196 



STEMS 



The vascular bundles of all Dicotyledons are very sim- 

 ilar to those of Monocotyledons in structure and function 

 of conductive vessels, but differ essentially in having cambium. 

 (Fig. 174-) The conductive tissue of the xylem consists 

 chiefly of annular, spiral, pitted, and scalariform vessels the 

 latter being so named because the thickened areas, separated 

 by slit-like thin areas, are so arranged, one above another, as 

 to resemble the rounds of a ladder. As in Monocotyledons, the 

 xylem vessels, probably assisted by the neighboring parenchyma 

 cells, are the passage ways through which the water and 

 dissolved substances absorbed by the roots are distributed 

 throughout the shoot. In addition to sieve tubes and companion 



FIG. 175. Cross section of a Flax stem, a, epidermis; d, bast fibers; 

 c, cambium; p, phloem; x, xylem, h, pith. Enlarged. 



cells, the phloem of Dicotyledons generally contains many thin- 

 walled parenchyma cells, which serve in conducting the carbohy- 

 drates and also as storage places for proteins. The sieve tubes 

 and companion cells conduct the proteins and a part of the carbo- 

 hydrates. The bast fibers, which commonly occur in connection 

 with the phloem of all Dicotyledons, are tough flexible strands 

 adapted to afford strength. In fiber plants, such as Flax and 

 Hemp, the bast fibers are well developed and their importance 

 in the manufacture of fabrics, as the manufacture of linen from 

 Flax, is well known. (Fig. 175.) 



In contrast to the stems of Monocotyledons, the stems of Di- 

 cotyledons and Gymnosperms have as their distinctive features 

 the circular arrangement of vascular bundles and the presence 



