122 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



bijugate capitula described, and tlie latter including all the anomalous 

 constructions yet observed. On the other hand, while it is probable that 

 such generalisations represent the general tendency of the phyllotaxis 

 phenomena in Helianthus with a considerable amount of accuracy, it is 

 not necessary to assume that they wiU give absolutely accurate results 

 in the case of individuals. Examination of the apices of a large 

 number of young plants is sufficient to show the possibility of a con- 

 siderable amount of variation, the causes of which are still unknown. 



The agreement of the preceding data with the observations of 

 Weisse shows that even if continuous expansion does not take place, 

 a margin of about twenty leaves sufSBces to cover all the members 

 formed in any Sunflower plant which do not belong to such a con- 

 struction, including the first leaves of the seedling and odd members 

 filling the gaps between transitional systems. 



These generalisations require therefore to be tested on actual 

 apices in which the transition is being effected. The difficulty of 

 the method will lie in the fact that only one section is obtainable 

 from any given plant, and there is thus no telling what the system 

 observed would have passed on to if the plant had been left alone 

 until the capitulum was reached. 



Owing however to the remarkable uniformity of growth in the 

 bud of Helianthus, it is possible to cut sections which practically 

 include the whole of the phyllotaxis system so far as it has gone. 



The first and most remarkable aberration observed in the Sun- 

 flower is that the apex commences in the plumule a definitely 

 symmetrical construction, and changes to normal symmetry at a 

 subsequent and varying date. 



Thus seedlings which have not as yet produced their first leaves 

 beyond the green cotyledons, show on sectioning two or three pairs 

 of decussating leaves constituting the terminal bud of the plumule 

 and contained in the cotyledon tube (fig. 48a, three pairs). 



Asymmetry may commence early ; at the 5th leaf while still en- 

 closed in the tube (fig. 48&), or it may be delayed until the first 

 leaves are well grown. 



Seedlings in which two pairs of leaves are well developed on the 

 elongating axis should afford suitable material for the onset of 

 asymmetry. In such buds most usually the (2-f-3) system is 

 directly assumed (fig. 4Sd, for 10 leaves), more rarely symmetry 



