ROOTLET BUNDLE. 31 



displayed by them, in common with the roots of living Lycopodiaceous plants and 

 of Ophioglossum. In most of the former and all the latter we only find one xylem 

 plate opposed to, or more or less surrounded by, one phloem element. Hence these 

 living roots are unmistakeably monarch. M. van Tieghem, finding that where a 

 rootlet dichotomises its bundle divides into two, one half going to each of the 

 secondary branches, came to the conclusion that the monarch appearance of such 

 a bundle was due to the fact that it had divided in order to supply two branches, 

 one of which had become abortive. M. van Tieghem's conception seems to be that 

 two xylem and two phloem plates initiated so near to each other that the two 

 xylems blended to form one, and that the two phloem strands did the same. I have 

 never been able to accept this explanation, because of the contradiction which the 

 rootlet bundle of Stigmaria gives to it. De Barry affirms that there is no basis of 

 fact for it. 1 



The history of the development of the xylem plate in Stigmaria makes it clear 

 that it is absolutely monarch. Every stage of that development, whether we 

 study its orientation in the vascular cylinder of the root, its appearance within the 

 root-cortex, or its final structure within the rootlet itself, leads us to the same 

 inevitable conclusion. And the establishment of this conclusion respecting what 

 was indisputably a primaeval Lycopodiaceous rootlet, may react upon our inter- 

 pretation of the same organisation in its living representatives. 2 



The rootlets of Stigmaria, springing from the axial root, always incline more 

 or less, as they grow, towards the growing end of that axis, enclosing a more or 

 less acute angle as they do so. The first formed Tracheids of the young rootlet 

 bundle (/" of PI. XI, figs. 55, 57, and 58) always originate on the side of the 

 rootlet nearest to the growing tip of the root. This relationship is absolutely 

 constant. We have already seen from PI. VI, fig. 9, /, that some of the vessels 

 and Tracheids successively added exogenously to the entire exterior of the xylem 

 cylinder, are prolonged into each rootlet bundle. Such additions to a bundle are 

 always made on the side of it that is turned from the growing tip of the root. Hence 

 in all sections of these rootlet bundles, like PI. V, fig. 16, the vessels /' represent 

 those first formed, whilst / indicates the newest additions. We have here a second 

 absolutely constant relation. The growth in thickness in each rootlet bundle was 

 steadily upwards and outwards from an inner and lower monarch starting-point. 

 The small Tracheids seen at/—/ of Plate IV, fig. 31, instead of being two additional 

 points of orientation of a triarch bundle, are really amongst the latest additions to 



1 ' Comparative Anatomy of the Phanerogams and Ferns,' English translation, p. 561. 



2 M. Renault having observed examples like my figs. 57 and 59, Plate XI, in which a few of 

 the Tracheids, /"", last added centripetally to the rootlet bundle of Stigmaria, were very small, 

 arrived at the conclusion that their relations to the surrounding bundle cylinder were as primary as 

 those of the Tracheids marked/" in my figures 57 and 58 ; in other words, he believes that these 

 bundles are triarch. Their entire history completely contradicts this interpretation. 



