MORPHOLOGY OF HIGHER PLANTS. 205 



roots of Veratrumviride (Fig. 113a). The uneven or corkscrew- 

 like appearance is due to a contraction, which arises as follows: 

 Some of the longitudinally elongated cells beneath the epidermis 

 as well as cells extending to and including the endodermis absorb 

 large quantities of water, which causes them to assume a spherical 

 form (as the cells of a potato are altered on boiling), the result 

 being a longitudinal contraction of the root at this point. In this 

 way the plant is fastened more securely to the earth, and at the 

 end of the season's growth the apical buds of plants, with upright 

 rhizomes, as of Vcratrum viridc, Dracontium, etc., are drawn 

 into the earth and thus protected during the winter season. 



Abnormal Structure of Roots. It is often difficult to 

 recognize the type-structure of dicotyledonous roots in drugs, 

 owing to the anomalous and abnormal secondary structure. 

 Sclerenchymatous fibers, while present in glycyrrhiza (Fig. 104) 

 and althaea, are not infrequently wanting. Wood fibers may be 

 sparingly developed, as in young belladonna roots or even want- 

 ing, as in gentian. In other cases the medullary rays are abnor- 

 mal, being replaced in calumba (Fig. 198) by wood parenchyma, 

 and in ipecac (Fig. 203) and taraxacum by sclerenchymatous 

 cells (Fig. 197a). In asclepias and calumba (Fig. 198) a layer 

 of stone cells occurs near the periphery; in gentian, sieve cells 

 develop in the xylem (Fig. 210) ; in senega the xylem is not 

 uniformly developed (Fig. 197), and in still other cases, as in 

 jalap (Fig. 195), pareira (Fig. 199) and phytolacca (Fig. 200), 

 successive cambiums develop, producing concentric series of open 

 collateral fibrovascular bundles. 



THE STRUCTURE OF THE STEM. 



If we make a transverse section of a young herbaceous stem, 

 we observe a differentiation of the tissues, which in several re- 

 spects agrees with that of the root, described in the preceding 

 chapter. In the primary structure of the stem the following 

 tissues are to be noticed : The outermost tissue is the epidermis 

 with a more or less distinct cuticle; the second is the cortical 

 parenchyma, frequently with strands of collenchyma near the epi- 

 dermis, often containing secreting ducts or cells, and not infre- 



