THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 169 



impermeability lies in roots and submerged plants in the external layer of the 

 protoplasm, whilst in aerial parts the impermeable cuticle prevents the washing 

 out of reserves by rain. It is very desirable that research should be undertaken 

 to determine more accurately than has hitherto been done, whether permeability 

 and impermeability are constant characteristics of the protoplasm of a definite 

 cell, or whether, as is more probable, the characters of the protoplasm'are capable 

 of variation according to the demands made upon it. [Compare NATHAN- 

 SOHN, 1904.] There is quite a number of phenomena which could, perhaps, 

 be interpreted in the former sense, but which might just as well be dependent 

 on some more complex influence of the protoplasm. At all events, the 

 facts cannot be explained merely by taking into account the principle of diffu- 

 sion, for we have to deal, not with a simple osmotic apparatus, but with a per- 

 petually changing organism. The facts to which we allude have been already 

 mentioned (p. 156). If the emptying of the storehouses of reserve were governed 

 by purely physical factors, then the artificial emptying of the endosperm in 

 PURIEWITSCH'S experiments could only have been arrested by such bodies 

 as are formed in the hydrolysis of reserves ; as a matter of fact, substances 

 also act in an inhibitory way, which are not subject to the laws of diffusion; 

 oxygen and chloroform especially have an influence on the emptying process. 

 Thus CZAPEK (1897), experimenting with leaf stalks, was able to show that 

 killing or stupefying with chloroform retarded the translocation of products of 

 assimilation, whilst an atmosphere of carbon-dioxide did not affect it. WORT- 

 MANN (Bot. Ztg. 1890), however, has arrived at an exactly opposite conclusion 

 in experimenting with carbon-dioxide. The possibility that phenomena of this 

 kind depend on variations in the quality of the plasmatic layer must be con- 

 ceded, but we must not forget the far-reaching influence of the whole proto- 

 plasmic machinery of the cell ; translocation is thus by no means so simple 

 a process as it has been hitherto considered. 



We are driven to the same conclusion for other reasons. Diffusion works 

 much too slowly to accomplish by itself the transportation of the materials 

 in the plant. DE VRIES (1885) has shown that, from STEPHAN'S calculations, 

 a milligram of sodium chloride, one of the most rapidly diffusible salts, takes 

 319 days, almost a year, in order to migrate from a 10 per cent, solution 

 into water a metre distant. The same result would be obtained in the case 

 of cane sugar in two and a-half years, and in the case of proteid in fourteen 

 years. The slowness of diffusion may be demonstrated very clearly in the follow- 

 ing way : Take a long glass tube, closed at one end, and place in it a solid 

 coloured salt such as copper sulphate, and then fill the tube with water or a not 

 too concentrated solution of gelatine. The rapidity of diffusion is quite as 

 great in the latter as in water. After a week, the copper sulphate will have 

 reached a height of 5 cm., after five weeks, 13 cm., after three months, 20 cm. 

 If the tube filled with stiff gelatine be inverted it will be seen that weight has no 

 influence on diffusion, a matter of importance in determining the translocation 

 of materials in the plant. 



This experiment leaves us in no doubt that simple diffusion cannot account 

 for the translocation of such quantities of material as migrate from the foliage 

 leaf in the course of a single night. Some means of accelerating the movement 

 must exist. One factor is the streaming movement taking place within the cell, 

 thus bringing about a mechanical mixing of the materials. Such rapid move- 

 ments result from unequal heating of different parts of the cell, perhaps, also 

 in consequence of electric currents which are widely diffused in the plant, and 

 also in consequence of protoplasmic movement. Although in this way, in a very 

 short time, a uniform degree of concentration of a certain substance may be 

 reached in a single cell, still diffusion is needed to account for the passage from 

 cell to cell and for the penetration of the cell-wall and the two layers of proto- 

 plasm which lie against it. It must not be supposed that the cell- wall places 



