226 



A YEAR IN SCIENCE 



the light and to the air. Stems exhibit a great variety 

 of form and habits. The upright, or erect stem, is the 

 most common, and altogether seems to be the best 

 device for properly bringing the leaves to light. The 

 most conspicuous erect stems are those of trees. The 

 branches of erect stems usually have a more or less 

 horizontal position, and are so arranged that one does 

 not shade another. Hard tissues are always present 



in the large erect stems to give 

 them support much as bones 

 give support to the bodies of 

 animals. 



In many plants the stem lies 

 prostrate on the ground, as in 

 the strawberry. Others have 

 developed a climbing habit. 

 Such stems can not stand alone, 

 but they have the ability to sus- 

 tain themselves by using sup- 

 ports. They have various ways 

 of attaching themselves to the 

 supports; the ivy by means of 



o 1ir ,],.j nO i Hkkj thp o-ranp hv 

 sucKing aibKs , me grape uy 



, ., , , 



tendrils ; beans, morning glories, 

 and hops twine their stems about the support. The 

 stems of desert plants are usually very short and 

 thick. This is quite in contrast to the enormously 

 developed root system of these plants. In the cactuses 

 the leaves are reduced to spines, and the short thick 



Fig. 93. Theivy climbs 

 by means of sucking 



disks, 8. 



