596 MOLECULAR FORCES IN THE PLANT. 



cause the production of new molecules of pellicle, which are deposited between those 

 already in existence. Growth therefore takes place by intussusception, and is caused by 

 the stretching of the pellicle, which stretching is on its part occasioned by endosmose. 

 That the growth takes place not only by stretching but also by deposition Traube proved 

 by replacing the tannic acid by water. As soon as this was done (i. e. as soon as the 

 formation of new molecules of the precipitate in the pellicle was prevented, the endos- 

 mose still continuing) the growth ceased. 



As long as the concentration of the contents of the artificial cell is everywhere the 

 same, the pellicle remains everywhere equally thick, and the cell retains its spherical 

 form. But when the contents are diluted, a denser solution is formed in the lower 

 part of the cell, a more watery solution in the upper part. The pellicle becomes 

 in consequence thinner above and therefore more extensible, because the difference of 

 concentration is smaller there ; hence the pellicle becomes more strongly stretched above 

 and increases more rapidly in superficies, and protuberances directed outwards are not 

 unfrequently formed. This may be expressed shortly by saying that endosmose takes 

 place principally in the lower part of the cell, growth in the upper part. The difference 

 however in the concentration in the interior of the cell which causes this is the con- 

 sequence of the water which penetrates by endosmose not mixing at once uniformly 

 with all parts of the interior solution, so that layers of different specific gravity lie 

 one over another. 



Further experiments showed that growing pellicle-precipitates having the form of 

 cell-walls are produced also by mixing colloids with crystalloids^; e.g. tannic acid with 

 copper and lead acetates, soluble glass with the same substances or with copper chloride, 

 or finally crystalloids with one another, as potassium ferro-cyanide with copper acetate 

 or chloride. Traube came to the conclusion that every precipitate the interstices of 

 which are smaller than the molecules of its components must assume the form of a pel- 

 licle when the solutions of its components come into contact with one another. Since 

 the pellicle-precipitates, as has already been mentioned, contain molecular interstices 

 but no perforations, they are peculiarly well adapted for the study of endosmotic pro- 

 cesses. They behave in this respect very diff'erently from other membranes, being 

 themselves often perfectly impermeable to the most diflfusible substances, but allowing 

 other chemical compounds to pass through them ; and every kind of pellicle has in this 

 respect its own peculiarities. Independently of the fact that every pellicle-precipitate 

 is impermeable to the fluids from which it is itself produced, the /3 gelatine tannate is, 

 moreover, impermeable for example also to potassium ferro-cyanide, but permeable to 

 ammonium chloride, barium nitrate, or water. The pellicle of copper ferro-cyanide 

 which is formed round a drop of copper chloride in potassium ferro-cyanide is imper- 

 meable to barium chloride, calcium chloride, potassium sulphate, ammonium sulphate, 

 or barium nitrate, but permeable to potassium chloride or water. Traube considers 

 that in the permeability of the pellicle-precipitates we have a means of determining the 

 relative size of the molecules of different solutions, since only those molecules can pass 

 through the pellicle which are smaller than its molecular interstices and therefore 

 smaller than the molecules of the solutions which produce it. 



If a small quantity of ammonium sulphate is added to a solution of /3 gelatine, and a 

 small quantity of barium chloride to one of tannic acid, and the two mixtures thus 

 obtained are themselves mixed, a pellicle is formed of calcium tannate, and on it a 

 precipitate of barium sulphate which diminishes the size of the interstices; the two 

 solutions which cause the deposit can no longer diifuse ; but the incrusted pellicle is still 

 permeable to the smaller molecules of ammonium chloride and water. 



^ [The term 'crystalloid' is here used in the sense in which it was first employed by Graham, 

 to indicate those substances— as opposed to ' colloids' — which may be susceptible of crystallisation, 

 and which are endowed with the power of diffusion through a porous septum. — Ed.] 



