STRUCTURE OF MONOCOTYLEDONOUS STEMS 191 



walls. The sieve vessels, assisted by the companion cells, which 

 are also thin-walled, elongated, living cells, conduct the foods 

 manufactured in the leaves, such as proteins and the carbohy- 

 drates of which sugar is the chief one. The strengthening cells, 

 which are more numerous at the outer margin of the xylem and 

 phloem, form a sheath around the vascular bundle. One peculiar 

 feature of the vascular bundles of Monocotyledons is that there 

 is no provision whereby the bundle can increase its tissues, and 

 for this reason it is known as a closed vascular bundle. In mono- 

 cotyledonous stems, where there is no special provision for growth 



A B 



FIG. 170. Cross sections of a Barley stem. A, section across the en- 

 tire stem showing the hollow (h) and the outer region (o) in which the vascular 

 bundles occur. B, a section of the outer region much enlarged, r, rind com- 

 posed of strengthening cells; v, vascular bundles. 



in diameter, growth is mainly in length, and often results in the 

 development of extremely slender trunks, like those of Palms and 

 Bamboos. 



In many Grasses the stems are hollow throughout the inter- 

 nodes, as shown in Figure 170, in which case the vascular 

 bundles are limited to a zone just within the rind. In most 

 Monocotyledons not belonging to the Grass or Sedge family, 

 the outer region of the stem is less firm in texture and in a few 

 Monocotyledons, as in the Yuccas and Dragon Tree, some of 

 the cells in the outer region of the stem divide like a cambium, 

 adding cells which form new vascular bundles and other tissues. 

 In this way the Dragon Tree may continue to grow in diameter 

 for thousands of years and attain a diameter of many feet. 



