408 



BOTANY 



B 



are transformed into the permanent elements of the wood and bast. 

 Connecting the cambium zones of the separate primary bundles, 

 there is formed a similar zone in the primary medullary rays, or the 

 ground-tissue between the primary bundles. There is thus devel- 

 oped a complete cylinder, composed of "fascicular" and "inter- 

 fascicular " cambium. The endodermis is not always clearly distin- 

 guishable. In such stems the primary cortex early disappears, and 

 is often replaced by a secondary bark developed through the activ- 

 ity of a special meristem, or " Periderm," developed in the cortex. 

 Very commonly part of the periderm assumes the character of " Cork- 

 cambium," or "Phellogen" (Fig. 386). 



As in the Gymnosperms, the tracheary tissue of the wood (Fig. 

 385) is interrupted by medullary rays (ra), which may be continued 



into the phloem. These are 

 sometimes very conspicuous, as 

 in the wood of various species 

 of Birch and Maple, where the 

 shining flakes, seen in radial 

 sections of the wood, are the 

 medullary rays. 



Anomalous Thickening. 

 While the secondary thickening 

 of the stem in Dicotyledons, 

 usually results in regular con- 

 centric growth-rings, like those 

 of the Conifers, there are & 

 good many exceptions. Some- 

 times a second cambium ring 

 is developed inside the ring of 



wood (Tecoma radicans) (Fig. 



Dicotyledons. A, Podophyllum pelta- 00 . x , . i_ ji 



turn. Numerous scattered vascular *) or each primary bundle 



bundles, vb (X 3). B, Tecoma radicans, may be surrounded by a sepa- 



secondary wood, *, formed inside the pate cambium ring g^g rise 

 primary wood, x 1 (X 2). C, Bignonia. 

 Unequal growth of wood, resulting in 



cor 



FIG. 387. Anomalous stem-structure 



to several masses of wood sur- 



four principal wood-masses with alternat- rounded by a common cortex, 

 ing masses of phloem (x 4) ; cor, cortex; Thig especially characteristic 

 /, bast-fibres; cam, cambium; ph, J 



phloem; x, xylem; p, pith. 1 some woody climbers (e.g. 



Serjania). Other variations 

 occur, but must be passed over here. See De Bary (2). 



The Bark 



The bark (Fig. 386) of most woody Dicotyledons consists of two 

 portions, the inner bark, or bast, belonging to the vascular bundles, 

 and the outer bark which belongs to the cortex. This outer bark is 



