136 BOTANY PART i 



Should thickening occur at an early stage, special eudodermal cells, directly 

 external to the xylem strands, remain unthickened and serve as TRANSFUSION 

 CELLS (Fig. 163 d). 



The outermost layer of cells of the central cylinder lying 

 immediately within the endodermis (Figs. 160^, 161 pc, 163 p) forms 

 the PERICYCLE ; this is usually a single layer and in rare cases is 

 wanting. The strands of xylem and phloem run longitudinally in the 

 central cylinder and in all roots form a radial vascular bundle ( 59 ) 

 (cf. p. 99). They are separated from one another by one or more 



Fm. 161. Transverse section of the radial bundle of the root of Ranunculus acer. R, Cortical 

 parenchyma; S, endodermis; pc, pericycle; ph, phloem; px, protoxylem; G, pitted vessels. 

 (x 200. ROTHERT modified from DIPPEL.) 



layers of cells that usually have the characters of conducting 

 parenchyma. The orientation of the strands of xylem in the root 

 contrasts with that found in the stem. In the stem the narrow 

 elements of protoxylem were situated internally, but in the root the 

 internal vessels are the widest, and the narrow elements of the proto- 

 xylem are found close to the periphery of the vascular bundle. 

 Annular, spiral, reticulate, and pitted vessels thus follow in order from 

 without inwards. The protophloem is situated at the outer margin of 

 the phloem strands, which are more or less circular in cross -section. 

 Roots are described as diarch, triarch, polyarch, etc , according to the 

 number of the vascular strands. Thus the root in Fig. 160 is octarch 



