348 MANUAL OF BOTANY 



orientation of the bundle or its positioii in the stele is such that 

 the narrow end of the wedge is directed towards the centre of 

 the stem, and its broad end towards the periphery. In such a 

 bundle the wood and the bast are generally separated by a band 

 or layer of cambium. In some cases the wood and bast in a 

 bundle of this type are in contact with each other, and there is 

 therefore no cambium. Such a bundle is said to be closed, in 

 contradistinction to the form in which all three are present. A 



Fig. 733. 



Fig. 733. Stele consisting of one concentric bundle from Fern stem. en. Endo- 

 dermis. p. Pcricycle, ph. PhloSm. px. Protoxylem. .v. Xylem. 



bundle possessing cambium, which is a meristem, is capable of 

 continued increase in size, and is said to be open. 



Some bundles of this type, occurring especially in Gueurhi- 

 tacece and a few other natural orders, have a second bast 

 bundle on the inner face of the wood. These are called 

 hicollateral. 



The second variety of collateral bundle is not so markedly 

 wedge-shaped. The wood is never separated from the bast by 

 a cambium layer, but tends to curve partially round it as in 

 fig. 732. The orientation of these bundles is similar to that 

 of the first type. 



