DIV. ii PHYSIOLOGY 241 



If the protoplasm were really quite impermeable to the salts that 

 have been considered above, not even traces of them could enter the 

 cell cavity. Practically, however, the impermeability of the protoplasm 

 is perhaps not absolute for any substance ; there are all grades, from 

 substances that pass through the protoplasm as easily as water, to 

 those that are almost incapable of passing through it. Alcohol, ether, 

 chloral hydrate, numerous organic pigments, and, lastly, very dilute 

 acids and alkalies, diffuse with special rapidity. 



The permeability of the protoplasm is not always the same, and 

 may be regulated according to the requirements of the cell ( 22 ). The 

 salts of alkalies, for example, determine an increasing impermeability 

 as regards themselves, and the salts of the alkaline earths can also 

 diminish permeability for the alkaline salts. The absorption or not of 

 a substance is determined not by the whole protoplasm but by its 

 external limiting layer. In the further passage of the substance, from 

 the protoplasm into the cell sap, the wall of the vacuole exercises a 

 similar power of selection. The cause of the SELECTIVE POWER, by 

 reason of which different cells can appropriate quite distinct con- 

 stituents or substances in different amounts from the same soil, is to 

 be sought in this most important property of the limiting layers of the 

 protoplasm. 



From the same soil one plant will take up chiefly silica, another lime, a third 

 common salt. The action of Seaweeds in this respect is especially instructive ; 

 living in a medium containing some 3 per cent of common salt and poor in 

 potassium salts, their cells, nevertheless, absorb relatively little common salt, but 

 accumulate potassium salts. 



Every substance to which the limiting layers of the protoplasm 

 are permeable must ultimately reach the same concentration in the 

 vacuole as in the solution outside the cell when its absorption would 

 cease. Practically it often enters in much greater amount than this. 

 Thus, for example, only a trace of iodine is present in sea- water, 

 but may be accumulated in such quantities in seaweeds for these to 

 become a source from which it is commercially obtained. The cell 

 has not only a selective power, but is also able to store up materials 

 by converting them into insoluble or indiffusible forms. 



Certain organic pigments ( 23 ) such as methylene blue are especially suited to 

 demonstrate the entrance and accumulation. Many cells contain tannins in their 

 vacuoles, and these substances form with the entering pigment a compound which 

 is indiffusible or quite insoluble. For this reason the vacuole becomes deeply 

 coloured or has blue precipitates, though the solution of methylene blue employed 

 is extremely dilute. It is noteworthy that the protoplasm itself remains un- 

 stained and is not in any way injured ; the pigment would be accumulated iu 

 dead protoplasm. 



Under natural conditions some plants absorb the nutrient salts 

 from water as do the plants in a water-culture experiment. This is 



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