68 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



Cortex. — In the fully-developed rhizome the cortex consists of 

 three zones — an inner, outer, and middle zone. The inner layer, 

 probably composed of delicate parenchyma, never seems to have 

 resisted decay in any specimens yet discovered. The middle 

 layer consists of radially-arranged dense prosenclrvmatous tissue 

 with small lumen. The dividing line between the middle and 

 outermost zones is very clearly seen in longitudinal sections — 

 the long, narrow, prosenchymatous fibres standing in marked 

 contrast to the outer layer of isodiometric, firm-walled paren- 

 chyma. 



This so-called middle zone is the product of a meristem layer 

 (phellogen) which appears at an early period, and lies between 

 the inner and outer zone. On the inner side of the phellogen, 

 the middle bark (phelloderm) is developed. Probably additions 

 were also made to the outer bark or zone, but to a less extent. 

 This outer zone differs in structure from the cork layer (phellem) 

 of recent plants, being a thin, though firm-walled, parenchyma. 

 To the exterior surface of the outer bark were attached the 

 rootlets. 



Rootlets. — The rootlet bundles spring from the first-formed 

 small vessels of the xylem cylinder, and pass out through the 

 basal angle of the primary medullary rays. At first they consist 

 of a single vessel, but as additions are made to the exogenous 

 zone, special radial lamina of small vessels are added to the 

 rootlet bundle, which not only increases vertically but also 

 laterally. The rootlet bundle therefore holds a certain propor- 

 tional size to that of the vascular cylinder. 



The increase in the volume of the rootlet bundles is well seen 

 in transverse sections of the xylem cylinder and in serial longi- 

 tudinal sections. Immediately after leaving the vascular cylinder, 

 the rootlet bundle is generally more or less deltoid in section, with 

 the angles rounded. Their size varies, but holds a certain rela- 

 tion to the size of the axis with which they are connected. The 

 earlier-formed vessels, or tracheides, are generally placed without 

 any order, but those subsequently added are usually radially 

 arranged. 



The rootlets of Stigmaria are long cylindrical bodies, varying 

 in length according to the age of the parent rhizome. Messrs. 

 Binney and Harkness state they traced them for over a distance 



