MORPHOLOGY OF HIGHER PLANTS. 199 



known as cambium, and (2) one which gives rise to the cork, 

 known as phellogen. The tissues formed from the secondary 

 meristems constitute the secondary structure of older dicoty- 

 ledonous stems and roots. 



While the point of vegetation in the higher plants' (spermo- 

 phvtes) embraces a number of cells, in the lower plants the tissues 

 can be traced back to a single apical cell. 



THE STRUCTURE OF THE ROOT. ' 



Primary Structure. — If we make a transverse section of the 

 root of a germinating plant, through the portion developing root 

 hairs, the following arrangement of tissues is observed (Figs. 



Fig. 109. A transverse section through the root of a germinating pea-plant (Pisum) 

 about 40 mm. from the tip' showing the origin of a root branch(RB); E, epidermis; CO, 

 primary cortex; X, xylem; P, phloem; EN, endodermis. ' ' - 



109, ito) : (i) An fepiDERMAL layer with root hairs (E, H) ; (2) a 

 HYPODERMis of a fcw layers of cells; (3) a cortex made up 

 of a number of layers of parenchyma cells (C) ; (4) a single layer 

 of more or less lignified cells, known as the EisrooDERMis (EN) ; 

 (5) a CENTRAL cylinder, consisting of parenchyma and alternat- 

 ing groups of ducts (X) and sieve (P), the nuinber of which 

 is more or less constant for certain genera. This arrangement 

 constitutes what is known as the primary structure of the root. 



