SECT. 1 



MORPHOLOGY 



121 



lueut of the tissue of these complicated strands is often similar to 

 that found in the vascular bundles of more highly oi'ganised plants. 

 This is a good example of a striking resemblance between structures 

 which are analogous but have arisen independently in the course of 

 evolution. 



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



Fio. 131. — Transverse section of a vascular bundle from the internode of a stem of Zcu Mais, a, Ring 

 of an annular tracheide ; sp, spiral traclieide ; to and m', vessels witli bordered pits ; v, sieve- 

 tubes ; s, companion cells ; cpr, compressed protophloem ; /, intercellular passage ; rg, sheath ; 

 /, cell of fundamental tissue, (x 180.) 



hisjh 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 

 generally united in one vascular bundle. Other terms often used 

 to designate the vascular bundles are fibro-vascular BUNDLES and 

 MESTOME. The \ascular portion is also termed the PRIMARY WOOD, 



