no 



General Botany 



type of tendril is especially effective in taking hold of the bark 

 of trees, rock chffs, and walls. Still other climbers, like the 

 trumpet creeper and poison ivy, have aerial roots that become 

 fastened to a support by growing into cracks and crevices ; or, by 

 the cementing action of the outer pectic layer of the epidermal 

 cell walls, they may become attached to quite smooth surfaces. In 

 the moist tropics, chmbing stems may attain a length of more than 

 looo feet. Thus the water transpired by terminal leaves has been 

 carried about a fifth of a mile within the plant. A cross-section of 

 such a stem will show not only that most of the stem is occupied 

 by water-conducting tissue, but that the individual tracheae are 

 very large and long when compared with those of upright stems. 

 Horizontal stems. Horizontal stems have httle woody tissue, 



and they display leaves to the 

 light advantageously only when 

 they grow in the open. There 

 are advantages in stems of this 

 type, however, because by grow- 

 ing horizontally on the soil or 

 beneath the surface of the soil 

 they spread the plant; because 

 they are in contact with the soil 

 and may take root at frequent 

 intervals; and because they are 

 better protected than upright 

 stems during the winter and 

 other unfavorable seasons. 



Underground stems. Many 

 plants, both herbaceous and 

 woody, possess underground 

 stems. They are particularly 

 useful as places of food accu- 

 FiG. 59. Young and old stems of Boston mulation and in vcgctativcly 

 ivy. T:_e young stems are held by spreading and multiplying the 



means oi tendrils, the older stems by ^ x ^ <-> 



means of adventitious aerial roots. plant. Their position renders 



