154 



BOTANY 



PART I 



Sapindaceae. This anomalous condition arises from the unusual 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, in- 

 dependently of the others, then gives rise to wood and bast (Fig. 162). A very 

 peculiar structure is exhibited by many lianes of the Bignoniaceae, the wood of 

 which is cleft by radially projecting masses of bast (Fig. 163). The primary stem 

 of the Bignoniaceae shows the oi'dinary circular arrangement of the vascular 

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

 manner, and an inner, normal wood cylinder of axial wood is formed. Such 

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

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



Fig. 162. — Transverse section of 

 the stem of Serjania Laruot- 

 te.ii.na. sk, Part of tlie rup- 

 tured sclerenchymatous ring 

 of tiie pericycle ; I and I*, bast 

 zones; Ig, wood; m, medulla. 

 (X 2.) 



Fig. 163. — Transverse section of the stem of one of the 

 Bignoniaceae. (Nat. size.) 



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 

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

 the outer so-called periaxial wood (Fig. 163). The originally complete cambium 

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

 Itrojecting 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. 



The Formation of Knots.— Tlie knots or streaks which add so greatly to the 

 technical value of certain woods depend on an unusually bent or interwoven course 

 of the elements of the wood. Their origin is due to the stimulus of wounding, 

 to the effects of parasites, the pressure exerted by lateral brandies which are in- 

 creasing in thickness, or to altered cambial activity. Larger knots are produced 

 by the origin of numerous adventitious buds, especially after wounding ; a finer 

 marking by the widening of the medullary rays, whicli may tlien appear circular 



