CONTAOT-PKESSURES. 243 



the type of the Pine-cone scales, really do give a system apparently 

 identical with the true phyllotaxis relations of the primary 

 members, affords a curious witness of the deep-seated faith of 

 observers in the laws of uniform growth ; and thus the Composite 

 capitulum, the Aroid spadix, and the Pine-cone have always been 

 favourite examples of theories with which they have after all only 

 a secondary connection (Schwendener, Jost, Leisering). 



I. The Pressure of Older Members on Progressively 

 Younger Ones. 



Hofmeister first put forward the view that the presence of older 

 members must affect the position of new ones; and that new 

 members in the vast majority of cases arise ontogenetically in 

 close contact with older ones is sufiBciently obvious ; it remained, 

 however, for Schwendener to make such close contact the basis of 

 a definite mechanical theory. But the value of contact-pressure 

 theories is greatly discounted if examples can be adduced in which 

 the primordia do not arise in contact at all, and yet present the 

 normal appearances of spiral phyllotaxis (Schumann). Thus, in 

 many large shoots with broad flat apices such is apparently the case 

 {Nynvphaea, Sem/pervivum, fig. 83) ; the latter may be taken as a 

 type of these constructions. The youngest visible primordia are 

 low elevations which show no boundary-line along the shallow 

 depressions between them ; but so long as the primordia show any 

 inclination to rhomboidal shape, a certain amount of contact must 

 be admitted, and contraction in the spirit-material allowed for. 

 In a broad apex such contraction may be greater in the longitudinal 

 direction, and in this and other cases frequently has the effect of 

 pulling down the growing-point into a slight depression. The only 

 evidence that can be accepted of complete absence of lateral 

 contact will be the retention by the primordia of their original 

 approximately circular outline. Such primordia occur noticeably 

 in the apices of species of Opuntia, where the leaves, though 

 rudimentary, are better developed than in most Cactaceae {0. 

 cylindrica, 0. leucotricha); but most remarkably and easiest of 

 observation in such Ferns as the common Aspidium Filix-Mas. 



