CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 6yi 



mucilage of quince and linseed, result from the decomposition of cell-walls; perhaps also 

 the formation of the substance of the cuticle must be included in this category. Viscin 

 is the product of decomposed cellulose ; the origin of colloidal pectin and caoutchouc is 

 still unknown ; but none of these substances are of any further use to the plant. 



(b) Traube's Artificial Cells ^ Among the most important of the phenomena belonging 

 to the growth of the plant are those connected with the cell-wall; and everything 

 which contributes to a more exact knowledge of its development must always be 

 welcome. The researches of Traube, of which an abstract is here given, are of great 

 interest from this point of view ; even though it may not always be possible to transfer 

 all the properties of his artificial cells to the real plant. 



Starting from Graham's observation that dissolved colloids cannot diffuse through 

 colloidal membranes, and from the empirical fact that precipitates of colloidal substances 

 are usually themselves colloidal, Traube found that a drop of a colloid A placed in a 

 solution of a colloid B must become surrounded by a pellicle. If ^ is also more con- 

 centrated (or rather if its attraction for water is greater) the cell must become turgid, 

 i.e. the precipitated pellicle must become stretched by the additional water that is 

 absorbed; and the micellae of the pellicle thus become separated to such an extent 

 that a fresh precipitate takes place between them which occasions increase in the 

 superficies of the pellicle. For a more exact study Traube chiefly employed cells the 

 pellicle of which consisted of a precipitate, of gelatine tannate. For this purpose the 

 tendency of the gelatine to coagulate was destroyed by boiling for thirty-six hours. 

 A large drop of this so-called 3 gelatine of the consistency of syrup was taken up by a 

 glass rod, allowed to dry for some hours in the air, and then plunged into a flask half 

 filled with a solution of tannic acid, into the cork of which the rod was fixed. The 

 portion of gelatine which undergoes solution on the outside of the drop immediately 

 forms a completely closed pellicle with the surrounding solution of tannin ; and the 

 water which penetrates through it constantly dissolves the gelatine within. In a dilute 

 solution of tannin of o"8 to i'8 p.c. a tense pellicle which is not iridescent and is there- 

 fore thick is formed; in a concentrated solution of from 3*5 to 6 p.c. (in which therefore 

 there is a smaller difference between the concentration of the two fluids) a thin flaccid 

 iridescent pellicle is formed^. Traube found that the cells which are at first thick-walled 

 go through various stages of development ; they remain spherical so long as the nucleus 

 of gelatine is not completely dissolved ; a turbidity then sets in from above downwards 

 owing to the solution of a part of the pellicle in the solution of gelatine which is more 

 dilute in its upper part ; the pellicle at the same time begins to collapse and to become 

 iridescent ; and finally the contents become clear and tension is again set up. After 

 the lapse of some weeks the cell still allows gelatine to escape when torn. The greater 

 the difference in the concentration of the two fluids, the firmer and more tense is the 

 pellicle; i.e. the greater the intensity of the endosmotic attraction the greater is the 

 number of layers of atoms which coagulate so as to produce the pellicle, and therefore 

 the thicker it is. 



With reference to the properties of the pellicle, Traube shows that all pellicles 

 hitherto employed in experiments on diffusion have perforations^, while the precipitated 



' Traube, Experimente zur Theorie der Zellbildung u. Endosmose, in Arch, fiir Anat., Phys., 

 u. wissensch. Medicin, von Reichert u. Du Bois, 1867, p. 87 et seq. : [also Pfeffer, Osmot. Unters.] 



"^ Only pellicles of gelatine behave in this way; all others are iridescent when tense. 



^ It is easy to convince oneself of the presence of actual perforations in pig-bladder, ox-bladder, 

 the pericardium, amnion, collodion-membrane, or parchment, with which experiments on diffusion 

 have hitherto usually been made, by stretching them over a wide glass tube, pouring in a column of 

 water from 20 to 40 cm. high, and repeatedly drying the free surface of the membrane with filtering 

 paper. Water is then almost always seen to ooze out at particular spots ; a piece of membrane 

 2 or 3 cm. square is seldom water-tight. The perforations are still more evident if the tube is filled 

 with a concentrated solution of common salt and the membrane dipped in water. Instead of a 



