Stems and Their External Features 



103 



being shaded, and each day 

 they are exposed to the sim- 

 shine during a longer period. 



The tall plant has an 

 additional advantage in 

 being able to expose to the 

 Ught a greater leaf area 

 over a given space of 

 ground, because it can dis- 

 play several or many layers 

 of leaves one above the 

 other. The rosette of 

 leaves formed by the bur- 

 dock illustrates the possi- 

 bihties of leaf display near 

 the soil. A large sunflower 

 plant covers no greater soil 

 area than a burdock, but 

 it is able to expose to the 

 simlight several times as 

 great a leaf area, because 



the sunflower leaves are placed at several different levels (Fig. 

 61). Trees have the greatest stem development and the greatest 

 leaf display. Rosette plants, like the dandeUons and plantains, 

 represent the opposite extreme of sHght stem development, 

 small leaf area, and a poor leaf display. One advantage in a 

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

 thereby makes possible a greater leaf display. 



Plants with upright stems have advantages also in connec- 

 tion with reproduction and seed dispersal. Wind-pollinated 

 flowers, like those of corn or the pine, are better exposed to 



Edu.in U Lincoln 

 Fig. 62. An American elm in the Berkshire 

 Hills, Massachusetts. The tall stem of such 

 a plant makes possible the display of a large 

 number of leaves to the light and facilitates 

 the production and dispersal of seed. 



