THE PRIMARY PERMANENT TISSUES 



43 



radially within, or on the pith side of the xylem, forming what 

 is known as the bicollateral bundle; in other instances the phloem 

 surrounds the xylem, or vice versa, making a concentric bundle; 

 while in roots it is the rule that the phloem and xylem of the 

 primary bundle are in strands alternating with each other, 



<y 



D 



FIG. 21. Different types of vascular bundles. A, the concentric type, with xylem, 

 k, surrounding the phloem, h. B, the collateral type, with phloem, h, standing in front 

 of the xylem, k. C, a portion of the radial type, shown complete in D, where the part 

 outlined at a, corresponds to C. Corresponding parts are lettered the same in both figures; 

 c, xylem; b, phloem;/, cambium ring; e, pericycle; d, endodermis. C and D are from the 

 tap root of Vicia faba. (After Haberlandt.) 



neither standing radially in front of the other, thus making 

 what is called a radial bundle. (See Fig. 21 for these types.) 



In leaves the primordial meristem becomes differentiated 

 into protoderm, ground meristem and procambium strands; 

 and, just as in stems, the protoderm gives rise to the epidermis, 

 the procambium strands to the vascular bundles which make 

 up the greater part of the veins, and the ground meristem to 



