MORPHOLOGY 



111 



Such bundles reach their highest differentiation in the Polytrichaceae. 

 In them the stem has a central cylinder composed of elongated 

 cells with scanty contents, of elements resembling sieve-tubes, and of 

 elongated cells ; the three kinds of element serve respectively to 

 transport water, albuminous substances, and carbohydrates ( 109 ). 

 The arrangement of the tissue of these complicated strands is often 

 similar to that found in the vascular bundles of more highly organised 

 plants. This is a good example of a striking resemblance between 



*p f /* 



I 



FIG. 1-2-2. Longitudinal section of a vascular bundle from the stem of Zea Mais, a, and a', Rings of 

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

 passage ; vg, sheath ; sp, spiral tracheides. (x 180.) 



structures which are analogous but have arisen independently in the 

 course of evolution. 



It is, however, in the Cormophytes, which possess roots, that a 

 high degree of differentiation of the vascular bundles is first attained. 

 Since the absorption of water is limited to the roots, the arrange- 

 ments in the conducting tracts require to be more perfect. Two 

 distinct components can be distinguished in these vascular bundles, 



the TRACHEAL Or XYLEM PORTION, and the SIEVE-TUBE or PHLOEM 



PORTION. While these may form independent strands, they are gener- 

 ally united in one VASCULAR BUNDLE. Other terms often used to 

 designate the vascular bundles are FIBRO-VASCULAR BUNDLES and 

 MESTOME. The vascular portion is also termed the XYLEM or 



