VARIATIONS IN STEM FORM 



39 



which the conducting system is ordinarily poorly developed. There 

 is, of course, no transpiration below the surface of the water but 

 above the surface transpiration is rapid. The length of the stems of 

 these water plants is thus governed by the depth of the water and 

 very great ^•ariations in the same species are sometimes found. 



Somewhat comparable to the variation in stem length in water is 

 the elongation of certain kinds of stems that are being gradually 

 buried by the sand of a moving sand-dune. Some woody plants, 

 such as wallows, poplars and dogwoods, grow very rapidly as the 

 sand piles up around them and the stems may become several times 

 as long as normally. Such plants, however, produce adventitious 

 roots from the stem in the sand so that the distance from the top of 



r 



Fig. 14. — Engleman spruce dwarfed at timber line and showing upright branches 



killed during the winter. 



the plant to the nearest roots, and thus to the water supply, is never 

 excessive. 



The dwarfing of plants is likewise to be explained largely on the 

 basis of a balance between absorption and transpiration. All desert 

 plants are more or less dwarf, that is, none of them grow very tall, 

 but the most typical dwarf plants are found in Arctic and Alpine 

 regions. In such places the low temperature of the soil makes ab- 

 sorption slow and difficult while the strong, dry winds tend to in- 

 crease the transpiration rate. The result is that some plants which 

 under more favorable conditions might develop into tall trees never 

 become more than a few inches high although they may live for 

 several hundred years. In many Arctic and Alpine regions the 



