io6 



General Botany 



upright stem is that it holds the leaves up to the light and thereby 

 makes possible a greater leaf display. Under certain conditions 

 upright stems may facihtate pollination and seed dispersal 



(Fig. 55)- 



The advantages of the upright stem are all dependent on its 

 capacity to support other organs. The stem must be strong 

 enough to support leaves, flowers, and fruits. The city sky- 

 scraper needs first of all a strong framework about which the 

 building is constructed. The stems of tall, erect plants must be 

 correspondingly strengthened by a mechanical structure. The 

 base of a tree is much smaller in proportion to its height than that 

 of the tallest and narrowest building, and it is possible for trees 

 to reach great heights only because their stems are composed in 

 large part of supporting tissues of great strength and pliabiHty. 

 The largest upright stems. Stems attain their best develop- 

 ment under medium conditions of 

 moisture, light, and temperature. 

 Such conditions are found in the 

 eastern United States, in the can- 

 yons of the western mountains, 

 and along the Northern Pacific 

 slope, and there the plants are 

 characterized by large leaf area. 

 In the East the vegetation cul- 

 minates in the forests of the rich, 

 well-watered soils of the river val- 

 leys. Here may be found oaks, 

 walnuts, elms, maples, sycamores, 

 and magnolias which reach 

 heights up to ico or i8o feet and 

 have trunks attaining diameters 



Fig. 55. Sunflower and burdock, showing of from 4 tO 14 feet. 



advantage of the tall, upright stem. The j^ ^^^^^ CanyOnS of the 



sunflower covers no greater soil area but -^ 



it displays more leaves to the light. Sierras of California the giant 





