366 ARTHUR J. MASLEN [may 



modern forms. It is characteristic of the primary vascular bundles in 

 an ordinary stem for the xylem to differentiate entirely in a centrifugal 

 manner, i.e., for the first formed elements (protoxylem) of the primary 

 xylem to occupy the inner limit of the bundle. But the examination 

 of sections of the stem of Lyginodendron at once shows that the smallest 

 first-formed elements of the xylem, lie neither at the outer nor at the 

 inner limit of the primary xylem, but occupy an intermediate position, 

 nearer the outer than the inner side. This being so, it follows that the 

 primary wood has developed both outwards (centrifugally) and inwards 

 (centripetally) from the protoxylem. To this type of structure Solms- 

 Laubach has applied the term mesarch, while Williamson and Scott 

 describe the same structure as mesoxylic. 1 



The interest of these mesarch collateral primary bundles in Lygino- 

 dendron, lies in its comparison with living forms. Until quite recently 

 mesarch bundles were believed to be peculiar to the foliar bundles of 

 Cycadeae, the stem bundles being of the ordinary type. However, in a 

 recent paper in the Annals of Botany, by Dr. Scott, we are told that — 

 " The frequency of mesarch structure in the stems of Palaeozoic plants 

 showing unmistakable affinities with Cycadaceae, renders it highly 

 probable that this character formerly extended to the stem, as well as 

 the leaf, of the Cycadaceae themselves. This consideration suggested a 

 renewed investigation of the anatomy of recent cycads, in order to 

 ascertain whether some vestiges of mesarch structure might not still 

 survive in the bundles of the stem." 2 Of the axial organs of cycads, 

 however, only the peduncles of certain forms (Stangeria paradoxa, 

 Bowenia spectabilis, Zamia Loddigesii, Ceratozamia mexicana, C. latifolia) 

 were found to still exhibit the structure in question. 



The secondary xylem of Lyginodendron (which is present with the 

 rarest exceptions in all known stems) was formed by a normal cambium 

 forming wood internally and phloem externally, and in well-preserved 

 specimens the actual cambium cells are quite evident. In general 

 characters the secondary xylem is quite similar to that of the stem 

 of cycads ; it consists of pitted tracheides and ray-parenchyma only, 

 and the xylem is split up into vascular plates only a few tangential 

 rows in thickness by the numerous medullary rays. 



Numerous structural anomalies in the stems of Lyginodendron have 

 been noted by the authors in the rich material with which they have 

 worked. The most interesting of these variations is that of the 

 appearance of a secondary meristem on the outer border of the pith, 

 and giving rise to wood and phloem with inverted orientation, i.e., with 

 xylem outside and phloem inside. This type of anomaly is exactly 

 that found in the stems of some modern plants, such as Tecoma 

 grandiflora, a root-climber belonging to the Bignoniaceae, and its 



1 Loc. cit. p. 713. 



2 "The anatomical characters presented hy the peduncle of Cycadaceae," Annals of 

 Botany, vol. xi. p. 401. 



