114 HISTOLOGY OF ANGIOSPERM STEM, BOOT, ETC. 



organised primary phloem, seen as patches of thick-walled 

 or crushed cells on radii alternating with the rays and 

 primary xylems ; (7) the cork-cambium, which has arisen 

 from the pericycle and produced a cork layer the eiido- 

 dermis, cortex, and piliferous layer have, of course, been 

 cast off. 



132. In Woody Dicotyledons secondary thickening in the root 

 begins as in herbaceous forms, but after the first year annual rings 

 are formed in the secondary xylem ; the primary xylem can, how- 

 ever, be recognised at the centre of the root owing to its radiate or 

 star-like appearance. The root of Horse Chestnut shows the tissues 

 clearly, but others should be tried. 



133. Roots of Monocotyledons. Suitable material is afforded 

 by the roots of Wheat, Maize, Oats, and other seedlings, some of 

 which should be grown in water or culture solution, others in soil ; 

 the roots may be mounted entire and examined as directed for Cress, 

 Mustard, etc. For sections, use the roots of Onion or Hyacinth 

 grown in water and also in soil ; Iris and other roots should also be 

 tried. 



The roots of Monocotyledons resemble those of Dicotyledons in 

 primary structure, but there is no secondary thickening. In Dico- 

 tyledons the number of xylem bundles varies from two to six, rarely 

 more ; in Monocotyledons a limited number five to eight is some- 

 times found (e.g. Onion, Hyacinth), but there are typically more than 

 often a very large mimber. The endodermis and pericycle are 

 usually very sharply defined, each consisting of a single layer. The 

 endodermis is often strongly thickened, especially on the lateral and 

 inner walls, but here and there we find a thin-walled "passage- 

 cell " in the endodermis, opposite a xylem-bundle this is usually 

 well shown in Iris. The exodermis, or hypodermal layer, the cells 

 of which become cutinised and persist after the piliferous layer has 

 disappeared, is usually well marked in Monocotyledons. 



134. Apical Meristem of Boot. For the structure 

 of the growing point of the root cut median longitudinal 

 sections of the radicle of the embryo in the seed of (1) 

 Broad Bean, (2) Sunflower, (3) Maize. 



In each case treat the sections with potash or eau de 

 Javelle, rinse in water, mount in glycerine, and note 

 (a) the root-cap, (fc) the "piliferous layer," (c) the "peri- 

 blem," (d) the " plerome." 



In Broad Bean all these tissues appear to arise from a 

 general mass of meristem, a and b being formed by cells 



