STRUCTURE OF STEM AS CONTRASTED WITH THAT OF ROOT 145 



at length upon the younger portion as not yet having made their way 

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

 is called the Primary, so its branches are called Secondary. Their 

 structural development is a repetition of that of the primary. 



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

 uniform — that is, there is no division of it 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 as Contrasted with that of the Root. — (The follow- 

 ing account of stem-structure 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 pui-poses of pharmacognosy.) 



The history of stem-development is best presented by contrasting it 

 with that of the root, which has already been gi^'en. 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 corresponding tissues of the root, and the details 

 pertain for the most part to histology and to the special treatment of 

 species or groups. 



The Epidermis. — The most important distinction between the epi- 

 dermis of root and stem may be mentioned as the presence in the latter 

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

 extra development from the dermatogen at the tip corresponding 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 be dissimilar in varying degrees. Rarely it is persistent, 

 being usually thrown oft' through the growth of the parts within it, 

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



The Cortex. — The periblem of the stem develops structures in general 

 similar to those of the root-periblem, the most important distinction 

 being the production of a chlorophyll-layer. A primary cortex, usually 

 somewhat thinner than that of the root of the same plant, 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-cortex, 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 ha^'e seen to be of rare occurrence. 

 In the case of the stem, howe\'er, it is of very general occurrence, so 

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