SECONDARY THICKENING. NORMAL DICOTYLEDONS. 



521 



the cambium, remained for a long time extremely obscure, and the accurate repre- 

 sentations given by Th. Hartig as early as 1837 failed to be understood, until Mohl, 

 in 1855^, brought them into deserved honour. For the same reasons later investi- 

 gations often leave much to be desired, and the special anatomy of the bast is but 

 insufficiently treated of by most authors. 



Sect. 162. The main fundamental mass of the medullary rays always consists 

 of parenchyma. The form and arrangement of its cells are identical with, or very 

 similar to those of the adjacent wood. It likewise occurs in the strands as a constant 

 constituent, and, like the parenchyma of the wood, it is usually derived from single 

 or repeated transverse divisions of the tissue-mother-cells, in those portions of the 

 cambium which correspond to the strands, and its original arrangement agrees 



Fig. 210. Fijj. 211. 



Fig. 210. — Cytisus Laburnum, tangential longitudinal section through the innermost layer of bast of the same branch as 

 Fig. 19S, under the same magnifying power as the latter; j members of sieve-tubes, / a sieve-plate lying deeper than the 

 surface of section, >« small medullary ray, two cells in height. The remaining elements are cells of the bast-parenchyma, 

 the origin of which from the transverse division of cambial cells becomes clear on comparison with Fig. 198. 



Fig. 211. — Juniperus communis, small stem. Transverse section through the autumnal wood, bast, and cambium, during 

 the winter's rest (end of September) ; h — h external rows of the autumnal wood, b^ b series of bast-fibres. At x there is only 

 one cambial cell between h and d ; *n — m medullary rays. 



with this mode of development (Fig. 210, comp. also Fig. 198, p. 465); more 

 rarely it arises without transverse division, from longitudinal division only of the 

 tissue-mother-cells, and then corresponds to the intermediate cells of the wood. 



The Sieve-lubes (Chap. V.) are constant, specific constituents of the strands in 

 the normal soft bast of Dicotyledons. They are always accompanied by parenchyma, 

 and in most woody plants are in general so arranged that the sieve-tubes form 

 single, biseriate, or pluriseriate, tangential rows, which may be interrupted by 

 parenchyma, and alternate with tangential rows of the same. The original radial 



' Compare note on p. 172. 



