85U REPORT — 1901. 



3. Remarks 7ipon the Wature of the Stele of Equmetum, 

 By D. T. Gwynne-Vaughan. 



The vascular buDdles of Equisetum are usually compared with those of a 

 monostelic phanerogam both in structural detail and with regard to their course 

 out into the leaf. The following observations made upon the stems of J3, Telma- 

 teja, kc, show that this comparison cannot be satisfactorily maintained. 



It was found that of the three strands of xylem present in each bundle of the 

 iuternode, the carinal strand aloue passes out at the node as a leaf-trace. The two 

 lateral strands join on to the xylem of the nodal ring, and in certain species {E. 

 Jdemale, and better still in E. 'ji<jantcum) they may be traced as e.\.ternally pro- 

 jecting ridges over the nodal xylem into the iuternode above. In passing through 

 the node they diverge from one another so that in the internode they are found on 

 the adjacent sides of two different bundles. At the node above they approach 

 each other, and in the next internode they both occur in the same bundle once 

 again. The leaf-trace protoxylem, having entered the bundle, runs downwards foi' 

 one internode between the two lateral strands : at the node below it divides into 

 two branches which curve to the right and the left ia order to fuss with the 

 neighlDouring leaf-traces that enter at this node. 



So the xylem of the so-called vascular bundle of Equiscluiti consists of three 

 strands, two of which are lateral and cauliue, while the median, or carinal, strand 

 is common to both stem and leaf. The fact that only a small portion passes out as 

 a leaf-trace, and not the bundle as a whole, constitutes an essential point of differ- 

 ence between it and the bundle of a phanerogam. 



The tracheides in each strand are very few, and consequently it is dithcult to 

 determine the direction of their development. However, as regardo the leaf-trace 

 and the carinal strand, it appears clear that they are not exarch but eudarch, or 

 perhaps slightly me.sarch on the adaxial side. The lateral strands, as a whole, are 

 differentiated later than the carinal strand (as might be expected from the close 

 relation of the latter to the leaves), but they do not seem to be a continuation of 

 its centrifugal development. On the contrary, in E. tjvjanieum, where as many as 

 ten to fifteen elements are present in each lateral strand, the smallest of them are 

 invariably at the outer extremity, and they gradually increase in size inwards. 

 Longitudinal sections show that the largest tracheides are coarsely reticulate 

 with large pits and very broad bands of thickening between them ; in the 

 smaller elements the reticulation becomes finer and more regular, and in the 

 smallest it closely resembles true spiral thickening. To state definitely whether 

 the lateral strands are exarch or not was not possible, because no incompletely 

 differentiated portions of the stem were available ; so the question must remain at 

 present imdecided, although the mature structure certainly gives a strong impres- 

 sion of centripetal development. Potonie ^ has established a comparison between the 

 secondary vascular tissues of the Calamavuv. and the Uphenophyllacem by mentally 

 doing away with the central mass of primary xylem that exists in the latter. ^ By 

 inverting this procedure, and considering it possible that the ancestors of the 

 Equisetums may have possessed a xylem that extended to the centre of the _stem,_ 

 one is led to derive their structure, as it exists at present, from the modification of 

 a stele with a solid central mass of centripetal xylem such as that of Hi)henophyllmn, 

 or of certain Lepidodendrese. To illustrate the nature of the modifications that 

 such a stele would have to undergo, a series of parallel developments may be 

 pointed out within the latter group {Leindodendron Ehodumneiise, Selaginoides, 

 Ilarcourtii, Siijillaria spinosa, and Menardi), in which parenchyma appears in the 

 xylem, and gradually increases in quantity until only an attenuated peripheral ring 

 of xylem remains, which then becomes more or less broken up into separate 

 strands. 



It is suggested that the lateral xylem strands in the vascular bundles of the 

 existing Equisetums may perhaps be taken to represent the last remnants of a 

 primitive central mass, and that this would be entirely in agreement with theii' 

 apparently centripetal development, and in particular with their cauline course. 



' P/{anzenpalaeontologiie,Tp. 205. 



