DIV. I 



MORPHOLOGY 



147 



woody plants is only found in the lianes. In those herbs '^in which the inner 

 portion of the medullary rays between the primary strands of xylem consists of 

 sclerenchyma (cf. p. 95), the interfascicular cambium forms similar tissue on its 

 inner side. 2. The stem as in the first type has a circle of collateral leaf- trace bundles 

 separated by broad medullary rays. Before the primary growth in thickness is 

 completed there arise from the still meristematic tissue of each medullary ray, 

 that now assumes the characters of a cambium, one or a number of small, cauline, 

 intermediate bundles which anastomose 

 tangentially ; the intervening meshes are 

 occupied by narrow primary medullary 

 rays that are spindle-shaped when cut 

 across (Fig. 170). The original medullary 

 rays become filled up in this way in many 

 herbaceous and woody plants. 3. In the 

 transformation of the primary meristem 

 to permanent tissue there arises, instead 

 of a circle of collateral bundles, a vascular 

 tube, which appears like a concentric 

 bundle with a central pith and internally- 

 situated xylem. There is a layer of meri- 

 stematic tissue between the xylem and 

 phloem that later becomes the cambium. 

 The vascular tube may be traversed by 

 very narrow spindle-shaped primary 

 medullary rays, or these maybe completely 

 wanting. This type is found in many 

 trees. 



The primary xylem of the bundles in 

 stems which have undergone secondary 

 thickening projects into the pith. 



2. Formation and Activity of 

 the Cambium in the Root. As 



has been seen (Figs. 160, 161), the 

 strands of xylem and phloem alter- 

 nate in the central cylinder of the 

 root ; they are separated by inter- 

 vening parenchymatous tissue. 

 When secondary thickening begins 

 in such a root cambial layers arise 



internal to the strands of phloem, and between these and the strands 

 of xylem, by divisions taking place in some of the parenchymatous 

 cells ; the cambium forms wood towards the centre and bast towards 

 the outside. These arcs of cambium meet in the pericycle just outside 

 the xylem strands and the cambial ring is completed from -the peri- 

 cycle. The wavy outline of this is shown in Fig. 171-4; by the 

 activity of the cambium in producing new tissues the depressions 

 in the ring are soon evened out (Fig. 171 B). Primary medullary 

 rays are absent from the wood and bast, but secondary medullary rays 



FIG. 171. Diagrammatic representation of the 

 growth in thickness of a dicotyledonous 

 root, pr, Primary cortex ; c, cambium 

 ring ; g', primary vascular strand ; s', 

 primary phloem strand; p, pericycle; e, 

 endodermis; g", secondary wood ; s", second- 

 ary bast; fr, periderm. (AfterSxRASBURGER.) 



