134 



MORPHOLOGY OF TISSUES. 



i 



phloem-bundle, consisting of radial rows of cells. Some of the innermost cambial 

 cells g^g'^ have already enlarged considerably, to form the first vessels of the secondary 

 xylem; these are arranged, in P/iaseo/us, in an annular zone continuous with the 

 inner youngest vessels of the primary xylem-bundles, a structure which is seen 

 nowhere else. Between and in front of these vessels other cambial cells develope 

 into prosenchymatous and parenchymatous wood-cells. Since therefore the cam- 

 bium-layer behind each phloem-bundle is continually producing vessels, wood-paren- 

 chyma, and wood-prosenchyma on its inner side, a four-rayed cross of woody tissue 

 is formed, as is shown in Fig. icy (less strongly magnified). The four arms of this 

 cross correspond in their position — as will be seen from what has been said, and 

 from a comparison with Fig. io6 — to the four primary phloem bundles d, h, b, b. 



Fig. io6. — Transverse section of the primary root of a seedling of Phaseolus miilHfloyns, from the upper thicker 

 part, at the time when the first leaves of the seedling have emerged above the ground and the first lateral roots have 

 already been formed. (The pericambium pc has been drawn too thick-walled.) 



On the outer side of each cambium-layer, but within the primary phloem, a zone of 

 new true bast-fibres (Fig. 107 <5') has already been formed in the secondary phloem. 

 Between the four arms of the secondary xylem and secondary phloem belonging 

 to it, lie four broad rays of tissue, consisting of large parenchymatous cells elongated 

 in the horizontal and radial directions. These four bands lie on the same radii as 

 the four primary xylem-bundles (Fig. 106, />) ; in front of each of these latter a 

 meristem-layer has been formed, which however has not produced xylem and 

 phloem like the cambium behind the phloem-bundles, but only radiate parenchyma 

 to tlie extent required by the growth of the adjacent masses of secondary xylem 

 and phloem. Fig. 107 further shows the tissue which surrounds the primary and 

 secondary phloem in the act of division ; radial rows of cells have been produced 

 here also, and form a layer of cork or periderm k on the outside of the cortex. 



