182 PRACTICAL BOTANY 



which have grown out, here and there, perpendicular to 

 the surface as root-hairs. 



2. Beneath this a layer of cells of larger size with 

 clearly-marked, cuticularized cell-walls : this is the 

 exodermis. 



3. A thick band of cortical parenchyma, consist- 

 ing of rounded cells with intercellular spaces : in old 

 roots the outer layers of this tissue become disorganized 

 and distorted. The inmost layer of this tissue differs 

 in structure from the rest, and is called 



4. The endodermis : the radial walls of this 

 layer present the characteristic appearance of a black 

 dot, and are cuticularized. 



5. Within this is a layer of thin- walled cells of 

 the pericycle which immediately surrounds 



6. The central vascular cylinder : two kinds of 

 tissue are specially to be distinguished in this cylinder, 

 viz., 



A. Xylem-tissueS; easily recognized by their dark 

 lignified walls : they are arranged in a series of groups 

 of indefinite number, which abut externally on the 

 pericycle, and extend inwards, till they meet in- 

 ternally, and form a central mass. The chief con- 

 stituents are vessels of various form. As may be seen 

 in transverse sections of young roots, the smaller peri- 

 pheral members of each group are formed first (proto- 

 xylem), and have spiral thickening ; then successively 

 the larger vessels towards the centre. 



B. The phloem-tissues, which are groups of ele- 

 ments with small cavity, and bright cellulose walls : 

 they lie between the peripheral groups of the xylem, 

 and alternate regularly with them. 



