144 



BOTANY 



I'ART I 



so-called PERIAXIAL WOOD (Fig. 154). 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 re- 

 spective cambium bands, they extend past each other without forming any lateral 

 connection. 



The Formation of Knots. The 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, 



Fin. 153. Transvrise section ol 

 the stem of Serjania Lnniot- 

 tenna. sk, Part of the ru]>- 

 tured .si-lerenchymatous rinj; 

 of the pericycle ; I and I*, bast 

 /ones ; Ig, wood ; m, medulla. 

 <x2.) 



}'i>:. 154. Transverse section of the stem of one <>t 

 the Bignoniaceae. (Nat. size.) 



to the effects of parasites, the pressure exerted by lateral branches 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, which may then appear circular 

 in tangential section, and influence the course of the adjacent elements of the 

 xylem ( 13 ). 



Secondary Growth of Monocotyledons. There are certain morio- 

 cotyledonous 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 fundamental tissue, it is a secondary meristem ; it does not, 

 as in Dicotyledons and Gymnosperms, produce continuously wood 

 and bast in opposite directions, but, instead, only new tissue to the 

 inner side. This later becomes differentiated into closed vascular 

 bundles embedded in a thick-walled parenchymatous ground tissue. 



The Gambia 1 ring in these cases is only active to one side ( IS1 ), and its initial 



