150 ANATOMY OF STEM, ROOT AND LEAF 



the root-cap dies off or is worn away by the soil in which the 

 root is growing, additions are being made to the interior of the 

 cap where it is in union with the meristem. 



A common arrangement of the tissues at the end of a root 

 is seen in Fig. 78. 



The innermost part of the 

 meristem which gives rise to the 

 vascular cylinder is the plerome 

 {b), while round it is the peri- 

 blem (fl), from which the primary 

 cortex of the root is derived. 

 In almost all respects these 

 portions of the apical meristem 

 are identical with those present 

 in the apex of the stem. The 

 outermost part of the meristem 

 is termed the calyptrogen or cap- 

 forming layer; instead of re- 

 maining a single layer as in the 

 stem it divides by walls parallel 

 to the surface as well as per- 

 pendicular to the latter, and thus 

 a many-layered root-cap (c) is 

 formed. 



In many instances the inner- 

 most single layer of cells pro- 

 duced by the calyptrogen be- 

 comes the piliferous layer : the rest of the cells which are 

 continually cut off towards the outside form the root-cap 

 proper. 



Fig. 78. — Longitudinal section through 

 the apex of a root. i5 Pierome ; a periblem ; 

 c root-cap ; d external dead and dying cells 

 of root-cap J e pericycle ; v vessels of the 

 protoxylem ; h root-hair. (Enlarged about 

 60 diameters. ) 



Ex. 76. — Soak some beans or peas, and allow them to germinate. As 

 soon as the tip of the radicle is visible through the micropyle, strip off the 

 coat of the seed and cut longitudinal sections of the young root. Place them 

 or half an hour in Eau de Javelle (see Ex. 75), then wash in water and 



