THICKENING IN DRAC^NA. 



117 



A 



V 



\ 



The vascular bundles of the Monocotyledons, if we exclude 

 immaterial modifications, reductions, and amalgamations, are 

 constructed upon the type of the two cases we have here studied, 

 and we need not therefore further study them. 



Thickening Stem of Dracanece. Closed vascular bundles are 

 not capable of increase in thickness, and therefore in the com- 

 paratively few 

 cases where such 

 occurs in the 

 M o nocotyledons, 

 it cannot be 

 brought about 

 through the me- 

 dium of the 

 vascular bundles 

 themselves. This 

 increase of thick- 

 ness results from 

 the action of a 

 Cambium - ring 

 which is found 

 external to the 

 vascular bundles ; 



/N 

 J) 



FIG. 43. Crystals. A, crystal of oxalate 

 of lime enclosed in a cell from the leaf of 

 7m ftorentitw, ( x 240). B-D, figures illus- 

 trating occurring forms of crystals. Ba, 

 Bb, and Z>, seen in optical longitudinal 

 section. C, projection, showing its planes 

 of symmetry. 



and is almost 



wholly confined to the families of Dracaenese and Alomeae, i.e., 

 the so-called "Arborescent Liliaceae," the Dioscorese, and some 

 of the Palms. 



For their study we select as a favourable object the plant so 

 commonly cultivated in gardens and nurseries as Draccena (more 

 properly, Cordyline) rubra, but any available species will equally 

 serve. The plant must be sacrificed for the purpose of the inves- 

 tigation. Examine the cut stem first with the naked eye ; we shall 

 notice externally a brown cork-layer and inside it the green soft 

 cortex, somewhere about ^V inch thick, and inside this the yel- 

 lowish hard tissue of the central part of the stem. Between 

 the two latter tissues lies the cambium-ring, which can be 

 distinguished from the general yellowish tissue of the central 

 cylinder as a lighter coloured ring. 



We now submit a cross-section to microscopical examination, 

 and first with a low power (Fig. 44). We then see, first, in the 

 central (stelar) portion of the stem, a ground-tissue composed of 



