90 DEVELOPMENT FROM THE GROWING POINT 



phloem (protophloem). As the strands are traced further and 

 further from the apex (i.e. from younger to older stages) 

 the differentiation of xylem and phloem proceeds towards the 

 middle of each. Ultimately all that remains is a narrow strip 

 of thin-walled cells between xylem and phloem, which in the 

 case of Dicotyledons gives rise to the cambium. In the root 

 alternating groups of protoxylem and protophloem arise at the 

 periphery of the procambial strand, whilst the later-formed 

 elements, and the pith if present, develop from the central region. 

 Both leaves and branches originate close behind the growing 

 point of the stem as superficial outgrowths (i.e. are exogenous, 

 Figs. 7, 8), a mode of development contrasting markedly with 

 the internal (endogenous) origin of lateral roots. Leaf -rudi- 

 ments, at first, consist merely of a mass of periblem enveloped 

 by a protrusion of the epidermis (Fig. 8), but as they enlarge 

 procambial strands are formed within them by division of certain 

 cells. The differentiation of these strands gradually extends 

 inwards till they ultimately join those of the stem. As a 

 result the vascular bundles of leaf and stem are continuous in 

 the mature condition (p. 114, Fig. 55). When vascular strands 

 occur in the stipules they are branches of those in the leaf -base. 

 The strands arising in the young branches also become connected 

 with those in the stem. 



