200 BOTANY AND PHARMACOGNOSY. 



and is essentially the same in both monocotyledons and dicoty- 

 ledons. The terms monarch, diarch, triarch, tetrarch, polyarch, 

 etc., are used to designate the number of plates or groups of ducts, 

 there being usually a larger number of groups in the roots of 

 monocotyledonous plants than in those of dicotyledons. 



Secondary Structure. — While monocotyledonous roots 

 grow in length, they do not grow perceptibly in thickness, so 

 that, for instance, a sarsaparilla root, which may be nearly three 



Fig. iio. Primary structure in the root. Transverse section of root of pea (Pisum) 

 about 40 mm. from the root-cap: H, epidermal cells, some of which are developed into 

 root hairs; C, primary cortex; EN, endodermis; X, xylem ray composed of tracheae; P, 

 phloem composed of sieve cells, the xylem and phloem forming a triarch radial fibro- 

 vascular bundle. 



meters in length, will show but little variation in thickness 

 throughout its entire length. Dicotyledonous roots, however, soon 

 begin to grow in thickness, as well as in length, the latter being 

 less marked than in the Monocotyledons. 



In Monocotyledons the primary structure of the root is for the 

 most part retained, the only change being an increase in thickness 

 of the walls of some of the cells. On the other hand, the increase 

 in diameter of dicotyledonous roots is accompanied by marked 



