Il8 PLANT ORGANS OR PARTS OF PLANTS. 



leaf scars are situated like cups straight upon the upper 

 side of the rhizome while the roots are found beneath. 

 In an oblique rhizome the disposition is the same, save 

 that the leaf scars, instead of being equal, are more or less 

 tilted, one side being deeper than the other. In vertical 

 rhizomes the scars are at the apex and the roots com- 

 pletely encircle the rhizome. 



The microscopical structure varies. In pharmacog- 

 nosy three types of rhizome are met with : the Vascular 

 Cryptogams, the Monocotyledons, and the Dicotyledons. 



The rhizomes of the vascular cryptogams are each laws 

 unto themselves, and as the pharmacist need study but 

 one, the description will here be found under Aspidium. 



The rhizomes of monocotyledons are more uniform in 

 their structure. In general they consist of an external 

 parenchymatous layer, in which a few fibro- vascular 

 bundles may be scattered, and an internal cylinder of 

 parenchymatous tissue in which a number of fibro- 

 vascular bundles are to be found. Separating the two 

 is a layer (or layers) of specially modified cells, the pro- 

 tective sheath or endodermis, the cells of which may be 

 cubical, square, thickened, etc. In general the fibro- 

 vascular bundles are more closely clustered near the en- 

 dodermis sheath and are built up upon either the con- 

 centric or the closed collateral type (no secondary bark 

 as a rule). In general, rhizomes and roots of mono- 

 cotyledons may show almost the same structure. This 

 is rarely the case in dicotyledons. 



The rhizomes of dicotyledons are quite uniform in 

 structure, closely resembling stem structure. In general 

 they consist of a cortical portion in which secondary 

 growth may have taken place to produce cork. The 

 thickness of this cortex varies greatly, and it generally 

 consists of parenchymatic cells; secondary structures 

 may, however, be found therein, sclerotic cells, etc. 



From the central woody portion the cortex is divided 



