MORPHOLOGY OF HIGHER PLANTS. 205 



Some of the longitudinally elongated cells beneath the epidermis 

 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 Vcratruni viridc, Dracon- 

 tium, etc., are drawn into the earth and thus protected during the 

 winter season. 



Abnormal Structures of Roots. — It is frequently difficult to 

 recognize the type-structure of dicotyledonous roots, owing to the 

 development of anomalous and abnormal secondary structures. 

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

 ■occurs near the periphery; in gentian, sieve cells develop in the 

 xylem; in senega the xylem is not uniformly developed, and in 

 still other cases, as in jalap (Fig. 195), pareira (Fig. 199) and 

 Phytolacca, successive cambiums develop, producing concentric 

 series of open collateral fibrovascular bundles. 



THE STRUCTURE OF THE STEM. 



Primary Structure. — If we make a transverse section of a 

 dicotyledonous stem a little below the growing point, we observe 

 a differentiation of the tissues, as follows, this arrangement rep- 

 resenting the primary structure : ( i ) The epidermis with gland- 

 ular or non-glandular hairs; (2) hypodermis; (3) cortex; (4) 

 endodermis; (5) open collateral fibrovascular bundles separated 

 Ijy primary medtiUary rays, and (6) a pith in the center. The 

 essential difference between the primary structures of the stems 

 of Dicotyledons and Monocotyledons, is that in the latter the 

 :fibrovascular bundle is of the Closed collateral or concentric type, 

 ;the medullary rays bdng wanting (Figs. 114, 115). 



