66 THE STRUCTURE OF ECONOMIC PLANTS 



In longisections, these cells can be distinguished from adjacent 

 parenchyma by their smaller caliber, greater length, and the density 

 of their protoplasts. 



At the level where the differentiation of the vascular elements 

 begins, the endarch character of the xylem is indicated by the 

 formation of protoxylem elements on the centrad faces of the pro- 

 vascular strands. Simultaneously, or even slightly in advance of 

 the development of the protoxylem, the first protophloem elements 

 are differentiated on the outer face of the strand. The strands form 

 common bundles which are direct continuations of those of the 

 leaf. The point of their initial differentiation is at the level of 

 divergence of the leaf primordium (at a node); and, from this 

 point of initiation, the development of the bundle proceeds in two 

 directions. Outwardly, it is continued into the developing leaf as 

 a part of its vascular system; and, downwardly, it becomes a 

 component part of the vascular system of the stem, where it ulti- 

 mately is anastomosed with the vascular bundles of leaves diverged 

 at a lower level. 



At a level where the differentiation of the primary tissue is 

 practically complete, the ring of vascular bundles is clearly de- 

 limited from the pith and cortex, and adjacent bundles are separated 

 from one another by medullary rays. At this stage, the collateral 

 bundle has an inner zone of endarch primary xylem, and an outer 

 one of primary phloem. Between the two, a region of meri- 

 stematic cells may remain undifferentiated, instead of maturing as 

 primary vascular tissue; and this may function later as a fascicular 

 cambium. 



Secondary Thickening of the Stem Axis. — When the vascular 

 bundle is collateral with an intervening zone of meristematic cells 

 between the primary xylem and primary phloem, it is an open type. 

 By continued activity, this meristem or cambium separating the pri- 

 mary vascular elements may give rise to cells which mature as 

 secondary tissues. 



In the formation of secondary vascular elements, the cambial 

 cell divides tangentially to form two daughter cells. One of these 

 remains meristematic while the other differentiates into either a 

 secondary xylem or phloem element, either directly or after a sub- 

 sequent division of the xylem or phloem mother cell. Such second- 

 ary division commonly occurs in the phloem where a new wall 

 divides the mother cell into two unequal daughter cells, one of 



