66 THE BIOLOGY OF FLOWERING PLANTS 



competition, Galitmi sylvestre completely suppresses Galium 

 saxatile on calcareous soil ; on sandy loam, Galium saxatile 

 becomes dominant. 



The effect of the presence or absence of lime in such 

 cases may be due to a variety of causes. In the first place, 

 calcareous soils tend to be well drained and warm. It is 

 in all probability because of this that they are preferred by 

 such plants as the box and the rock-rose, for these grow 

 perfectly well on well- drained siliceous gravels in sunny 

 situations. In the second place, the hydrogen ion concen- 

 tration of calcareous soils is low, that of soils poor in lime 

 tends to be high. Olsen (1923) has found a remarkable 

 correlation between the occurrence of Swedish plants and 

 the ^" value of the soil. Sahsbury (1921) has found a 

 similar correlation for some English plants. Olsen has also 

 shown experimentally that different species react differently 

 to the same /)" value in identical nutrient solutions. When 

 we find plants like Calluna vulgaris or Galiufn saxatile 

 showing marked signs of weakening in calcareous soils, 

 it is very likely that we are witnessing the effects of an 

 unsuitable hydrogen ion concentration. In the third 

 place, excess or deficiency of calcium may produce an 

 " unbalanced " and toxic soil solution for a particular plant. 

 It has been shown (Schimper, Jost) that excess of calcium 

 depresses the absorption of potassium and iron, with the 

 result that the broom, the sweet chestnut, and the maritime 

 pine become stunted and chlorotic, forming little chloro- 

 phyll. Watering these plants with potassium and iron salts 

 enables them to grow on calcareous soils. Mevius (1921) 

 states, however, that for the broom and pine the hydrogen 

 ion relation is important. For further details the critical 

 survey by Salisbury (1920) should be consulted. 



Absorption o£ Salts. — The regulation of the entiy of the 

 salts into the cell is carried out by the protoplasm or its 

 limiting plasma membranes. On the classical de Vries- 

 Pfeffer theory of the semi-permeability of the protoplasm, 

 the proof of non-penetration by various substances is given 

 by the production of plasmolysis in solutions of sufficient 



