STEMS, THEIR FORMS AND FUNCTIONS 71 



1. Escape from grazing animals. 



2. Escape from crushing by being stepped on. 



3. Crowding away neighbors by the wide, close leaves. 



4. Water is retained near the root, by the cover of leaves above. 

 Creeping Stems. The creeping stem is another type, with 



common examples, such as the strawberry, in which a plant, 

 though having a weak and slender stem, is, with great economy 

 of wood tissue, enabled to spread its leaves widely. By this habit 

 it also escapes injury from wind, cold, or storms, since it is closely 

 attached to the earth at frequent intervals. Besides, these 

 " runners," as the horizontal branches are called, furnish a valuable 

 means of propagation, since they send out roots at the nodes, 

 and grow even if separated from the parent plant. 



Climbing Stems. Many stems succeed in exposing their leaves 

 to the light without producing much more supporting tissue than 

 do the creepers. These are the climbing stems which use supports 

 outside of their own structures to lift themselves into the light. 

 One means of climbing is by twining round some supporting plant, 

 as in case of hops and pole beans. Another similar method is by 

 means of tendrils, which are usually leaves reduced to the mere 

 skeleton of veins, as in the grape, wild cucumber, etc. 



The coiling of tendrils or twining stems is a curious process, 

 for it frequently seems as though a plant or tendril had started 

 straight for a certain support and deliberately coiled about it. 

 This is not the case though the real process is scarcely less wonderful. 

 The tip of the twiner or the tendril grows unequally on different 

 sides, causing it to swing through the air in circles, as it grows. 

 Thus it has a chance to reach anything within the radius of its 

 swing, which is often several inches. 



Having reached a support, the growing point can no longer 

 swing as a whole, but the tip coils about the support as it grows, 

 enabling it to rise as high as its sturdier neighbors. Tendrils also 

 coil between the support and the plant, raising the latter and hold- 

 ing it by a spring which will yield to wind pressure without break- 

 ing. This later coil usually reverses midway to avoid twisting the 

 tendril off. 



