Rosaceæ. 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 few 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-ofï 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/^. 

 The outer layers of the cortex are somewhat collenchymatous 

 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 consist- 

 XXXVII. 2 



