122 



BOTANY 



1»ART I 



XYLEM or HADROME, and the sieve -tube portion the primary 

 PHLOEM or LEPTOME (^-^). The distinction of the two components 

 of the vascular bundle is most evident from transverse sections 

 (Figs. 131, 133), with which the longitudinal section (Fig. 132) 

 should be compared. The vascular portion contains tracheae and 

 TR.ACHEIDES as most essential for the fulfilment of its function of 

 water conduction {a, sp, m, Figs. 131, 132), or tracheides alone, 

 and, in addition, living, elongated parenchymatous cells that may be 



r/ 



^^* 



Fig. 132.— Longitudinal section of a vascular bundle from the stem of Zea MtiU. a, and ai, Rings 

 of an annular tiacheide ; r, sieve-tubes ; s, companion cells ; ej), protophloem ; I, inteicellular 

 passage; vg, sheath; «;>, spiral tiacheides. (x ISO.) 



designated XYLEM or WOOD parenchyma. . In the j^hloem portion 

 the most essential elements are the sieve-tubes (v\ which serve for 

 the conveyance of albuminous matter. They are always accompanied 

 by other living cells ; either by the so-called companion cells (s), or 

 in addition by elongated parenchymatous cells, or by the parenchyma 

 alone. Companion cells only occur in the phloem of Angiosperms. 

 They are sister cells of the cells composing the sieve-tubes, having 

 arisen by longitudinal division from the same mother 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 



