ANATOMY 



189 



leaf-trace protoxylem, having entered the bundle-, runs downwards for one 

 internode between but internally to the two lateral strands : at the node 

 below it divides into two branches, which curve to the right and the 

 left in order to fuse with the neighbouring leaf-traces that enter at this 

 node (Fig. 212). So the xylem of the so-called vascular bundle of 

 Eqitisetitm consists of three strands, two of which are lateral and cauline, 

 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 difference between 

 it and the bundle of a Phanerogam. The general conformation of the 

 vascular tissue at the node, according to the 

 above description, is shown in the diagram 

 (Fig. 213). 



The tracheides in each strand are very 

 few, and consequently it is difficult to deter- 

 mine the direction of their development. 

 However, as regards the leaf-trace and the 

 carinal strand it appears clear that they are 

 not exarch but endarch, or perhaps slightly 

 mesarch on the adaxial side. The lateral 

 strands, as a whole, are differentiated later 

 than the carinal strand, but they do not 

 seem to be a continuation of its centrifugal 

 development. On the contrary, in E. gigan- 

 teum, 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 in this 

 species, because no incompletely differentiated portions of the stem were 

 available : so the question must remain at present undecided, although 

 the mature structure certainly gives a strong impression of centripetal 

 development. 



It is suggested by Gwynne-Vaughan that the lateral xylem-strands in the 

 vascular bundles of the existing species of Eyuisctitm may perhaps be 

 taken to represent the last remnants of a primitive central mass : this 

 would be in entire agreement with their apparently centripetal develop- 

 ment, and in particular with their cauline course. The probability of 

 this suggestion can best be gauged by comparison with the fossil Equisetales, 

 and with other Pteridophytes. For Calamites the case has been succinctly 



I 



FIG. 21 ;. 



Diagram constructed by Mr. Gwytine- 

 Vaughan to represent the conformation 

 of the vascular tissue at a node of Equi- 

 setuin. Above and below the node the 

 groups of three strands, the median leaf- 

 trace and the lateral cauline strands, arc 

 shown. In the centre the entry of a leaf- 

 trace is shown, and its passage inwards 

 to take its place nearest to the pith, 

 while the cauline strands are lateral, and 

 form the external parts of the composite 

 vascular bundle. 



