NOTES AND ERRATA. 353 



parent shoot before this segmentation of the growth-centre took place. That 

 is to say, the " genetic-spiral " may work out as homodromous or hetero- 

 dromous, and the two capitula may be true "twins," i.e. images one of the 

 other, or they may not ; and the latter is possibly the commoner case. 



When the systems are homodromous, in the long-stalked form (Case I.), the 

 close agreement of the position of the foliage-leaves is readily checked ; the 

 only variations being clearly due to the secondary unequal elongation of the 

 different internodes. Where heterodromy occurs, the point of bifurcation will 

 be associated with a pair of equal leaves close together. In the case of the 

 capitula themselves, homodromy or heterodromy is readily checked by noticing 

 the course of the long and short curves of the disk, though this is often 

 rendered impossible owing to the addition of structural irregularities. 



The fact that phenomena of dichotomy, identical with those obtaining 

 normally in Lycopodium, should occur as anomalous constructions in such a 

 plant as Helianthus, would thus appear to suggest, not so much the extreme 

 antiquity of the dichotomous method, as that this represents an alternative 

 system of ramification which is worked out equally thoroughly in accordance 

 with certain definite mechanical laws ; and different plants have at different 

 periods selected that method which in the long run proved most satisfactory 

 to them. Thus Helianthus, like other Phanerogams, exhibits normally a 

 system of axillary branches, but stUl retains the power to arrange an alter- 

 native method of construction, just as L. Selago is typically dichotomous in its 

 assimilative region, but presents in addition a copious formation of true 

 axillary shoots which are subsequently utilised as gemmae for a secondary 

 biological purpose. 



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