INCREASE IN SIZE OF STEMS. 



137 



primary bundles brings the cambium into the form of a circle, 

 sometimes broken, but frequently uninterrupted. If the cam- 

 bium circle is substantially unbroken, a new compact ring of wood 

 is laid upon the wood 



of the primary bun- B A. 



die, and a new ring 

 of liber forms within 

 the older liber. This 

 action may be indefi- 

 nitely repeated ; and 

 in a climate where 

 there are notable dif- 

 ferences either in tem- 

 perature or moisture 

 between the seasons, 

 the concentric circles 

 are records of the 

 years. 



If the primary bun- 

 dles are not in con- 

 tact, the new wood 

 added }*ear by year 

 simply increases the 

 size of the wedges at 

 their outer part. 



393. New bundles 

 may be intercalated 

 directly between those 

 already present, and 

 grow in much the 

 same manner as the 

 primary ones ; or the} 1 

 may arise at new points of activity and produce great changes 

 of form. In the same way tertiary changes and those of a 

 higher order may follow the secondary ones, giving rise to stems 

 which have a very complicated structure. The most puzzling 



FIG. 112. Diagrams showing the secondary increase in thickness of a normal dicoty- 

 ledonous stem : 11, cortex ; p, phloem with three fascicles of hard-bast fibres ; x, xylem ; 

 M, pith. A shows only primary structure; B exhibits formation of the ring of cam- 

 bium ;/c, fascicular cambium; ic. inter-fascicnlar cambium; f>, l>,b. fascicles of hard 

 bast; C, at the end of the year, affer the formation of the secondary tibro- vascular ring; 

 p, liber ; fh, secondary wood of the bundle; ifj>, inrer-fascieular liber; ifh, inter-fas- 

 cicular secondary wood; the entire ring is subdivided by medullary rays of different 

 lengths. (Sachs.) 



