324 THE PRIMARY STEM 



xylem is formed which, so to speak, grows in length at 

 each end, extending up into the leaf and down into the 

 stem, and these strands form the first water channels 

 of the shoot. Later, protoxylem is formed in the smaller 

 bundles, and the strands extend till they meet those of 

 other bundles, and thus form a continuous water- 

 conducting- system. 



Elongating Region of the Stem. The formation of the 

 protoxylem takes place at the beginning of the elongating 

 region of the stem, which is much longer than that of 

 the root, and often extends over several centimetres or 

 even several inches. It is the internodes which elongate, 

 separating the successive leaves from one another, the 

 nodal regions growing mainly in diameter. 



Meanwhile the other tissues of the shoot are rapidly 

 differentiating. The cuticle as it is exposed to the air 

 by elongation of the internodes becomes thicker ; the 

 outer layers of the cortex frequently become collenchy- 

 matous by the thickening of the longitudinal walls at 

 the corners of the cells ; chlorophyll develops in the 

 plastids ; and intercellular spaces between many of the 

 cortical cells. 



In the vascular cylinder the fibres of the pericycle 

 are clearly marked at an early stage, but their walls 

 are not thickened till relatively late, generally after 

 elongation has ceased. Narrow sieve tubes (protophloem) 

 usually appear early just below the pericyclic fibres and 

 opposite the protoxylem of each bundle. When growth 

 in length has ceased, the development of the primary 

 vascular bundles continues by the centripetal forma- 

 tion of the larger sieve tubes and companion cells 

 (metaphloem] , and of metaxylem, consisting of larger 

 scalariform or some other type of pitted vessels. Differ- 

 entiation progresses outwards (centrifugally) in the 



