STEMS 



651 



enormous number of leaves. Their height and size in turn are made 

 possible by the perennial nature of their aerial stems, which permits each 

 season the resumption of growth where it ceased the year before, and 

 also by their diametral increase, which permits the development of a 

 .supporting trunk sufficient to bear the weight of the constantly increas- 

 ing branches. Trees dominate the vegetation in most regions where they 

 grow, other plants being eliminated except such as 

 can endure the shade; among trees those dominate 

 ultimately whose seedlings can germinate in the 

 shade. In prairies and in treeless swamps, how- 

 ever, there is a predominance of sun-requiring herbs, 

 among which rhizomatous monocotyls, such as 

 many grasses, sedges, and rushes, take a prominent 

 place. Perhaps from the standpoint of world dom- 

 inance plants with grasslike foliage may be placed 

 second only to the trees. From the foregoing the 

 conclusion must not be drawn that plants other 

 than trees and grasses are without comparative 

 advantage. Of all vegetative organs the leaves 

 of trees are the farthest removed from the water 

 supply and the most exposed to the dangers of 

 transpiration. Thus the tree habit is possible only 

 through the development of extensive conductive, 

 mechanical, and protective tissues. 



Lianas in relation to leaf display. Definition 

 of lianas. Lianas are plants that ascend by climb- 

 ing or by leaning upon other plants or upon any 

 adequate support, their mechanical tissue being 

 insufficient to permit them to stand erect, though 

 they are prophototropic and apogeotropic. 



Twiners. Possibly the most specialized climbers are these that twine, 

 for in them the growing tip of the main stem executes movements, 

 known as revolving nutations, whereby a widening circumference comes 

 within the sweep of the elongating stem. After coming into contact with 

 an erect stem, the continuation of the nutatory movements results in 

 twining. 



If the lower part of a coil appears from behind the support at the observer's 

 left and the upper part disappears at his righff^ie twiner is called sinistrorse (as 

 in the bean, fig. 957, and in the dodder, fig. 1081). If,-as is more rarely the case, 



FIG. 957. Apart 

 of the climbing stem 

 of a scarlet runner 

 bean (Phaseolus ntul- 

 tiflorus), a representa- 

 tive sinistrorse twiner. 



