138 



BOTANY 



position of the primary vascular bundles, which are not arranged in a circle but 

 form a deeply lobed ring ; so that, by the development of interfascicular cambium, 

 the cambium of each lobe is united into a separate cambium ring. Each of these 

 rings, independently of the others, then gives rise to wood and bast (Fig. 152). 

 An even more peculiar structure is exhibited by many Lianes of the Bignoniaceae, 

 the wood of which is cleft by radially projecting masses of bast (Kg. 153). The 

 primary stem of the Bignoniaceae shows the ordinary circular arrangement of the 

 vascular bundles. Wood and bast are at first produced from the cambium ring in the 

 usual manner, and form an inner, normal wood cylinder of axial wood. Such 

 normally formed axial wood cylinders are common to many, otherwise abnormally 

 developed Lianes. The cambium ring of the Bignoniaceae, after performing for a 

 time its normal functions, begins, at certain points, to give off internally only a 

 very small quantity of wood, and externally a correspondingly large amount of 



Pig. 152. — Transverse section of 

 the stem of Serjania Lanioi- 

 teana. sk, Part of the rup- 

 tured sclerenchymatous ring 

 of the pericycle ; I and £*, bast 

 zones ; Ig, wood ; m, medulla. 

 (X2.) 



Fig. 153. — Transverse section of the stem of a Bignonia. 

 (Nat. size.) 



bast. As a result of this, deep wedges of irregularly widening bast project into the 

 outer so-called periaxial wood (Fig. 153). The originally complete cambium 

 becomes thereby broken into longitudinal bands, which are broader in front of the 

 projecting wood than at the apices of the bast wedges. As the periaxial wood 

 is always developed from the inside, and the wedges of bast from the outside of 

 their respective cambium bands, they extend past each other without forming any 

 lateral connection. 



Secondary Growth of Monocotyledons. — As we have already 

 seen, Palms grow in thickness only as the result of the increase 

 in size of the individual tissue elements. There are, however, certain 

 monocotyledonous plant families and genera, especially Dracaena, Yucca, 

 Aloe, and the Dioscoreaceae, in the stems and roots of which a cambium 

 ring is developed. As in such cases, the cambium ring generally arises 

 in the pericycle, outside the scattered vascular bundles and from the 



