374 BOTANICAL GAZETTE [November 



not centripetal, as one would interpret from the treatise cited. It has 

 been shown that the transition from endarch to exarch is carried 

 through in the metaxylem, both centripetal and centrifugal wood 

 occurring long before secondary wood is developed. It is very hard 

 to tell where protoxylem ends and metaxylem begins, and where 

 metaxylem ends and secondary wood begins. The separation of 

 metaxylem from secondary wood by means of the thin-walled par- 

 enchymatous cells is not a safe guide in the region of transition, and 

 only becomes well marked above this region, where the separation 

 of secondary from primary wood is well marked by the thin-walled 

 parenchymatous cells, as noted by many authors. The metaxylem 

 above this point gets to be relatively very bulky, while the secondary 

 wood is represented merely by a few elements. These were the few 

 pitted cells which presented to Von Mohl (i) a situation without a 

 counterpart, now known to be the herald of the secondary wood 

 which has gradually crept up into the petiole, a transformation 

 begun in its early ancestry, according to Scott, the "new wood" 

 driving out "the old," the former being the only wood present in the 

 higher gymnosperms and angiosperms. 



Matte (6) argues in very much the same way as does Mettenius, 

 an argument which would hold good if the protoxylem and a large 

 part of the metaxylem were left out of account. Matte says that 

 the bundles of the cotyledons have centripetal wood throughout, 

 centrifugal wood only below the upper region of the petioles, and 

 centripetal and centrifugal wood equally well developed at the bases 

 of the petioles. In the present investigation it has been pointed out 

 that there is no centripetal wood at first, and that it gradually increases; 

 while the centrifugal wood diminishes in bulk to the upper extremities, 

 where it is less than the centripetal but does not disappear entirely. 



Matte further says that what has been said of the cotyledonary 

 traces applies equally well to the foliar traces, except that there is no 

 trace of centrifugal xylem in the youngest leaves. It can be shown that 

 as soon as there are enough xylem elements to show the direction of 

 development the centrifugal wood is present, but gradually disappears, 

 and the centripetal wood increases in the same ratio, until in the upper 

 extremities there is only centripetal wood. This also agrees with 

 Bertrand and Renault, except that their statement that centripe- 



