RHIZOMES AND BOOTS. 213 



Epidermis or Cork. 



In some monocotyledonous rhizomes (sweet flag) the epi- 

 dermis is persistent and no cork is formed, but in others 

 (ginger) it may be thrown off comparatively early, and 3,11 

 abundant development of cork may take its place. In some 

 drugs the cork cells of the root offer certain points of distinction 

 from those of the rhizome (white hellebore). In dicotyledonous 

 rhizomes the epidermis is usually replaced by a varying number 

 of layers of cork cells. Occasionally the epidermis both of rhi- 

 zomes and roots is found to be provided with hairs ; these may 

 be numerous and persistent (valerian root) or scattered and often 

 broken off (zedoary rhizome). 



Phelloderm. 



The arrangement of the cells of this tissue is the same as that 

 of the cells of the cork; they may be distinguished by their 

 walls, which are not suberised. 



Collenchyma. 



Several rhizomes contain a layer of collenchyma abutting 

 upon the epidermis (or cork) ; it presents the usual characters. 



Cortical Parenchyma. 



In most roots and rhizomes the cortical parenchyma consists 

 of tangentially elongated cells ; sometimes, however, these cells 

 are axially elongated. The cortex of rhizomes is usually tra- 

 versed by leaf traces, and may contain sclerenchymatous cells 

 secretion cells, etc., of considerable diagnostic value. 



Bast. 



The arrangement of the bast in monocotyledonous rhizomes 

 differs materially from that which obtains in dicotyledonous. 

 In the former this tissue usually consists of a few small elon- 

 gated cells, closely adherent to the vessels of the vascular bundle, 

 which may be supported or surrounded by a crescent-shaped 

 bundle of fibres. In dicotyledonous rhizomes the bast forms a 

 much more considerable portion of the drug and of the powder. In 

 many dicotyledonous roots the bast ring contains no sclenchy- 

 matous fibres, but in others such may be present, and they then 

 constitute a valuable diagnostic feature. 



Endodermis. 



Most monocotyledonous rhizomes and roots contain a very 

 obvious endodermis, the cells of which may furnish an impor- 

 tant means of identifying the powder. In dicotyledonous 

 rhizomes it is usually of little importance, and among the 

 dicotyledonous roots those of aconite and valerian are the only 

 drugs in which this tissue is to be detected. 



