THE ANNALS 



MAGAZINE OF NATURAL HISTORY, 



[THIRD SERIES.] 

 No. 30. JUNE 1860, 



XLVI. — On Cyclostigma, a new Genus of Fossil Plants from the 

 Old Red Sandstone of Kiltorcan, co. Kilkenny; and on the 

 General Law of Phyllotaocis in the Natural Orders Lycopo- 

 diacese, Equisetacese, Filices, ^t. By the Rev. Samuel 

 Haughton, F.R.S., Professor of Geology in the University 

 of Dublin. 



The extremely imperfect condition in which fossil plants are 

 usually found, and the almost total absence of their more im- 

 portant organs, lead us naturally to lay stress on such characters 

 as are found persistent in the fossil condition. Among these, 

 one of the most important, as I believe, is the geometrical 

 law of arrangement of their leaves. A careful examination of 

 this arrangement leads me to conclude that the leaves of Palaeo- 

 zoic fossil plants are arranged according to a different law from 

 that which prevails in the ordinary Exogens and Endogens, and 

 usually described in elementary text-books of botany. 



The law of arrangement is very simple, and may be thus 

 expressed : — 



" The leaves, or leaf-scars, are arranged in whorls, so placed 

 that each leaf is directly above or below a leaf of the alternate 

 whorls, and intermediate to the leaves of the adjacent whorls." 



The development of leaves following this law may be easily 

 conceived by imagining the whorl to ascend spirally on the 



/ 1 80° \ 

 stem, traversing an angle ( j between each of its resting- 

 places ; n denoting the number of leaves in each whorl. This 

 is the same as supposing each leaf to have an independent law 

 of development, of the ordinary kind, expressed by 



Divergence = — -. 

 "n 



The leaves, according to this view, are developed in simultaneous 



Ann. S^ Mag. N. Hist. Ser. 3. Vol. v. 29 



