6S THE BIOLOGY OF FLOWERING PLANTS 



(19 1 5) on seaweeds, yeasts, aquatic plants (Azolla) and land 

 plants (bean, lupin, chick-pea). Analysis of the solutions 

 in which these plants were grown showed great differences 

 between the two ions of a salt, both in the rate of absorption 

 and in the equilibrium finally reached. Stiles and Kidd 

 (1919^) have shown that chlorides and nitrates are more 

 rapidly absorbed than sulphates, and that, of cations, 

 potassium is absorbed most rapidly, then sodium, and 

 then calcium and magnesium. Into the mechanism of 

 this relation we cannot go here, though we may note that 

 these investigators agree that adsorption by colloids is 

 involved. It may be noted that these results are a step in 

 the analysis of the phenomena grouped under the head of 

 the " selective permeabihty " of the protoplasm. A 

 familiar example of this was given by the known facts that 

 the constitution of the ash of a plant does not agree closely 

 with that of the minerals in the water or soil in which it 

 grows, and that many plants are able to accumulate large 

 quantities of certain elements, the richness of the kelps in 

 iodine and of the tobacco in nitrates being instances. 



Solvent Action of Roots. — ^We may here refer to the 

 action of the plant in making minerals available for absorp- 

 tion. In a classical experiment Sachs showed that if seed- 

 lings are grown with the roots touching a sloping plate of 

 marble, the course of the root branches was revealed as an 

 etching on the polished surface. The inference was that 

 an excretion of the roots was responsible for dissolving the 

 marble. This action might be due to a solution of carbon 

 dioxide in water, and the carbon dioxide respired by the 

 roots would be sufficient to explain it. Using a mixture of 

 calcined gypsum and various other insoluble minerals, 

 poured on a glass plate to obtain a polished surface, Czapek 

 found that only those minerals which were soluble in water 

 containing carbon dioxide in solution showed etching, 

 i.e. carbonates, and phosphates of calcium, magnesium, and 

 iron. Aluminium phosphate, which is insoluble in carbon 

 dioxide water, was not etched. 



]\lany investigators have endeavoured to show that other 



