RECENT ADVANCES IN SCIENCE 575 



and a sandy loam respectively. It was found that these 

 organic solutions gave no reaction either with phenolphthalein 

 or methyl orange, but nevertheless had a pronounced effect on 

 the solubility of the calcium and phosphoric acid in these soils. 

 The amount of phosphoric acid dissolved from the soil 03^ the 

 organic solvents was from \"j to 5*4 times that extracted by 

 distilled water. The solubility of magnesium and iron was 

 also considerably increased by their use. It is suggested 

 that the beneficial effect of mulches may be in part due to these 

 facts. 



PLANT PHYSIOLOGY. By Walter Stiles, M.A., University, Leeds. 

 (Plant Physiology Committee.) 



Permeability. — There are a number of expressions* in common 

 use in plant physiology which serve very largely to screen 

 our ignorance of the problems under discussion. Among these 

 is the term permeability. This word is used in a variety of 

 senses by different writers, and cases can be cited of the same 

 writer using it in different senses in the same paper. The 

 simplest sense in which the word permeability may be used 

 is in regard to the capacity of a membrane for allowing a sub- 

 stance to pass through from one side to the other. In the case 

 of the cell, however, we are dealing with a structure much 

 more complex than a membrane. Outside we have the cell 

 wall, pressed against this is the cytoplasm of the cell, and 

 within this is the vacuole. At the surface of separation of the 

 cell wall and cytoplasm, and also of the cytoplasm and vacuole, 

 we may suppose layers (the ectoplast or plasmatic membrane, 

 and the tonoplast or vacuole wall respectively) with properties 

 different from those of the main part of the cytoplasm. The most 

 general sense then in which the expression permeability can be 

 used in regard to the living cell is when it indicates simply the 

 capacity of the substance to pass into or out of the cell, and 

 this is the sense in which it is generally used. Some writers, how- 

 ever, speak of permeability when they mean the capacity of a 

 substance to pass from the external medium through the cyto- 

 plasm, into the vacuole, or even from outside the cell through 

 the limiting layer (plasmatic membrane) into the cytoplasm. 

 It is sometimes not clear what a writer does mean. There is 

 thus a necessity for definition of the term permeability, and in 

 this article it will be taken in its ordinary general sense as 



