46 CARNEGIE INSTITUTION OF WASHINGTON. 



stantly being formed in the protoplasm of living cells and in part by the 

 mineral salts which have penetrated to these vacuoles. If the plant cell 

 were a simple osmometer, the problems of enlargement would be simple; 

 but being a living mechanism, all of its parts are in a state of continual 

 change. 



Our present knowledge of permeability rests chiefly upon experiments 

 upon the eggs of fishes, starfish, and other marine organisms, on vegetative 

 cells of algae, and upon the tissues of a few higher plants. The animal eggs 

 used are of a type in which the passage of material into and out of the cell is 

 controlled by a thin external membrane. This membrane is variously taken 

 to be a protein, a lecitho-protein, or a protein-lipin-soap combination by 

 various observers. 



The importance of permeability is very great in the relations of the plant 

 to the soil as carried out by root-hairs, these structures being the tubular 

 extension of epidermal cells on the young and actively growing part of the 

 root, although they sometimes persist and are active for 70 or 80 days. All 

 exchanges between the soil and the medium must take place through these 

 cells. The root-hair is at first a dense mass of protoplasm limited by a fairly 

 definite membrane, but its development quickly carries it to a stage in which 

 the interior is occupied by an enormous vacuole, while the protoplasm forms 

 a thin layer lying against the wall. Permeability here becomes a question 

 of the passage of material through both wall and protoplasm. The entire 

 thickness of the wall and of the layers of protoplasm which intervene between 

 the vacuole and the outside of the cell is to be taken into account in all 

 studies of growth in which at no stage can the wall be considered as dead, 

 or as anything but a colloid, the mesh or lattice of which may be altered by 

 many factors. 



The wall, outside of its lipoidal component, in the stage in which it is a 

 control of permeability, has a skeletal structure of cellulose, swelling slightly 

 in water but entirely insoluble. A mixture of lipins, manosans, glucosans, 

 and pectins, all liquefying or going into solution in water, occupies the spaces 

 of mesh-work. Pectates tend to accumulate in the middle portion of the 

 wall between joined cells stratifying as a middle lamella. In the case of a 

 root-hair, this material forms the external part of the wall. 



Measurement of Changes due to Hydration of Colloids, to Altered Permeability, 



and to Growth, hy D. T. MacDougal. 



Determinations of permeability have been made by a wide variety of 

 methods, including the study of visible and micro-chemical changes in the 

 cell, analysis of sap and of the medium, measurement of hydrogen-ion con- 

 centration in the sap and in the external solution, measurements of the elec- 

 trical conductivity of the cell and of the medium, estimations of metabolism, 

 plasmolysis, plasmometry, measurements of tissue tensions, and of diffusion, 

 and finally by measurements of changes in weight and volume of living cell- 

 masses induced by the entrance or loss of water and of other substances. 



In my own studies, determinations of the action of various substances upon 

 the swelling of biocoUoids and upon living and dead tissues have been made by 

 identical technique with the auxograph designed for this purpose.^ The 

 composition of the living material has been made the subject of extensive 



' H. A. Spoehr, Carbohydrate economy of the cacti. Carnegie Inst. Wash. Pub. No. 297, 1919. 



