40 THE ECOLOGY OF STEMS 



height of the woody plants is controlled largely by the depth of the 

 snow. They grow upward as far as they can during the favorable 

 season but during the winter all parts that project above the snow, 

 and so are unprotected, are killed through desiccation (Fig. 14). 



Tubers, such as those of the potato and many other plants, are 

 very much shortened and thickened stems and are, therefore, some- 

 what comparable to dwarf stems. The causes of tuber formation 

 are not well understood. Although tubers are most abundant in dry 

 regions yet many tuber-producing plants grow where there is an 

 abundance of water so that desiccation cannot be the only cause of 

 tuber formation, and, until a great deal more experimental work has 

 been done, it is not possible to go beyond the unsatisfactory state- 

 ment that the production of tubers is an inherited characteristic of 

 the plants on which they are produced. 



REFERENCES 



Cooper, William S. : The Broad Sclerophyll Vegetation of California, Carnegie 



Inst. Publ., 319, 124, 43 fig., 21 pL, 1922. 

 Coulter, J. M., Barnes, C. R., and Cowles, H. C: A Text-book of Botany, 



Vol. II. Ecology, Chapter III. Stems, American Book Company, 1911. 

 Hayden, Ada: The Ecologic Subterranian Anatomy of Some Plants of a 



Prairie Province in Central Iowa, Am. Jour. Bot., 6, 87-106, 1919. 

 Larson, J. A.: Natural Spreading of Planted Black Locust in Southeastern 



Ohio, Jour. For.. 33, 616-619, 1935. 

 McDougall, W. B., and Penfound, W. T.: Ecological Anatomy of Some 



Deciduous Forest Plants, Ecology, 9, 349-353, 1928. 

 Priestley, J. H., and Woffenden, Lettice, M.: Physiological Studies in 



Plant Anatomy. V. Causal Factors in Cork Formation, New Phytol., 21, 



252-268, 1922. 



