Stem and Root Structures 81 



although there is considerable variety in the growing point of 

 different roots. In Fig. 36 D, the dermatogen (or calyp- 

 trogen) produces the root cap which is free from the root ex- 

 cept at the growing point. The cells P, the periblem, give 

 rise to cortex and piliferous layer, those marked PI, plerome, 

 to the central stele. The bean root in Fig. 78 shows a portion 

 of the cortex ; it is composed of parenchyma with large air 

 spaces between the cells. Its innermost layer, the endodermis, 

 does not store starch as in the stem ; thickenings on its radial 

 walls serve to exclude air and prevent the tissue within from 

 drying. 



Around the pith in the centre of the stele, the strands of 

 wood and bast lie separated by conjunctive tissue and in dif- 

 ferent radii ; the arrangement is radial, not collateral, as in the 

 stem. Another contrast to the stem is shown in the position 

 of the small elements of the wood (protoxylem) which lie out- 

 side of the large ones (metaxylem). 



The outermost layer of the stele, the pericycle lying just 

 within the endodermis, may escape notice. By division of 

 its cells, just outside of the wood strands lateral roots take 

 their rise, so that there are, as a rule, as many rows of lateral 

 roots as there are strands of wood. 



In the carrot where a resin gland extends just outside the wood 

 strands which are two in number, roots arise on either side of each gland. 

 This accounts for the four rows of roots seen on the surface. 



In the sunflower the endodermis becomes double layered, and resin 

 is stored in large cavities between the cells ; these cavities lie outside the 

 bast strands. 



Roots are assisted in pushing their way through the cortex 

 by an enzyme secreted by some of the endodermal cells which 

 dissolves the cell walls in the path of the young root. The en- 

 dodermal cells themselves divide for a time and form a pro- 

 tecting pocket within which the young root is contained. 



When- growth in thickness takes place in a root (which is 



seldom in monocotyledons), a wavy ring of cambium is formed. 



This arises partly from the pericycle outside the wood strands 



together with the ground tissue just inside of the phloem where 



6 



