254 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



have been wholly withdrawn from the testa and are beginning to 

 expand, present a very remarkable appearance (fig. 86, XL, III., IV.). 

 Parastichies are still wanting, as indicating any definite system, and the 

 primordia assume irregular forms under pressure ; so that the resem- 

 blance to a section of ordinary packed parenchyma is very close. 

 Growth continues to be irregular in the individual primordia (III.), 

 but as the plants become older it appears more regular and parastichies 

 begin to appear. How far these appearances are partly due to ir- 

 regularities in the phyllotaxis system itself is thus obscured ; but the 

 irregularity in the phyllotaxis is associated with irregularity in the 

 shape of the members. That the phyllotaxis system is itself irregular 

 is rendered probable by the comparison of other types (Cedrus Atlanti- 

 cus), but this would not necessarily lead to irregular shapes in the 

 members. As the contact-parastichies become increasingly obvious, 

 they give very anomalous results : for example, (IV.) is apparently a 

 system (6 + 1) with irr^ular packing among the first leaves; but 

 when the cotyledons fully expand, and the plumule becomes visible 

 between them, the presence of a definite system of the normal series 

 becomes clear for the first time. The central portion of the bud is now 

 unmistakably (5 + 8) (V.), although in the example figured this 

 appears to have been only rendered normal by the opening up of a new 

 curve by the member numbered 1. 

 Seedlings vary from (5 + 8) to (6 + 10), (5 + 8) being the usual type. Section 



of a plant in which the primary 

 shoot had reached the length of 

 6 inches, shows a normal (5 + 8) 

 system with remarkable perfection, 

 the members retaining to a v6ry 

 considerable degree the form of the 

 quasi-square of the theoretical con- 

 struction, owing to the very small 

 extent to which progressive bilater- 

 ality has been carried. Such a bud, 

 however, grown in a warm house, 

 retains the primary construction 

 to a much greater degree than the 

 foliage-buds produced on older plants 

 in the open air and exposed to desic- 

 cation : these primary shoots lacking 

 the protection of the bud-system 

 Fig. S7.—Coleochaetescutata. Young plant, subsequently developed in the adult 

 showing radial and circular symmetry. condition of the plant. 



A very instructive case is afforded by the arrangement of the 

 florets on bractless Composite capitula, and may be well observed 



