256 RELATION OP PHYLLOTAXIS TO MECHANICAL LAWS. 



the inflorescence receptacle, and they remain so far dormant 

 growth-centres. * 



But the discussion of the mutual relations of the florets of 

 Gynara does not enter into the question of primary phyllotaxis at 

 all, except in so far that it is a derivative arrangement, which 

 follows the primary non-developed system so uniformly and 

 closely that it may be taken as a perfect guide to the original con- 

 struction. These florets, though not primary phyllotaxis elements 

 at all, are not in any close lateral contact, but are packed round 

 with hairs so that each develops independently and maintains its 

 own normal orientation, without any angular alterations produced 

 by mutual pressures. The curves are perfect Fibonacci systems, 

 and two features of special interest may be noticed : — 



I. The rotation of the peripheral florets (fig. 536) owing to 

 secondary pressure against the smooth, firm involucre 

 edge; no slipping is involved nor displacement, only a 

 readjustment. 

 II. The elongated oval shape of the ovaries, in the slightly 

 spiral " median plane," due, again, not to any mutual 

 pressure, but to the inherent structural tendency of 

 two "median carpels" to build an oval rather than a 

 circular organ. 

 Comparing Helianthus now with Gynara, it will be seen that the 

 curve-systems are identically accurate in both types, but Helian- 

 thus differs (1) in having subtending bracts present and visible, 

 although pressed out of their original positions by their axillary 

 florets ; (2) the ovaries of the florets are definitely rhomboidal by 

 mutual pressute ; (3) those of the decadent ray-florets change 

 their shape but not their position, and thus become packed into 

 triangular facets. The angular ovaries are of special interest: 

 theoretically they should have presented the same oval shape as 

 in Gynara, since they are constituted by the same two " median 

 carpels." But in consequence of growth-pressure, each oval has 



» Gf. the interesting case of the missing subtending bract of the flower of 

 Nymphaea ; the axillary flower thus appears to fall in the normal phyllotaxis 

 system, as if the flower replaced a leaf. But the young flower-bud does not 

 fill the quasi-square left empty, and is packed into it with wooUy hairs. 



