6o THE STRUCTURE OF ECONOMIC PLANTS 



xylem. In the bicollateral type, the bundle is organized like the 

 collateral one; but it develops an inner phloem as well as an outer 

 one so that the xylem lies between two primary phloem groups. 

 This type occurs in the Cucurbitaceae, Solanaceae, and Con- 

 volvulaceae; but there is some difference of opinion as to the 

 interpretation of the bundle in certain members of the last two 

 families. 



The pericycle which lies outside the outer phloem is a continuous 

 zone of cells one to several layers in width. In some instances, 

 it can be clearly distinguished from adjacent tissues because of the 

 differentiation of a complete ring of pericyclic fibers; but, in other 

 stems, the fibers may be formed only over the phloem portion of 

 the bundles where they serve as protective caps. The intervening 



A B c J) 



Fig. 13. Types of vascular bundles: A, collateral; B, bicollateral; C, amphivasal; D, 

 amphicribral. The stippled areas represent phloem, the black ones, xylem. 



sectors of the pericycle may remain parenchymatous so that they 

 cannot be separated on any histological basis from ray tissue; or, 

 late in ontogeny, they may mature as stone cells. Because of the 

 relationship of the pericyclic fibers to the vascular elements in 

 many plants, the term fibro-vascular bundle has been employed; but, 

 since fibers are not a constant feature of the vascular unit and in 

 many cases often occur as distinct and isolated strands, the term 

 vascular bundle is more commonly used. 



Except in some underground stems and those of certain woody 

 plants, the endodermis is generally not as clearly defined as it is in 

 roots, since it often lacks characteristic Casparian strips. It may 

 be distinguished in some cases by the size and regularity of its 

 component cells, which are oval or rectangular in transection, by 

 the presence of starch and other substances in the lumen of the 

 cells, or by its characteristic physiological reactions. The cortical 

 region consists mainly of thin-walled parenchymatous cells, the 

 outermost layers sometimes containing chlorophyll. Frequently, 



