650 ECOLOGY 



Lombardy poplar), the stem contour is a narrow, elongated cone; it has 

 been suggested that such shapes are well-fitted for protection from in- 

 tense light. The display of foliage also is facilitated in high degree 

 by the variability of the internodes in respect to length, a variability com- 

 parable to that of petioles and attended with similar advantages (see 

 p. 725 for discussion of causes). Internodal elongation makes possible 

 not only the prevention of shading in large-leaved plants, but also, in 

 many cases, the elevation of plants above their surroundings. In hori- 

 zontal stems, leaf display often is facilitated by stem twisting, through 

 which the leaves are brought into a common plane transverse to inci- 

 dent hght (fig. 781). 



Stem contours other than conical. — Though the prevailing tree contour is conical, 

 there are many exceptions, particularly among deliquescent trees {i.e. trees whose 

 main stem is replaced by the diverging upper lateral branches), such as the bur 

 oak (fig. 956), the silver maple, and especially the American elm, whose shape is that 

 of a flaring vase. A prevalence of horizontal branching characterizes the cedar of 

 Lebanon, while some hawthorns and acacias have an almost umbrella-like contour, 

 owing to the numerous descending branches. The extreme drooping of the branches 

 in the weeping willow (due, perhaps, to a slight development of mechanical tissue) 

 gives a characteristic rounded contour to the crown. Many trees, especially 

 willows, are asymmetric, the main trunks curving to one side as they develop. 

 So far as known, these diverse tree shapes are not inherently advantageous. The 

 causes are quite as Httle known, though much more likely to repay investigation. 

 In some cases, as in Araucaria and Pinus Strobus, the branches appear to be in 

 whorls, the tree thus being divided into stories ; this habit results from the develop- 

 ment at intervals of numbers of lateral buds. An interesting habit is that of the 

 rosette-bearing trees (as in the ferns, cycads, and palms, fig. 951), characteristic 

 of tropical forests. While the lack of branches in such plants appears disadvan- 

 tageous, the elevation of the crown of leaves makes possible a relatively favorable 

 display of foliage. The palmetto often has a short trunk or none at all in dry 

 open habitats, while it has a tall slender trunk in deep woods. 



Foliage display in shrvbs and herbs. — Most shrubs and herbs that have aerial 

 stems exhibit essentially the same methods of display as do trees, their branches 

 ascending or diverging variously and exhibiting conical and other contours. Herbs 

 with stems wholly underground are poorly situated for leaf display, though some 

 forms (as Pteris) have greatly elongated petioles, which raise the leaf blades well 

 into the light. Rosette plants also are situated somewhat poorly for light reception, 

 though the leaves originate above the soil level instead of below it. 



The significance of trees and grasses in foliage display. — Trees 

 are the culminating forms of the plant world from the standpoint of 

 stem development and the display of foliage to light, their great height 

 and size making possible the development and adequate display of an 



