STRUCTURE OF STEM AS COXTRASTE /) WITH Til A T OF ROOT 145 



at ItMifftli uj)()n the younger })()rti()n as not yet having made tlieir way 

 througli the overlying tissues to the surface. As the root first formed 

 is called the Primary, so its l)ranches are called Secondary. 'J'heir 

 structural develoi)ment is a rejx'tition of that of the ])rimary. 



Continuity of Root-growth.- The continuity of growth in the root is 

 uniform — that is, there is no di\ision of it into joiuts or ])hytomers. 

 There are hence no regular distances at which it hranclus, and when 

 buds are produced ui)on it, as they are in rare cases, their points of 

 origin are not so regulated. 



Structure of the Stem as Contrasted with that of the Root. — (The follow- 

 ing account of stem-structure refers only to the ordinary i)lants of the 

 flowering class. At its close a brief reference will be made to such others 

 as require attention for the pur])()ses of ])harmacognosy.) 



The history of stem-devel()j)ment is best presented by contrasting it 

 with that of the root, which has already been given. The three elemen- 

 tary tissues, dermatogen, periblem, and plerom, are also found in the 

 young stem-structure. The epidermis and other tissues of the stem are 

 more variable than the corresj)onding tissues of the root, and the details 

 l)ertain for the most i)art to histology and to the special treatment of 

 species or groups. 



The Epidermis. — The most im])ortant distinction between the ej)i- 

 dermis of root and stem may be mentioned as the i)resence in the latter 

 of stomata, to be studied in connection with the leaf. There is no 

 extra development from the dermatogen at the tij) corresi)onding 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 the latter, 

 they may l)e dissimilar in varying degrees. Rarely it is i)ersistent, 

 being usually thrown off through the growth of the parts within it, 

 as has already been considered in the case of the root. 



The Cortex. — The i)eril)lem of the stem develops structures in general 

 similar to thos(> of the root-pcM-ibU^n, the most imj^ortant distinction 

 being the ])r()(lucti(in of a (■hloro])hyll-hiyer. A ])riniai"y cortex, usually 

 somewhat thinner than that of the root of the same i)lant, is bounded 

 externally by a hypoderm and internally by an endoderm, and may 

 de\-elo]) tubes similar to tlio-e mentioned as fre(iiientl\- ])ei-taining to 

 the root-cortex, but, as in that case, no true \a>cular bundles. The 

 effects of growth within the ])rimary cortex of the root, leading to the 

 formation and casting off of bork, we h;t\e seen to be of rare occurrence. 

 In the case of the stem, howc\er, it is of \ cry general occurrence, so 

 10 



