242 The Phenomena of Morphogenesis 



tissues are appropriate in character for the place where they now are and 

 are much like those normally present in such regions. 



Somewhat more complex is the redifferentiation of vascular bundles 

 in places where these have been severed. Simon (1908; Fig. 9-3), Nemec 

 (1905), Sinnott and Bloch (1945), and Jacobs (1952) have studied this 

 in various herbaceous stems. If a bundle or group of bundles is severed 

 by a lateral incision and the region examined in longitudinal section after 

 a week or two, a new vascular strand can be seen developing behind the 

 incision and connecting the severed upper and lower ends of the bundle 

 ( p. 193 ) . This strand is formed by the conversion of large, squarish paren- 

 chyma cells of the pith into xylem cells with reticulate lignified thicken- 



Fig. 9-4. Left, regeneration of connection between severed vascular bundles in stem 

 of Coleus. Right, differentiation of parenchyma cells into reticulate xylem cells in the 

 development of this strand. Arrow shows the direction of its development. Note 

 new walls parallel to it. ( From Sinnott and Bloch. ) 



ings (p. 194 and Fig. 9-4). Differentiation seems always to be basipetal, 

 suggesting the downward passage of a morphogenetic substance, pre- 

 sumably auxin, from the upper bundle toward the lower. The regener- 

 ated strand is not directly at the cut surface but about the same distance 

 behind it that the normal bundles are from the uninjured surface. Jost 

 ( 1942) caused the plant to form these vascular bridges in manv ways and 

 finds that, although their general course is basipetal, they may develop 

 acropetally for a while in passing around an obstacle. They do not always 

 take the shortest route. The position of the strand seems to be determined 

 by a gradient of some sort from the wound surface inward. It is significant 

 that the conversion of parenchyma to xylem is an "all or none" reaction, 

 with no cells intermediate in character. 



