70 THE STRUCTURE OF ECONOMIC PLANTS 



shoot is established. The transitional region may involve several 

 internodes (three or four in Pisum), or it may be very short, occur- 

 ring within a vertical distance of but a few millimeters in the 

 hypocotyl (Beta). In the majority of cases, the transition is 

 hypocotyledonary if the cotyledons are brought above ground in 

 germination; but it may involve the first nodes and internodes 

 above the cotyledonary node when the cotyledon or cotyledons are 

 hypogeal. 



In the terminology of the seedling axis, the hypocotyl, where 

 vascular transition most frequently occurs, is regarded as that 

 portion of the axis immediately below the first node (cotyledonary 

 node), in contrast with the epkofyl, which is the whole portion of 

 the axis above that point. The two terms express axial relation- 

 ships with respect to the first node of the plant from which the 

 first leaves (cotyledons) are diverged; but they do not connote any- 

 thing with respect to the structural organization of the portion of 

 the seedling axis to which they are applied. Thus, the hypocotyl 

 may be almost entirely root-like in structure, as in many hypogeal 

 forms (Zea and Pisum). On the other hand, the upper portion of a 

 hypocotyl may be stem-like in structure as in many epigeal forms 

 where the transition occurs in the lower and middle hypocotyl. 



The manner in which the vascular transition is accomplished is 

 constant for a given species, except for minor variations. It is 

 brought about in ontogeny by a progressive reorientation of the 

 vascular tissues which are differentiated in the embryo and by the 

 apical and intercalary meristems of the hypocotyl. In a represen- 

 tative case, the vascular elements at the upper limits of the transi- 

 tion region are differentiated as endarch collateral bundles; and, at 

 successively lower levels, the vascular strands are oriented in a 

 radial relationship with an exarch development of the primary 

 xylem. 



While descriptions of specific vascular transitions tend to imply 

 motion, the cells are laid down and differentiated in place so that 

 the vascular reorientation is not accomplished by cell movement, 

 but by a gradual change in the vascular pattern of the stele at each 

 succeeding level of the transition region. In many cases, the 

 changes involved are described as proceeding from the root and 

 extending up the axis to the point where stem structure occurs, but 

 this is done for reasons of convenience and clarity of description. 

 In the actual ontogeny, the processes of growth and differentiation 



