35] 



CHAPTER II. THE TISSUES. 



139 



tangential division, those lying on the inner side of the cambium, 

 jtre transformed into the elements of the wood (Fig. 108 B /i 2 ), 

 those on the outer side, into the elements of the bast, while the 

 cells of the intermediate zone continue to be capable of dividing. 

 The activity of the cambium thus gives rise t6 secondary wood 

 and secondary bast, as distinguished from the primary con- 

 stituents of the bundle, which existed previously to, and indepen- 

 dently of, the activity of the cambium. The primary wood of the 

 bundle is thus the innermost part of it, and the primary bast the 

 most external. 



Not only does the fascicular cam- 

 bium add secondary wood and bast 

 to the primary bundles of the stem, 

 but the interfascicular cambium 

 generally forms (except Cucurbita- 

 cese, Aristolochia, and some other 

 plants, where it only forms conjunc- 

 tive tissue) new secondary bundles 

 between the primary, and in this 

 way a compact ring of wood and of 

 bast is formed. These secondary 

 bundles are of course destitute of 

 protoxylem and protophloem. 



In roots the secondary vascular 

 tissue is developed in essentially the 

 same manner as in the stem; the 

 wood inwards, the bast outwards, 

 from the cambium-layer ; and the 

 same forms of tissue are produced. 

 It is, however, only in certain cases 

 (e.g. Taraxacum, Rubia, Taxus, Cu- 

 pressus, etc.) that the cambium of 



the root produces Wood internally, bundles. B Transverse section of an 



and bast externally, over its whole older root of * be * ame plant ' which 



J ' is growing in thickness : 



surface, so that a complete ring of 

 secondary vascular tissue is formed : 

 in most cases secondary vascular 



tissue is formed Only Opposite to wood-bundles. (Slightly magnified; 



the primary bast-bundles, whereas, ' 



opposite to the primary wood-bundles, the cambium produces only 

 ground-tissue, thus giving rise to broad medullary rays opposite 

 to these bundles (Fig. 110). 



FlG . no.-^ Transverse sectiou of 

 young root of Phaseoius muiti/ion** . 



secondary 

 bast ; k periderm : the four rays ex- 



tendin & to near the centre consist 



of secondary ground-tissue, and cor- 

 respond in position to the primary 



