TRANSITION FROM STEM TO ROOT 



89 



protoxylem-strands are directly continuous, whilst the metaxylem 

 is developed towards the exterior in the stem and towards the 

 interior in the root, and occupies a more or less intermediate 

 position in the region (i.e. the hypocotyl) where the transition 

 from the one to the other type of structure takes place. The 

 phloem is similarly continuous, and the transition from root to 

 stem mainly involves a lateral and outward displacement accom- 

 panying the enlargement of the vascular cylinder (Fig. 42). 

 The transition between the two organs is, however, often 

 very complex, being accom- 

 panied by splitting or union 

 of conducting strands, as a 

 result of which the number of 

 protoxylem-groups in stem and 

 root do not necessarily corre- 

 spond. 



The development of the 

 different regions of the axis 

 from the growing points has 

 already been described in 

 Chapter II, but some additional 

 details as to the manner of 

 origin of the vascular tissues 

 will serve to amplify the 

 picture. These first appear in 

 the plerome a short distance 

 behind the tip as so-called 

 procambial strands, composed 

 of very narrow, elongated, thin-walled cells with pointed ends, 

 dense protoplasmic contents, and prominent nuclei. They 

 develop by repeated longitudinal divisions in the cells of the 

 plerome, in which transverse division almost ceases at an early 

 stage, the narrow segments thus formed subsequently elongating. 



In the stem each procambial strand gives rise to a vascular 

 bundle, whilst in the root there is usually a single strand from 

 which the whole vascular cylinder is differentiated. The inner- 

 most elements, in each of the procambial strands of the stem, 

 become the spirally thickened protoxylem- vessels, whilst simul- 

 taneously the outermost elements develop into the first-formed 



FIG. 42. Diagrams showing one 

 type of transition from the 

 vascular structure of the root 

 to that of the stem . The dotted 

 areas represent phloem and the 

 shaded xylem ; protoxylem 

 shown black. R., root; S., stem; 

 the remaining cross-section, 

 hypocotyl. 



