66 



ESSENTIALS OF VEGETABLE PHARMACOGNOSY. 



into joints or phytomers. There are 

 hence no regular distances at which it 

 branches, and when buds are produced 

 upon it, as they are in rare cases, their 

 points of origin are not so regulated. 



STRUCTURE OF THE STEM. 



(The following account of stem strnc- 

 ture refers only to the ordinary plants of 

 the flowering class. At its close a brief 

 reference will be made to such others as 

 require attention for the purposes of 

 plmrmncognosy.) 



The history of stem development is best 

 presented by contrasting it with that of 

 the root, which has already been given. 

 The three elementary tissues, dermato- 

 gen, periblem and pleroni. are also found 

 in the young stem structure. The epi- 

 dermis and other tissues of the stem are 

 more variable than the corresponding tis- 

 sues of the root, and the details pertain 

 for the most part to histology and to the 

 special treatment of species or groups. 

 The most important distinctions between 

 the epidermis of root and stem may be 

 mentioned as the presence in the latter of 

 stomata. There is no extra development 

 from the dermatogen at the tip corre- 

 sponding to the root-cap, nor of hairs 

 similarly aggregated to those of the root, 

 although hairs of many forms abound 

 upon the epidermis of the stem. Stem 

 epidermis may consist of one or of several 

 layers, and if of the latter they may be 

 dissimilar in varying degrees. It may be 

 persistent or it may suiTer the same fate 

 til rough the growth of the parts within it, 

 which has already been considered in the 

 case of the root. 



The periblem of the stem develops 

 structures in general similar to those 

 from the root periblem, the most im- 

 lK)rtant distinction being the production 

 of a chlorophyll-layer. A primary cortex, 

 usually somewhat thinner than that of the 

 root, is bounded externally by a hypoderm 

 and internally by an endoderm, and may 

 develop tubes similar to those mentioned 

 as frequently pertaining to the root cor- 

 tex, but, as in that case, no true vascular 

 bundles. The effects of growth within 

 the primary cortex of the root, leading to 

 the formation and casting off of bork, we 

 have seen to be of rare occurrence. In 



common occurrence, so that the entire ac- 

 count which has been given of the devel- 

 opment and disposition of periderm and 



phelloderni may be applied with especial 

 force in the case of the stem. 



The principal differences between root 

 structure and stem structure are found in 

 the developments from the plerom. Al- 

 though, with the single exception among 

 important medicinal stems of the male 

 fern, there is but a single stele, in the form 

 of a central cylinder, yet the develop- 

 ment of its structure is markedly different 

 from that of the root. Leaving out of con- 

 sideration exceptions which are unimpor- 

 tant in pharmacognosy, we find that two 

 distinct types of structure characterize 

 respectively the monocotyledons and the 

 dicotyledons and gymnosperms. The form 

 characterizing the latter tw^o will be first 

 considered. 



Vascular bundles originate in the plerom 

 in the form of a circle, just as in the case 

 of the root, the important difference being 

 that each bundle consists, even in its 

 primary state, of both phloem and xylem, 

 with a cambium between. The typical 

 form is that which in the root constitutes 

 the secondary structure— namely, a bun- 

 dle consisting of xylem within and phloem 

 without the cambium arc, and this consti- 

 tutes what is known as the Open Col- 

 lateral Bundle. Secondary growth here 

 consists in the addition by the cambium to 

 each kind of tissue, and, in almost all 

 cases, the development of new interme- 

 diate bundles and new medullary rays, as 

 has been described in the case of the root. 

 The result is that the general plan of 

 structure attained is identical with that 

 already recorded as ultimately attained 

 by the most highly developed woody 

 roots. There are, however, several dif- 

 ferences which must be noticed. The 

 most important is that the primary xylem 

 bundles do not progress toward and meet 

 one another at the centre, so that there is 

 always left there a cylinder of the funda- 

 mental tissue, constituting the Medulla 

 or Pith, which is connected through the 

 primary medullary rays with the pericy- 

 cle, or, after the disappearance of that 

 and of the endodermis, with the cortex. 

 The whole structure in transverse sec- 



t 



the case of the stem, however, it is of very t^Z ^n^ --— « m transverse sec- 



, uvvcvti, iLisut ver^ tion may now be roughly compared with 



