6o8 THE STRUCTURE OF ECONOMIC PLANTS 



of the designation "bicollateral." Baranetsky (i) regarded the 

 inner phloem as of independent origin, and Worsdell (41) stated 

 that "the medullary phloem represents, probably in all cases, a 

 vestigial structure, the remnant of a former system of medullary 

 vascular bundles in which the xylem has disappeared." 



With respect to the interconnection of inner and outer phloem, 

 there has also been some difference of opinion. Gerard (ii) traced 

 the course of the bundles in the root and stem, finding that the 

 inner phloem consists of branches from the outer. Fischer (11) 

 stated that, in the transition region, the inner phloem ends blindly 

 at its lower extremity; but distinguished four kinds of phloem, 

 one called commissural because it connects the other types. The 

 three categories in addition to commissural phloem are: (i) hypo- 

 dermal or ectocyclic phloem, occurring between the epidermis and 

 the stele; (x) fascicular phloem, an integral part of the bundle; 

 and (3) entocyclic phloem, formed within the stele. Holroyd 

 (18) adopted the terminology of Fischer, attributing the origin 

 of the internal, or entocyclic, phloem to the development of what 

 he designated "perixylary" cambium, stating that "through imme- 

 diate activity of the inner cells of this cambium . . . patches of 

 internal phloem originate in front of each bundle." 



Whiting has investigated the situation in C. maxima in connec- 

 tion with the ontogeny of the hypocotyledonary bundle. The 

 provascular strand at first consists of small elongated cells without 

 intercellular spaces. (Fig. 316, A.^ The annular or loosely 

 spiral protoxylem elements develop first; but they do not arise 

 at the extreme inner margin of the procambial strand, one or more 

 of the centrad layers remaining undifferentiated. The outer 

 protophloem is formed at the same time, followed by the meta- 

 phloem, which consists of sieve tubes and companion cells that 

 can be identified in seedlings five days old. (Fig. 316, B.) By 

 the sixth day, the small protophloem cells are crushed and 

 resorbed; and, in the meantime, the inner phloem differentiates 

 in the procambial tissue on the centrad margin of the provascular 

 strand, forming sieve tubes and companion cells similar to those 

 of the outer metaphloem. 



The development of the inner phloem is centripetal, and a 2one 

 of undifferentiated parenchyma remains between it and the primary 

 xylem. In this region, the inner cambium arises later in ontogeny, 

 but it remains inactive during the primary growth. (Fig. 316, C.) 



