33^ 



PRIMARY ARRANGEMENT OF TISSUES. 



fluitans. The structure is similar to that of the preceding species, but simpler, ^x trachea 

 with annular and loosely spiral fibres, bordering on an intercellular passage in which the 

 letter is written, g wider tracheae with densely spiral and reticulated wall, j- five relatively 

 large sieve-tubes between narrow cambiform cells. The whole bundle is surrounded 

 by narrow elongated prismatic cells, showing non-lignified cellulose walls, and these pass 

 over gradually on the outside into large-celled parenchyma with abundant starch. As 

 regards the surrounding layer marked a it remained doubtful whether it has an indi- 

 cation of endodermal structure on those walls which are radial with reference to the 

 bundle. 



Figs. 154 and 155, from the fully elongated hypocotyledonary stem of Ricinus communis 

 (from Sachs' Textbook), represent that form of bundle, with radially disposed xylem, 

 which predominates among Dicotyledons, together with the initial stages of the secondary 

 growth in thickness, which here follows immediately on the formation of the primary 

 bundles. In the cross-section (Fig. 154)^,*,? are the rows of vessels alternating with rows of 

 thick-walled cells, beginning at the inner edge of the bundle with the primitive elements, 

 which are distinguished by the thick shaded walls, t narrower, g wider pitted vessels. 

 y phloem consisting of sieve-tubes, cambiform and delicate parenchyma, with three bundles 

 of sclerenchymatous fibres on its outer border, at b. On the boundary between xylem 

 and phloem, the formation of the zone of cambium and secondary growth {c-c) has begun 

 by means of tangential divisions, and is continued, starting from the sides of the vascular 

 bundle, over the parenchymatous zone cb, cb: cf. Chap. XIV. m parenchyma of the 



Fig. 153. 



pith, r of the outer cortex. Between the layers containing the letters b and r is the 

 parenchymatous sheath containing starch-grains (Starch-ring, Chap. IX). Fig. 155 is the 

 radial longitudinal section through a bundle of the same structure as Fig. 154. The 

 letters r, b, c, m indicate the same things as in the former figure, p bands of parenchyma 

 from the phloem, s innermost and narrowest spiral vessel. / a wider one. / scalari- 

 form reticulated vessel. / mature vessel with bordered pits ; at q the perforated trans- 

 verse wall between two elements. / a pitted vessel still immature, the borders of the 

 pits not yet developed, h, h' thick-walled cells of the xylem ; on the wall of t and /' the 

 boundaries of cells which have been cut through are visible, h", h'" narrow tracheides(?). 



