COURSE OF VASCULAR BUNDLES 319 



outside it, e.g. by possessing specially large and 

 conspicuous starch grains, when it is called the " starch 

 sheath." 



The vascular cylinder of the stem thus differs in many 

 ways from that of the root, and this is related to its very 

 different conditions of life and to the fact that its 

 bundles are primarily the direct downward continuation 

 of those of the leaves (Fig. 52, D). They are identical 

 in structure with the leaf bundles, and their orientation 

 is the same, the outward position of the phloem in the 

 stem clearly corresponding with its lower position in 

 the leaf. 



The stem bundles do not pursue a completely in- 

 dependent course. A certain number come in from each 

 leaf, they often branch, and they always sooner or later 

 join on to neighbouring bundles which have come 

 down through the internode above from higher leaves 

 (Fig. 52, D). Traced downwards through the internode 

 the bundles fuse with one another laterally at various 

 levels, so that they leave room for those entering the 

 cylinder at the next node below. As the base of the 

 stem is approached more fusions take place, so that 

 comparatively few bundles enter the hypocotyl, and 

 these usually fuse with the cotyledon traces, so that 

 only the latter are directly continuous with the vascular 

 cylinder of the root. In the hypocotyl or at the top of 

 the primary root the cyhnder narrows, the pith dis- 

 appears, the pericycHc fibres die out, and the xylem and 

 phloem strands change their relative positions, so that 

 they come to be alternate, i.e. situated on different radial 

 planes. At the same time the cortex changes its character 

 and the shoot epidermis is replaced by the pilif erous layer. 



Maintenance of the Erect Position. — The herbaceous 

 stem is maintained in an upright position partly by the 



