is clear that if growth proceeds at the same rate from the 

 lateral growth-centres as well, these will never make any relative 

 progress nor produce any visible result, although they may have 

 been mapped out in the construction system. Nor is a rate of 

 growth in the lateral members at first slower than that of the 

 parent axis conceivable, since the insertion of the lateral members 

 constitutes the surface of the axis itself. In botanical phrase- 

 ology, therefore, so long as the rate of growth in the primordium 

 and axis is equal, the lateral growth-centres remain "dormant." 

 No visible effect, then, can be produced by growth from a lateral 

 growth-centre unless its rate of growth be greater than that of 

 the system as a whole. In such case the expansive development 

 of each lateral centre will be continued until contact is established 

 with adjacent members. Thus, in the simplest conception of a 

 growing system of stem and leaves, uniform growth may be 

 postulated for the main shoot, and uniform growth, but at an 

 increased rate, for all the lateral members, the result being that 

 the growth of the lateral member becomes visible as a disturbance 

 of the original equable system, and protuberances are formed 

 which come sooner or later into close lateral contact. 



Observation of the plant shows that such methods of arrange- 

 ment actually prevail, and the regularity of the construction, 

 especially as indicated by the contact-lines, is its most fundamental 

 and important feature. Nor again is it possible that any such 

 regularity can ever be a secondary effect ; comparison of systems 

 in which primordia are less regularly formed, and exert unequally 

 distributed contact-pressures on one another, as in the case of 

 the growth of fasciated shoots, and in the apparently centrifugally 



growth-centre at its apex as a perfectly independent system. This view 

 further suggests that the imperfectly individualised growth-centre which 

 gives rise to a leaf outgrowth remains at the point of its insertion, and the 

 apparent presence of an apical cell in certain leaves would thus appear to have 

 nothing to do with their space-form, but is, as in the case of the shoot itself, 

 only a part of the mechanism by means of which the architectural form is 

 worked out. 



