SECT. I 



MORPHOLOGY 



103 



cell. The companion cells are not so large as the sieve-tubes, and 

 may be distinguished from them by their more abundant protoplasmic 

 contents, and especially by the fact that they retain their nuclei, while 

 the nuclei of the sieve-tubes soon disappear. In Monocotyledons (Figs. 

 117, 118), and in the Ilununmlareae among the Dicotyledons (Fig. 119), 

 the phloem consists solely of sieve-tubes and companion cells ; in the 

 other Dicotyledons parenchymatous elements are also present, and these 

 are accordingly distinguished as phloem or sieve parenchyma ; 

 no companion cells are found in Gymnosperms and Pteridophytes, 



cp v s- 



Fig. IIS. — Longitudinal section of a vascular bundle from the stem of Zm Mais, a and a', Rings of 

 an annular tvacheid ; v, sieve -tubes; s, companion cells; cp, protophloem ; I, intercellular 

 ge ; vg, sheath ; sp, spiral tracheids. ( x ISO.) 



and in addition to sieve -tubes the phloem contains only phloem 

 parenchyma. 



The bundles of the Phanerogams (Gymnosperms and Angiosperms) 

 are generally collateral in structure, that is, the xylem and phloem 

 are in contact on one side only. In stems the most usual arrangement 

 of the two portions of a collateral bundle is that in which the xylem 

 lies nearest the centre ; in leaves the xylem portion lies nearer the 

 upper, and the phloem portion nearer the lower surface. The xylem 

 and phloem of roots generally form separate strands (Fig. 120, s, r), 

 and in that case the arrangement of the vascular portion is somewhat 

 altered ; for, while in stems the narrow vessels of the xylem are 



