104 Science of Plant Life 



air currents when they are borne on an upright stem, and the 

 display of insect-pollinated flowers high up on a stem is 

 advantageous because the flowers are more readily found 

 by insects. Moreover, seeds are likely to be distributed 

 more widely when they fall from tall plants. For example, 

 a maple seed dropped from a height of 50 feet is exposed 

 to air currents during its fall and is almost sure to reach 

 the ground at some distance from the point directly below 

 the starting place. A second advantage in an upright stem 

 is that it facilitates the production and dispersal of seed. 



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 pos- 

 sible for trees to reach great heights only because their stems 

 are composed in large part of supporting tissues that have 

 great strength. Mechanical or supporting tissues are neces- 

 sary in upright stems. 



Advantages of horizontal stems. Horizontal stems have 

 little or no mechanical 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 

 growing 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 win- 

 ter and other unfavorable seasons. 



