236 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



III. Contact-Pressures. 



The existence and great importance of contact-pressures has been 

 emphasised by Schwendener, as also the fact that the contact-lines 

 follow those of what has previously been described as a " concen- 

 tration-system." It has also been seen that such growth-pressures 

 may be referred to an increased rate of expansion in the lateral 

 primordium as compared with that of the parent axis. This 

 increased rate of growth implies that growth initiated from a new 

 growth-centre extends radially and equally in all directions until 

 contact is made with adjacent centres of growth distribution ; and 

 in the great majority of cases it would appear that the visible rise 

 of a primordium has some relation to the formation of contact 

 surfaces ; although in other cases (c/. Aspidium, fig. 35) there can 

 be no doubt that the primordia rise from a central region before 

 any lateral contacts are effected. 



It is clear that the existence of such undoubted cases of the 

 complete absence of any lateral contact whatever, combined with 

 the production of perfectly normal Fibonacci phyllotaxis, com- 

 pletely puts out of court all theories of phyllotaxis which demand 

 the close lateral contact of primordia as being of fundamental 

 importance in determining the initiation of new growth-centres, 

 which has been such a favourite standpoint from the time of 

 Hofmeister to that of Schwendener and his pupils: The construc- 

 tion of such an apex as that of Aspidium (fig. 35) is alone sufficient 

 to disprove any contact theory, whether it be taken in the original 

 form of mechanical contact-action, or in the diluted and still more 

 hypothetical form of contact-stimulus. The essential point, how- 



