TRANSACTIONS OF THE SECTIONS. 131 



silicle, that is, acporcling to this explanation, to the apex of the enveloping leaves. 

 The term " false " dissepiment is therefore no longer necessary, the fruit being a 

 normal growth, though of an unusual construction. 



On the S2}ecies of Grimmia {including Schistidium) as re/presented in the 

 neighbourhood of Edinhurgh. By J. Sadler. 



Observations on the intimate Structure of Spiral ducts in Plants and their 

 relationship to the Flower. Bg Neil Stevtaet. 



An Inquirg into the Functions of Colour in Plants during different Stages of 

 their Development. By Neil Steavart. 



On the Classijication of the Vascular Cryptogamia, as affected by recent Dis- 

 coveries amongst the Fossil Plants of the Coal-measures. By W. C. William- 

 son, F.R.S., Professor of Natural History in Owens College, Manchester. 



The author described the structure of the stem of Calamite explaining, his inter- 

 pretation of its structure, viz. that it consisted of a central fistular medulla, sur- 

 rounded by a ring of woody wedges, each one of which grew by additions to the 

 exterior of its surface until it often became a woody cvlinder of considerable thick- 

 ness. The Lepidoilendra and Siffillarice were next reviewed, beginning with Lepi- 

 (hdendra, in which the central axis was a mixture of cells and vessels surrounded 

 by a very thin, and often scarcely appreciable ligneous ring, and which gave off 

 vascular bundles to the leaves. Other forms were then noticed in which the central 

 medulla became differentiated into a central cellular portion, and an outer vascular 

 one, the latter existing as a modified medullary sheath. One of the types described 

 by Mr. Binney as Sigillaria vascularis, exhibits these features; and the development 

 was traced still further through Diploxylon and Sigillaria, where the woody zone 

 became yet more fully developed, the medullary rays more distinct, and the dif- 

 ferentiation of the two elements of the pith, viz. the vascular and the cellular, yet 

 more complete. The origin of the vascular bundles going to the leaves in some 

 forms of Diploxylon was shown to be, not in the medullary vessels, as described by 

 Corda, but in a cellular layer separating the ring of the medullary vascular cylinder 

 from the more external vascular cylinder of the true woody zone. The relation of 

 these various structures to those seen in Stigmaria was pointed out. In the latter, 

 as was to be expected in a root, the vessels of the meduUary axis disappeared, the 

 pith being in direct contact with the inner surface of the woody zone. The 

 vascular bundles given off to the rootlets were shown to originate in the ligneous 

 cylinder, and to pass outwards through large lenticular spores, occupied by mural 

 cellular tissue, separating the woody wedges, whilst in addition to these spores, 

 there exist a complete system of minor medullary rays, the entire structure ex- 

 hibiting, in the author's opinion, an exogenous arrangement. 



The conclusion to be drawn from the study of the structure of these fossil cryp- 

 togamic stems is, that, so far as their medullary axis and ligneous zone is concerned, 

 they are not in any sense Acrogens, but Exogens ; that they have a pith consisting 

 of the less developed Lepidodendroid forms of a mixture of cells and vessels ; that 

 as we ascend in the series of forms the cells become separated from the vessels, the 

 former occupying the interior, and the latter the exterior of the medullary axis ; 

 that the woody zone surrounding the medullary axis consists of a cylinder com- 

 posed of radiating lines of A'essels, which increase by successive additions to the 

 external surface of the zone, the laminre of which vessels are separated by mural 

 arrangements of cellular tissue constituting two kinds of medullary rays ; con- 

 sequently when such a process of growth has gone on until the result was a tree 

 with a stem two, three or more feet in diameter, the application of the term 



9* 



