Limitation by Nutrients in Nature 285 



it plays in influencing pH and other aspects of the environment. Al- 

 though some plants do not germinate if more than a trace of calcium 

 is present, many more species are dependent upon a generous supply 

 of this element. Plants such as Aster amellus and Libanotis montana 

 that require a high percentage of calcium in the soil are referred to as 

 obligate calcipliijtes or calciphiles, and their occurrence is limited to 

 limestone or dolomitic regions. Many species are facultative calci- 

 phytes. Other plants such as Calluna (heather) and Vaccinium 

 (blueberry) that occur where the soil contains less than 3 or 4 per 

 cent calcium, are known as calciphobes. Investigation shows that 

 these correlations may be primarily due to the physical structure of 

 the soil, its general richness, or its pH as indirectly influenced by the 

 presence of lime, rather than to a direct nutritive dependence upon 

 calcium. The occurrence of Sphagnum, formerly always mentioned 

 as a strong calciphobe, is now believed to be adversely affected by high 

 concentration of hydroxyl ions rather than of calcium. This compli- 

 cated subject is discussed more fully by Lundegardh (1931, Ch. 7). 



Soils containing large amounts of gypsum (hydrated calcium sul- 

 phate) or of serpentine (hydrated magnesium silicate) support 

 peculiar or impoverished floras. Some kinds of plants require gyp- 

 sum but others are intolerant of it. Johnston ( 1941 ) reported that in 

 the deserts of northern Mexico the complex pattern of distribution in 

 certain plants was controlled with remarkable rigidity by their de- 

 pendence on gypsum coupled with very successful powers of dis- 

 semination. Haploesthes Gregii (a grass) and Nama Stewartii, for 

 example, were often found in widely separated patches of gypsum 

 soils no more than 2 or 3 sq m in extent. Within the same genus one 

 species, Foiiquieria shrevei, was confined to gypsum areas whereas 

 another species, F. splendens, occurred only on non-gypsum soils. 



Serpentine can be tolerated by only a restricted group of plant 

 species, and the areas of sparse vegetation growing on soils rich in 

 this material are known as "serpentine barrens." South of Phila- 

 delphia barrens of this type may be seen as areas about half a mile 

 wide and several miles long in which pitch pine, black-jack oak, 

 scrub oak, and cat briars are the most prominent species, whereas 

 surrounding regions support good stands of large red and white 

 oaks and gums with few if any pines (Wherry, 1932). A Venezuelan 

 orchid, Epidendrum O'Brienanwn, exhibits a similarly interesting as- 

 sociation with iron ore deposits; the plant grows within areas con- 

 taining this ore but is not found beyond their peripheries (Buck, 

 1949). 



In other localities deficiencies in trace elements control plant 



