174 TA e Botanical Gazette. U une 



• 



ullary cellular expansion caused the vessels of the vascular 

 bundle to form a ring increasing in size, the number of ves- 



sels also increasing correspondingly and changing their re- 

 spective positions. This meristemic process is repeated ' 'until 

 the medulla and its surrounding vascular ring attain to their 

 ultimate magnitude — a condition which was probably coinci- 

 dent with the first appearance of the more external exogenous 

 zone." It is probable that the new vessels are produced cen- 

 trifugally, on the cortical side of the vascular cylinder, though 

 it is possible that some of the young medullary cells assumed 

 a procambial form and were converted into vessels. In any 

 case he considers that the enlargement of the medullary vas- 

 cular cylinder is mainly, if not wholly, effected "through the 

 internal tension occasioned by the subsequent multiplications 

 and expansions of the medullary cells — -a condition that has 

 no existence among the exogenously-grown trees now living. 

 He regards the occurrence of an exogenous growth at some 

 time in the development of all the Carboniferous Lepidoden- 

 dra as more than probable. 



The existence of an exogenous growth among the arbores- 

 cent Lycopods, Gymnosperms, and Cahimarias has long 

 been known. Part XVII of these memoirs has the important 

 office of making known the existence of an exogenous devel- 

 opment among the Carboniferous ferns. The anticipation 

 expressed by the author, in Part IV, that Dictyoxylon (Ly- 

 ginodendron) Oldhamtum, there described as belonging to the 

 paleozoic Proto-gymnosperms, might be identical with th< 

 petioles described in the same memoir as EdraxyloiL and later 

 (Part VI) as Rachiopteris aspera, is now confirmed, and the 

 two are conclusively proved to be trunk and petiole of the 

 same plant. The study is thorough; the steps in the growth 

 of the petiole from the trunk arc observed with the accuracy 

 and minuteness of detail characteristic of the author's former 

 memoirs. The pairs of vascular bundles so characteristic 

 and conspicuous in the middle cortex of L. Oldhatniutn are 

 shown to pass outwards through the outward cortex and be- 

 come the tracheal bundles of the petioles of R. aspera. The 

 clusters of tracheae, in the small stems, which at first formed 

 one united axial cluster, are separated, the space thus pro- 

 duced at the center being occupied by a steadily expanding 

 parenchymatous medulla. This process is accompanied by a 

 corresponding enormous increase in the number of the vascu- 



