PLANT STRUCTURES 



Another striking feature is that in the stele there is 

 organized a single solid vascular cylinder, forming a tough 

 central axis (Fig. 277), from which the usually well-devel- 

 oped cortex can be peeled off as a thick rind. In this vas- 

 cular axis, which is called " a bundle " for convenience but 

 does not represent the bundle of Seed-plant stems, the ar- 

 rangement of the xylem and phloem is entirely unlike that 



FIG. 276. Cross-section of the vascular axis of a root, showing radiate type of bundle, 

 the xylem (p) and phloem (ph) alternating. After SACHS. 



found in stems. The xylem is in the center and sends out 

 a few radiating arms, between which are strands of phloem, 

 forming the so-called radiate bundle (Fig. 276). This 

 arrangement brings the tracheary vessels (xylem) to the 

 surface of the bundle region, which is not true of either 

 the concentric or collateral bundle. This seems to be asso- 

 ciated with the fact that the xylem is to receive and conduct 

 the water absorbed from the soil. It should be said that 

 this characteristic bundle structure of the root appears only 



