292 TEXTBOOK OF BOTANY 



and hence secondary growth, so evident in dicots and conifers, does not 

 occur. The bundles are composed of xylem and phloem surrounded by a 

 bundle sheath of more or less sclerenchyma-like cells. Growth in length 

 occurs at the apex and also at the bases of several of the uppermost in- 

 ternodes. The epidermis is composed of hard, thick walls and may be 

 subtended by several rows of thick-walled cells, the htjpodermis. The 

 epidei-mis, together with the hypodermis, is sometimes called the rind; 

 this is evident in the corn stem. 



The remainder of the stem consists of parenchyma. It is usually im- 

 possible to distinguish pericycle or cortex or pith. In some few monocots 

 the central region is devoid of bundles, and the central parenchyma 

 resembles a pith. Many monocots have hollow stems. 



Because of the absence of a cambium, stems of most monocots do not 

 increase in diameter after the cells are mature. The bamboo and corn, 

 for example, grow to considerable heights but remain slender. Many 

 perennial monocots have underground stems that elongate each year and 

 enlarge the area occupied by the plant. Such aerial stems as those of 

 palms, dragon tree, and yucca have secondary thickening. Cells in the 

 pericvcle or parts of the undifferentiated parenchyma become meri- 

 stematic, and from them additional parenchyma and new vascular 

 bundles develop. 



Healing of wounds. As a result of the injurious effects of winds, ice, 

 insects, and fungi, areas of living tissue on trees and branches may be 

 killed or completely removed. Girdling a tree is another way of destroy- 

 ing its living tissue. It is perhaps evident by now that "recovery" from 

 such "wounds" is possible only through the activity of the vascular cam- 

 bium. Expert "tree surgeons" are able to save valuable trees, apparently 

 ruined by mechanical injury, disease, and decay, by removing the in- 

 jured areas and covering the exposed tissue with wax, tar, or other sub- 

 stances that reduce water loss and prevent the entrance of destructive 

 organisms. If the area involved is not too large, the scar may in time be 

 covered completelv by tissue that develops from a cambium. 



Grafting and budding. The importance of the cambium has long been 

 recognized in grafting and budding. Many species of plants, especially 

 fruit trees, do not "come true to seed" and must be propagated vege- 

 tatively. Sometimes it is commercially advisable to use vegetative mul- 

 tiplication, instead of seeds, for other reasons. One common practice used 

 in perpetuating plants without the use of seeds is grafting. 



In grafting, a twig or stem ( the scion ) of one plant is attached to the 



