464 ME. W. C. WORSDELL ON THE COMPARATIVE 



structure of the same parts in fossil plants identical with or closely allied to the 

 Medullosese. The formation of the successive vascular rings would have been brought 

 about as follows: — The original structure in the Medullosese "consisted," to quote my 

 former words, "of rings or layers of concentric vascular strands. ... As time x^ent on, 

 and greater specialization in the conducting-tissues arose, and a need for the formation 

 of a larger amount of this tissue became urgent, the cambium of the inner portion of 

 each such concentric strand gradually became less and less functional, that of the outer 

 portion, on the contrary, more and more active, so that a much larger quantity of wood 

 and bast became formed on the outer side of each strand than on the inner side, for this 

 was the surest and best means of economizing both space and expenditure in the building 

 up of an efficient conducting-tissue for the stem. The result is, finally, the structure, 

 as we at present know it, in the stem of Cycas, JEncephalartos, and Macro zamia.'" 



Dr. D. H. Scott, in his extremely interesting and valuable paper on the structure of 

 Medullosa anglica, holds the above view to be " fallacious," on the ground that " the 

 primary ground -plan of the stem-structure of a polystelic Medullosa was fundamentally 

 different from that of the monostelic Cycadaceae." For myself, however, and with all 

 deference to the authority and experience of the author just quoted, the primary ground- 

 plan of the two structures cannot be regarded as " fundamentally different," inasmuch as 

 I hold (which Dr. Scott does not) that in the " monostelic " genera — Stangeria, Cerato- 

 ::cnnia, Zamia, Boicenla — there is evidence for the derivation of the central cylinder of 

 the stem from a ring of steles or concentric bundles. What I regard as a relic of the 

 ancestral structure is here found in the most primitive cauline organ of the plant, viz., 

 the peduncle, and consists of tracheides occurring in considerable numbers on the inner 

 or ventral side of the protoxylem of each bundle, and, what is more remarkable still, 

 occasionally accompanied, as was seen in the case of Stangeria^ by phloem on the inner 

 side of these tracheides, /. ^., on the side nearest the centre of the peduncle. Once or 

 twice an entire bundle, with inverted orientation, was observed on the ventral side of, 

 and in close proximity to, one of the bundles of the cylinder. This centripetal xylem of 

 the central cylinder of the peduncle of the four genera mentioned was first discovered 

 by D. H. Scott, and has been carefully and minutely described by him in a valuable 

 paper*. 



The curved, in some cases almost horseshoe-shaped. in one or two cases perfectly 

 concentric, contour of the bundles composing the cylinder in the lower part of the 

 peduncle at once suggests the idea of their derivation from concentric bundles or steles. 

 The same may also be said of the bundles of the cylinder in the axis of the male 

 cone of Ceratozamia niexicana, Brongn. f, and C. latifolia, Miq. %. Here also occurs 

 another piece of evidence for the Medullosean ancestry of these plants, in the form 

 of concentric and collateral bundles in the pith of the same size as the bundles of 

 the cylinder, as also much smaller collateral ones of inconstant orientation. That all 

 these bundles are vestiges of an ancestral condition is shown by their inconstant structui'e 



* Scott, Aim. Bot. vol. xi. 1897, p. 403. 



t Thibout, Recherches sur TAppareil Male des Gymnospermes, 1896. 



t Worsdell, Ann. Bot. vol. xii. 1898, p. 232. 



