252 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



curve-system of the vascular bundles supplying them, as they 

 pass down the axis. Further, there can be no doubt that any 

 production of an " expansion system " will give the appearance of 

 irregularity, when the number of members developed before 

 the change takes place is not sufficient to give the appearance of 

 definite parastichies. 



Thus Pinus sylvestris * commences very commonly with an 

 approximation to a (2 + 3) system, although the older shoots show 

 (5 + 8); and P. Pinea after initial irregularity settles down to 

 (5 + 8). In such specialised seedlings, as in the case of species of 

 Helianthus, for some reason, the phyllotaxis is irregular at first, 

 though this is by no means the general case for all plants. But 

 the effect of pressure against the cotyledons so long as the 

 plumule is enclosed between their bases, and these again by the 

 endosperm, is well seen in the case of the first leaves which are 

 initiated while the seedling is wholly within the endosperm and 

 testa. Sections of such seedlings show very marked irregular 

 packing shapes produced by pressure, much as in the bud of 

 Cedrus Lihani. Such pressures add, therefore, to the complexity 

 of the determination of the systems as they appear at any given 

 time ; but they clearly have nothing to do with the origination 

 of the first impulses which determined the formation of the 

 leaf-members in the substance of the broad embryo apex (fig. 86). 



Comparison of the broad apex of a seedling in which the radicle has alone 

 protruded (fig. 86, I.), with that of older seedlings, suggests most 

 strongly that primordia are already being formed within it, but have 

 not yet arisen above the surface. The space between the bases of the 

 cotyledons is usually somewhat elliptical, and the primordia at the 

 ends of the ellipse are distinctly more advanced than the others, so that 

 here, as in other oases in which true centric growth does not obtain, the 

 actual ontogenetic order of appearance gives no clue to the order of 

 formation. There is as yet no regular system observable, since the 

 number of leaves already in sight is insuflficient to show contact- 

 parastichies : the arrangement thus appears somewhat irregular, and, 

 owing to the conical shape of the apex, is not readily observed in 

 transverse section. Slightly older seedlings, in which the cotyledons 



* Gf. Schwendener, Bot. Mittheilungen, i. p. 89, Taf. v. The number of 

 leaves seen in section being too small to give any reliable pattern. 



