died shortly after gemination or grew very slowly. A few man- 

 aged to make it through to flowering, but only as dwarfed plants. 

 Seedlings from serpentine populations grew very well on serpen- 

 tine soils and also did very well on non-serpentine soils. One con- 

 clusion that was drawn from this set of simple experiments is 

 that G. capitata is made up of at least two genetically different 

 ecological races adapted to different soils. One of these races is a 

 serpentine race which tolerates serpentine soil (as well as non- 

 serpentine soil) and the other is a serpentine-intolerant race which 

 can grow only on non-serpentine soils. 



One question that might be asked, however, is that if the ser- 

 pentine races can grow not only on serpentine soils but off 

 serpentine soils as well, why is it that apparently all plants of 

 Gilia capitata that occur off serpentine in the wild are, in fact, 

 serpentine intolerant? The answer to this seems to be that, for 

 some reason, the serpentine races are not only tolerant of serpen- 

 tine soils but they are generally restricted to these soils because 

 they are susceptible to pathogenic soil fungi that occur in non- 

 serpentine soil. Whenever seeds of the serpentine race stray over 

 to non-serpentine soils the resultant seedlings die because they 

 are attacked by pathogenic soil fungi present under field condi- 

 tions, or are otherwise unable to compete successfully with 

 plants on these soils. In contrast, the serpentine-intolerant races 

 of G. capitata are not only intolerant of serpentine but are tol- 

 erant of the soil fungi. One might characterize G. capitata as a 

 species made up of serpentine-tolerant, fungus-intolerant races 

 and serpentine-intolerant, fungus-tolerant races. R. B. Walker, a 

 contemporary of Kruckeberg, suggested that the physiological 

 basis of serpentine tolerance lay chiefly in the ability of serpen- 

 tine tolerant races to extract sufficient calcium for their meta- 

 bolic needs against the magnesium gradient. 



Regardless of the physiological interpretations that can be 

 made from the series of experiments and observations described 

 above, it is clear that soils do have an important influence on 

 plant distribution, and it can be demonstrated clearly that some 

 species or races of plants can survive only on specific soil types 

 and are genetically and physiologically adapted to them. 



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