BALANCED SOLUTIONS : TOXIC ACTION 65 



of magnesium. Osterhout (1922) considers that an undue 

 increase of the permeabiHty of the plasma membranes is 

 caused by single salts. True (1922) supposes that insoluble 

 calcium-pectin compounds in the cell walls of the root 

 and root-hairs are changed into soluble potassium and 

 magnesium compounds by excess of these elements. The 

 relation is complex, and more than one effect maybe involved. 



Mineral salts may also neutralise the toxic effect of 

 organic compounds formed as the result of decomposition 

 processes in the soil. Schreiner and Skinner (1910, 1912) 

 have shown that dihydroxystearic acid, which they isolated 

 from the soil, has a toxic action which is prevented by 

 ammonium salts, while the harmful action of cumarin is 

 corrected by phosphates. 



Lime and the Plant. — ^The importance of calcium, not 

 only as an essential nutrient, but as an antitoxic agent, must 

 be specially noted. We may here refer to one of the most 

 discussed problems of plant biology, the question of the 

 so-called calcifuge and calcicole species. Some plants, like 

 the heather or ling, the sweet chestnut, the broom, the 

 foxglove, grow only on soils poor -in calcium carbonate 

 or lime — they are calcifuge. As they therefore grow on 

 markedly siliceous soils they have been termed silicole. 

 Others, like the box, the rock-rose, the kidney vetch, and 

 the bee orchis, occur only or chiefly on soils rich in 

 calcium carbonate — they are calcicole. The classical case 

 is that of two Swiss alpine species of Achillea : Achillea 

 vioschata is characteristic of siliceous soils, and Achillea 

 atrata of calcareous soils. Each is, however, capable 

 of growth on either type of soil in absence of the other ; 

 in competition, Achillea moschata completely suppresses 

 Achillea atrata on siliceous soils, and vice versa. A 

 similar case has recently been described by Tansley (19 17) 

 for two English bedstraws. Galium sylvestre is a calcicole, 

 Galium saxatile a calcifuge. Each can germinate and 

 produce mature plants on both types of soil, although 

 Galium saxatile is markedly weaker on calcareous soil, and 

 Galium sylvestre somewhat weaker on sandy loam. In 



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