Rosacea?. 17 



strong, but otherwise the thick cortex is rather loose and 

 perishes early. The central cylinder which is surrounded by an 

 endodermis contains only a lew primary bundles and a homo- 

 geneous pith of long duration. A continuous cambium de- 

 velops during the first period of vegetation a considerable 

 amount of wood as in Pot. palustris. This — the secondary 

 wood — consists, in addition to large vessels, of one-layered 

 medullary rays usually one layer high, and also of about 

 equal quantities of thick-walled stereom-cells and woody 

 parenchyma. Some indistinct annual rings may be distin- 

 guished in the older rhizomes which may attain an age of 

 at least about ten years. The periderm is thick and consists 

 of series of layers of usually 3-layered phelloid separated by 

 one layer of cork. The outer tangential wall of the cork- 

 cells is especially very thick. Some of the older phelloid- 

 cells in rhizomes from several localities had very thick walls 

 which were lignified (Fig. 6, H). These strengthening cells 

 form connecting bands between the vigorous cork-cells in 

 the periderm and probably thereby delay the peeling-off of 

 the latter. There are large intercellular spaces in the peri- 

 derm; these are, however, absent from the inner side of the 

 corky layer. 



The often rather tall flowering shoot is stiffened partly 

 by a strong bast cylinder situated outside the sieve-tissue, 

 and partly by the secondary wood formed by a continuous 

 cambium the peripheral part of which consists chiefly of 

 thick-walled tracheids. The tangential walls of the epidermis 

 are strong, the outer one measured 4.5« and the inner 3.3 fx. 

 The outer layers of the cortex are somewhat eollenchymatous 

 and as the otherwise loose cortex is rather few-layered, the 

 outer stereom-ring is close to the periphery of the stem. An 

 endodermis is present. 



The pith consists of a narrow, small-celled peripheral part 

 formed of living cells only, and a larger central part consi-i- 

 x.xxvji. 2 



