Conditions and Principles of Absorption 75 



and other cells are quickly colored blue. It is evident that 

 there has been penetration, and further that there has 

 been accumulation of the dye. In some cases this accu- 

 mulation is particularly noticeable, due to the formation 

 of a granular precipitate, as in Spirogyra. These facts 

 give some faint idea of the complexity of the problems of 

 cell absorption. 



In accordance with the foregoing statements it is pos- 

 sible to assume that when a mixed solution is presented 

 to a root-hair, certain substances, independent of their 

 concentration in the environment, may be absorbed, while 

 others, whether dilute or concentrated, will fail to enter. 

 Upon this ground the relatively abundant occurrence of 

 iodine in seaweeds may be explained. In seawater iodine 

 is present at very great dilution, about .000001 ; yet it 

 is accumulated in marine algaB to such an extent that it 

 has yielded (and still yields upon the coasts of Japan) a 

 commercial source of this material. Similar and striking 

 examples may be found from a study of the ash content 

 of any plant ; thus the content of potash, iron, phos- 

 phoric acid, etc., may be greater than the ratio of these 

 substances in the soil solution, whereas other substances 

 may be absorbed in relatively less quantity. The ash 

 content of plants, however, is discussed at greater length 

 later. 



It is scarcely practicable to consider here some of the 

 factors which have been found to affect permeability and 

 selective absorption. It is necessary to observe, however, 

 that Overton has developed an interesting theory of 

 absorption based upon certain facts. One of these facts 

 is that substances may be assembled into diverse groups 



