16 



POSSIL PLANTS. 



axis lias been assumed to be composed of lax cellular tissue, like Sigillaria and Stipnaria 

 were supposed to have been. Mr. Dawes, as well as Petzholt, had noticed the transverse 

 divisions in the pith or central axis of Calamites at the nodes, and both authors con- 

 sidered the pith to be composed of cellular tissue, with a few vascular bundles in it. 



Searchers after fossil plants showing structure must be aware, if they have any great 

 experience, that large specimens have their tissue generally very much distorted and 

 disarranged, and seldom afford any evidence of the central axis or pith, except a mere 

 line, or a cast of mineral matter, in an amorphous condition. My endeavours have been 

 to find very small specimens, for these appear to have undergone less alteration from 

 pressure than larger ones. They vary in size from yg,; of an inch in diameter to three 

 inches ; and it is only in the very smallest specimens that we find every part of the plant 

 preserved. Transverse sections of small specimens are met with which show only a 

 radiating cylinder of pseudo-vascular tissue of two tubes in breadth, whilst some of the 

 larger ones exhibit a radiating cyhnder of upwards of one hundred tubes in breadth. 



In the small specimens we have not much chance of seeing the external characters of 

 the stem, and for this we have to resort to larger specimens. 



In order to reconstruct the whole of the Plant, it is necessary to build up its parts 

 from different individuals of various sizes. The outside of a small Calamites is ribbed 

 and farrowed, and shows nodes or joints ; whilst a larger specimen, in its decorticated 

 state, is nearly smooth, or slightly marked with fine longitudinal strise. The cell-walls of 

 the tubes composing the pseudo-vascular cylinder of the former are thin, and the oval 

 openings not so well defined ; whilst in the latter, the cell-walls are stronger, and the 

 elongated openings in them are of a more distinct form. In both large and small speci- 

 mens the central axis or pith is divided at the joints by horizontal diaphragms. Both 

 have a thick carbonaceous bark, and their pseudo-vascular systems are wedge-shaped, 

 springing from orifices^ or openings around the central axis. 



In this Monograph no attempt will be made to distinguish the genus Calamodendron 

 from the old genus of Calamites ; but, as all my chief specimens show structure, they 

 will be named Calamodendron. It was formerly supposed that the larger specimens 

 belonged to the latter genus, and the smaller ones were classed with the former: but 

 my specimens, both large and small, afford only evidence of one kind of structure ; and 

 I am, therefore, induced to class both provisionally under one genus in this Memoir. 



Other observers may bring forward fresh grounds, from structure, to show that 

 Calamites is a distinct plant from the Calamodendron herein described. My specimens 

 I have named Calamodendron commune. 



1 As I know of no similar arrangement in living plants, I have used the words " orifices" or " openings" 

 in the descriptions, in preference to the terms nuclei or areolae. 



