MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 



369 



367. In the Stems of dicotyledonous Phanerogams, on the other hand, 

 ~we find a method of arrangement of the several parts, which must be re- 

 garded as the highest form of the development of the axis, being that in 

 which the greatest differentiation exists. A distinct division is always 

 seen in a transverse section (Fig. 252) between three concentric arese, 

 the pithy the wood, and the bark; the first (a) being central, the last (b) 

 peripheral, and these having the wood interposed between them, its circle 

 1>eing made up of wedged-shaped bundles (d, d), kept apart by the bands 

 (c, c), that pass between the pith and the bark. The pith (Fig. 253, a) 

 is almost invariably composed of cellular tissue only, which usually, pre- 

 sents (in transverse section) a hexagonal areolation. When newly formed 

 it has a greenish hue, and its cells are filled with fluid, but it gradually 

 dries-up and loses its color; and not unfrequently its component cells are 

 torn apart by the rapid growth of their envelope, so that irregular cavities 

 are found in it; or, if the stem should increase with extreme rapidity, it 

 becomes hollow, the pith being reduced to fragments, which are found 

 adhering to its interior wall. The pith is immediately surrounded by a 



Transverse Section of Stem of Rham- 

 nus (Buckthorn), showing concentric 



layers of wood. 



Portion of the same, more 

 highly magnified. 



delicate membrane consisting almost entirely of spiral vessels, which is 

 termed the medullary sheath. 



368. The woody portion of the stem (Fig. 253, I, b), is made up of 

 woody fibres, usually with the addition of ducts of various kinds; these, 

 however, are absent in one large group, the Coniferce or Fir tribe with 

 its allies (Figs. 258-260) in which the woody fibres are of unusually large 

 dameter, and have the peculiar markings already described ( 361). 

 In any stem or branch of more than one year's growth, the woody 

 structure presents a more or less distinct appearance of division into 

 concentric rings, the number of which varies with the age of the tree 

 (Fig. 254). The composition of the several rings, which are the sec- 

 tions of so many cylindrical layers, is uniformly the same, however 

 different their thickness; but the arrangement of the two principal ele- 

 ments namely, the woody fibre and the ducts varies different species: 

 the ducts being sometimes almost uniformly diffused through the whole 

 layer, but in other instances being confined to its inner part; while in 

 other cases, again, they are dispersed with a certain regular irregularity 

 (if such an expression may be allowed), so as to give a curiously figured 

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