408 



MEMOIRS OP THE AMERICAN ACADEMY. 



of these two, and if the latter were placed at the same heights we should 



have a sector of three whorls; that is, two leaves of the highest and two 



of the lowest whorl, and one leaf of the intermediate whorl. But such an 



arrangement disregards or sacrifices in the structure of the whorl itself the 



advantage, if it be one, of such an alternation. It cannot be reasonable to 



suppose that a leaf on an intermediate line would seek distance and isolation 



from those of the lines beside it, and, at the same time, seek close connection 



horizontally with those of its own whorl. This would be directly opposed to 



the accommodation of uses in spiral arrangements. The structure of whorls, 



and the alternation in successive ones, appear, therefore, to be of distinct 



origins. Whatever advantage there is in the former appears to be sacrificed 



by this alternation, and by the spiral arrangements; or, if it be a disadvantage, 



it is avoided by these. It is probably on the whole a disadvantage; since it 



is ill-fitted for great extensions and branchings in stems, for which the simpler 



spiral arrangements appear peculiarly fitted. This contrast, however, cannot be 



regarded as the origin of the contrasted types themselves, and the soundest 



conclusion appears to be, that, whatever adaptations they may have, these 



bud: th 



only incidental, and are not concerned in their origination, either directly 

 through physiological laws of growth, or indirectly by Natural Selection. They 

 are properly genetic characters. This is confirmed by the fact that the par- 

 ticular arrangement for each plant is provided for, or already completed in the 



nofc a result of laws of development in general, but of the 

 special nature of the plant, or the predisposition of its vital forces. In regard 

 to the causes which I have supposed to control the so 



duction of buds or leaves, it should not be supposed fr* «*^«** 

 actual plants any considerable influence ; though the plant's particular laws of 

 growth are probably not in opposition to them. They should only be considered 

 as modifying agencies reacting on the formative forces; but they fail, as we 



© 



called adventit 



P 



seen, to account for the spiral and verticil arrangements, and their 



con- 



trasts through any utility which could modify these forces. But in concluding 



therefore that these general types of arrangement ought . to be regarded as only 

 genefc characters in the higher plants, and as presenting no important adyan- 

 tage or d lsa dvantage, independently of the special forms which they have 

 acqu.red or in present forms of life ; we are not precluded by such a con- 

 clus.on from the further inquiry as to what former advantage there could have 

 been m less specialized forms, before these genetic characters had lost their 



