THICKENING IN DRAOENA. 119 



growing tissue produced on the inner side of this cambium-ring 

 are numerous vascular bundles in all stages of development. The 

 youngest consist of a group of thin-walled cells, the oldest are 

 already perfect at their inner edge, while the thin- walled outer 

 edge, i.e., the portion towards the periphery, is still immersed in 

 the cambium-ring and in course of development (the outer /"). 

 From the position where the vascular bundles appear crowded 

 together, and the cells lying between them have acquired a radial 

 arrangement, the tissues are Secondary tissues produced by the 

 activity of the cambium-ring. External to the cambium-ring, and 

 also resulting from its activity, is the Secondary cortex (cr), 

 consisting of rounded cells. Between these, especially in the 

 inner part of this cortex, occur single cells, in which lie fine 

 needle-like crystals, closely packed together into a bundle (r). 

 These are the bundles of so-called Raphides, composed of oxalate 

 of lime. They are here seen in end view. Individual raphides- 

 containing cells are sure to be opened by the razor in cutting, 

 and the fine needles will therefore be found lying scattered over 

 the section. The rest of the cortical cells contain chlorophyll 

 grains. In the cortex are also visible occasional circular cross- 

 sections of bundles (/'") which are passing outwards into the 

 leaves. They are the so-called leaf -traces. Next to the 

 secondary cortex comes a thick layer of thin-walled, colourless 

 cells, arranged radially (Z), which on its outer side passes over 

 into a brown, less regular tissue. This is the Cork-layer, con- 

 sisting of colourless developing cork-tissue in its more internal, 

 but of old irregularly elongated and discoloured cork-tissue in its 

 outer parts. The nature and mode of development of cork we 

 will study hereafter. 



If a thin section is treated for some time with chlorzinc iodine 

 very beautifully differentiated figures are obtained. Examined 

 with a high power the rounded ground-tissue elements have 

 violet walls, on which the numerous pits show as colourless 

 spots. In the vascular bundles of the central part of the stem 

 the centrally placed bast is stained violet, the surrounding xylem 

 is yellow-brown to a muddy-brown. The bundles are concentric, 

 with external wood ; a condition much more common in rhizomes 

 than in aerial stems. The bundle is surrounded by a generally 

 double layer of lignified elements stained brown, and with simple 

 rounded pits ; these form the sheath, and make the bundle air- 

 tight to the ground-tissue. In the vascular bundles of cambial 



