1902] 



EVOLUTION OF VASCULAR TISSUE OF PLANTS 



219 



m 



hammm, Poroxylon, and others, each concentric stele compos- 

 ing the cylinder of Medullosa underwent a reduction of the tissue 

 on its inner side, whereby the phloem and the whole of the 



secondary wood of that side vanished, leaving behind 

 is known as a mesarch bundle, consisting of a central 

 external protoxylem, with a group 

 of primary metaxylem on both its 

 inner and outer side, or solelv in 

 the former region ; as a rule, a 

 greater or less development of 

 secondary wood and phloem occurs 

 on the outer side of each bundle. 

 The secondary wood usually extends 

 across the gaps separating the bun- 

 dles, toform a continuous solid cvlin- 

 der enclosing a pith, thus tending 

 eminently to obscure the original 

 and primitive condition of a ring 



what 



or an 



ph 



/ 



— X 



p 





2 



Fig. 5. — The primitive mesarch 

 type : reference letters as before ; 

 x^, secondary xyleui. 



ot reduced concentric strands i^fig. 5). That these latter are 

 each of them really a vestige of a concentric bundle, such as 

 that of Medullosa, is shown very well in the case of Lygifwden- 

 dron Oldhamiiim by the arc- or horseshoe-shaped outline of the 

 bundles, as also by the occasional occurrence of secondary xylem 

 and phloem on the i?i?ier sid^ of some of the bundles, exhibiting 

 an inverted orientation, as if to remind us that the concentric is 

 the original type of structure of these strands. Dr. D. H. Scott^ 

 supposes that the stem cylinder of Lyginodendron has been 

 derived from the single solid stele of Heterangium (another of 

 the Cycadofilices from the Coal-measures), by means of the 

 extinction of the whole of its central solid xylem with the excep- 

 tion of the primary tracheides on the immediate inner side of 

 each protoxylem group. But the w^riter fails to see much ground 

 for holding this view. The very compact stele of Heterangium, 

 with the neatly and evenly circular outline of the inner limits 



*^ Scott, D. H., Studies in fossil botany, p. 340, 1 900. 





