150 GENERAL STRUCTURE OF ROOT AND STEM 



The Nucleus-sheath. — The endodermis of such plants is commonly 

 known as a Nucleus-sheath (b). 



Polystelar Stems. — Finally, we note that in many plants, represented 

 among drugs by the ferns, the stem possesses a number, usually definite 

 for the species, of vascular bundles, or groups of them, each invested by 

 its own endodermis, each being thus a stele. Such stems are, therefore, 

 called Polystelar. In such plants no epidermis is developed, the 

 hypoderm, developed from the periblem, being superficial. 



The Bark. — Its Nature. — The Bark is everything external to the 

 cambium. It has been proposed to remove the word "bark" from 

 common language, or to ignore its fixed common meaning, and to 

 convert it into a technical name for the bork. Experience with English- 

 speaking people leaves no hope that they will' consent to give up a word 

 employed so widely and in such important ways, and its technical use 

 can apparently result only in the introduction of a confusion, which is 

 more wisely avoided by the coining of some new name, if that of bork 

 is seriously objectionable, which does not appear to be the case. 



Im/portance of the Bark in Pharmacognosy. — Viewed from the stand- 

 point of pharmacognosy, the bark, especially when detached from the 

 remainder of the root or stem, is one of the most important portions 

 of the plant. As has been seen, it is not a simple structure, but develops 

 in part from the plerom, as well as from the periblem, and bears fre- 

 quently, although this is not true of any detached medicinal bark, the 

 epidermis as well. 



Layers of the Bark. — In practice, the bark is commonly differentiated 

 into three layers — the Endophloeum, that portion resulting from the 

 plerom; the Mesophloeum, which is either the primary cortex, or the 

 products of a phellogen developing external to the endophloeum, or 

 both when they exist together; and the Exophloeum, consisting of a 

 primary periderm. If, as is not the case in any medicinal bark, the 

 epidermis persist, it will form the exophloeum. It has already been 

 made clear that a bark can come to want successively its exophloeum, 

 mesophloeum, and even the outer part of its endophloeum, as is seen in 

 some Cinchona bark, from old trees. 



The study of barks includes a close examination of the cellular 

 elements, as a preparation for which histological work is absolutely 

 necessary. Examination of its gross characters invoh'es, as the more 

 important features, its extreme and a\erage thickness, its manifest 

 layers, as seen with a lens on transverse or radial section, their relative 

 thickness, color, markings, consistency as shown by fracture, their 



