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 
Fie. 170.— Cross sections of a Barley stem. A, section across the en- 
tire stem showing the hollow (A) and the outer region (0) in which the vascular 
bundles occur. 8B, 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. 
