VARYING GROWTH IN LATERAL MEMBERS. 325 



(2) Diminution in the radial direction, giving rise to phenomena 



of " dorsiventral " bilaterality, including sliding-growth 

 effects. 



(3) Secondary elongation of the main axis in passing through 



the " Second Zone of Growth." 



(4) Intercalary growth of petiole formation. 



(5) Cessation of growth-activity, leading to the production of 



members of a constant bulk in the adult condition. 



(6) All irregular and local secondary pressure relations in 



members approaching maturity, producing distortion of 

 spirals in asymmetrical systems, or the true orthostichies 

 in symmetrical constructions {of. fig. 79). 



(7) Special differentiation of individual members or portions 



of them. 

 So greatly is the discussion of theoretical phyllotaxis limited 

 in its general application to descriptive purposes, that these 

 generalisations may be almost taken as suggesting that the 

 original use of the term phyllotaxis as applied to all arrangements 

 and effects, as seen by the eye on young or adult shoots, may 

 after all be retained with great advantage for such phenomena ; 

 and the word be still used to express the general relations of 

 members as presented to the eye, and judged either by the loose 

 and approximate method of Schimper and Braun, or preferably 

 by the observation of the more obvious parastichies at any given 

 point; while the true primary system, which is the first visible 

 sign of the hidden forces which initiate new growth-centres in 

 the actual substance of the protoplasm, and which can only be 

 satisfactorily determined by examination of a transverse section 

 at the level of the growing apex, may possibly be preferably 

 restricted to such a term as mechanotaxis, in that it indicates 

 in the most concise form the actual growth-mechanism, while the 

 parastichies of such a section may further represent paths of 

 equal distribution of growth-energy, existing in a system in 

 which vital energy follows general laws of orthogonal distribution, 

 comparable with those which obtain in the case of manifestations 

 of forms of physical energy, the discussion of which is brought 

 within the range of mathematical conceptions in the mathematical 



