176 



Dr. H. Airy on Leaf- Arrangement. [Feb. 27, 



M. Pisani has ascertained that the variety from Montebras yields 



Fluorine 2-27 



Phosphoric acid 34-30 



Alumina 38*25 



Water 26-60 



101-42 



Specific gravity 2-33 



February 27, 1873. 



WILLIAM SPOTTISWOODE, M.A., Treasurer and Vice- 

 President, in the Chair. 



The following communication was read : — 



" On Leaf-Arrangement." By Hubert Airy, M.A., M.D. Com- 

 municated by Charles Darwin, F.R.S. Received January 21, 

 1873. 



(Abstract.) 



Assuming, as generally known, the main facts of leaf -arrangement, — 

 the division into the whorled and spiral types, and in the latter more 

 especially the establishment of the convergent series of fractions, J, 

 h I' "ft"' TT' TO ih ih t¥t' & c -> as representatives of a corresponding 

 series of spiral leaf-orders among plants, — we have to ask, what is the 

 meaning that lies hidden in this law ? 



Mr. Darwin has taught us to regard the different species of plants as 

 descended from some common ancestor ; and therefore we must suppose 

 that the different leaf-orders now existing have been derived by different 

 degrees of modification from some common ancestral leaf-order. 



One spiral order may be made to pass into another by a twist of the 

 axis that carries the leaves. This fact indicates the way in which all the 

 spiral orders may have been derived from one original order, namely by 

 means of different degrees of twist in the axis. 



We naturally look to the simplest of existing leaf-orders, the two- 

 ranked alternate order j, as standing nearest to the original ; for it is 

 manifest that the orders at the other extreme of the series (the condensed 

 arrangement of scales on fir-cones, of florets hi heads of Composite, of 

 leaves in close-lying plantains, &c.) are special and highly developed 

 instances, to meet special needs of protection and congregation : they 

 are, without doubt, the latest feat of phyllotactic development ; and we 

 may be sure that the course of change has been from the simple to the 

 complex, not the reverse. This point will be illustrated by experiment 

 below. 



