204 The Phenomena of Morphogenesis 



seen, in most cases, is the elongate form of the procambial cells in longi- 

 tudinal section. This form is due either to fewer transverse divisions in 

 these cells as compared with their neighbors or to more frequent longi- 

 tudinal ones. From groups of these elongated provascular cells arise the 

 vascular bundles of the stem. There is a close relation between the differ- 

 entiation of these bundles and of the leaf primordia near the apex, for the 

 young leaf traces that enter the base of each primordium are continuous 

 with the differentiated vascular tissue below. 



There has been some difference of opinion as to just how the pattern 

 of vascular differentiation originates. It is now rather generally agreed 

 (Esau, 1953Z? ) that the procambial strands develop acropetally, continu- 

 ous with the mature vascular tissue below and pushing up into the bases 

 of the primordia themselves. In a transverse section of the axis, the pro- 

 cambium forms a ring which may be continuous or consist of a series of 

 bundles. On the outside of a procambial strand the first phloem differen- 

 tiates, and on the inside, the first xylem. The developmental history of 

 these tissues is different, however. The phloem, like the procambium, 

 develops continuously from the base toward the tip. The xylem, on the 

 contrary, differentiates first in the base of the enlarging leaf primordium 

 and then both upward and downward. In its downward course it meets 

 the upward developing xylem in the axis below (Miller and Wetmore, 

 1946). Jacobs and Morrow (1957) traced the downward differentiating 

 xylem strands and found that they did not always make connection with 

 the normally opposite ones below. In the root, the procambium, phloem, 

 and xylem all differentiate acropetally and continuously ( Heimsch, 1951; 

 Popham, 1955k). 



The physiological significance of these facts is not clear, but morpho- 

 genetically they are concerned with the important question as to whether 

 the course of initiation and development of structures at the apical meri- 

 stem, notably the position of the leaf primordia and the pattern of internal 

 differentiation, results from stimuli proceeding up from the mature struc- 

 tures below or whether in its development the tip is independent of what 

 has gone before. This problem is discussed elsewhere in the light of some 

 experimental results (p. 238). Torrey (1955), working with root tips cut 

 off and grown in culture, presents evidence that the pattern of vascular 

 differentiation (triarch, diarch, or monarch) just back of the tip is not 

 induced by the tissue farther back but is related to the dimensions of the 

 apical meristem at the time the cylinder is differentiated. 



Much work has been done on this problem of differentiation at the 

 apical meristems, and it is well covered by Esau ( 1953b ) . Among other 

 recent publications are an extensive review by Esau ( 1954 ) and papers 

 by Rathfelder (1954), Young (1954), Wetmore and Sorokin (1955), 

 McGahan (1955), and Jacobs and Morrow (1957). 



