MORPHOLOGY OF HIGHER PLANTS. 319 



The root branches arise as the result of the development of 

 primary meristems in the pericambiiim (Figs. 161, 172). The 

 tissues forming the branches are directly connected with the 

 fibrovascular tissues of the root and protrude through the over- 

 lying tissues w^ithout having any connection with them. The 

 structure of the branches thus formed corresponds to the primary 

 structure of the roots, and in the case of dicotyledonous roots 

 may also subsequently develop a secondary structure. Goebel 

 states that in plants which grow in moist soil, or whose roots func- 

 tion only for a short time, the branches may be altogether sup- 

 pressed, as in Colchicum, Arisaema, etc. 



Contraction of roots is observed in both monocotyledons 

 and dicotyledons, it being most apparent in the former, as in the 

 roots of Veratrum viride (Fig. 178). 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 Veratrum viride, Dracontium, etc., are drawn 

 into the earth and thus protected during the winter season. 



Abnormal Structure of Roots. — It is often difficult to recog- 

 nize the type-structure of dicotyledonous roots in drugs, owing 

 to the anomalous and abnormal secondary structure. Scleren- 

 chymatous fibers, while present in glycyrrhiza (Fig. 176) and 

 althaea, are not infrequently wanting. Wood fibers may be spar- 

 ingly developed, as in young belladonna roots (Fig. 177), or even 

 wanting, as in gentian. In other cases the medullary rays are 

 abnormal, being replaced in calumba by wood parenchyma, and 

 in ipecac and taraxacum by sclerenchymatous cells. In asclepias 

 and calumba a layer of stone cells occurs near the periphery ; in 

 gelsemium sieve cells develop in the xylem ; in senega the xylem is 

 not uniformly developed, and in still other cases, as in jalap, 

 pareira, and phytolacca (Fig. 177, A), successive cambiums de- 

 velop, producing concentric series of open collateral fibrovascular 

 bundles. 



