October 17, 1912J 



NATURE 



intercrvstalline boundaries is clearly visible on the 

 specimens. An example of the channels formed in 

 this way is shown in Fig. i, reproduced from the 

 paper. An interesting confirmation of the authors' 

 views is further found in the tact that while the 

 boundaries between adjacent crystals always exhibit 

 this deep channel, the boundaries of twin-crystals do 

 not show such a groove ; since the crystal units on 

 either side of a "twin " boundary fit into one another 

 in a regular manner there is no room for amorphous 

 matter in these boundaries, hence the absence of the 

 groove. With the metals antimony, cadmium, and 

 aluminium the authors obtained irregular losses of 

 weight which neither confirmed nor refuted their 

 views ; thev put these views forward merely 

 as a suggestive working hypothesis, and do not claim 

 to have furnished a valid proof of its truth. 



Incidentally, the method of heating metals in high 

 vacua furnishes an interesting means of developing 

 their micro-structure, and this method has been used 

 bv the authors to study the conditions under which 

 twinned crystals are developed in silver. One of their 

 photomicrographs showing the structure of a twinned 

 crvstal of s.ilver is reproduced in Fig. 2. 



THE SURFACE-TENSION OF LIVING 

 CELLS.^ 

 TDROF. CZ.'\PEK'S pamphlet contains most 

 ^ important experimental work upon one of the 

 fundamental physical attributes of the living proto- 

 plasmic cell, namely, the surface-tension of its 

 external hmiting layer. He makes it clear that the 

 tension conditions obtaining in this layer, which 

 intermediates between each metabolic unit and its 

 environment, are of great significance for secretion 

 and for absorption, and he has established that the 

 surface-tension of the cells of the higher plants is 

 maintained fairly constant at the value of about 0685, 

 the surface-tension of water in contact with air being 

 taken as unity. 



This very important conclusion is the outcome of 

 a line of research which began at an apparently re- 

 mote point, the successive stages of which may be 

 briefly set out. The work started with the investiga- 

 tion of the curious precipitates that could be pro- 

 duced in the living cells of many plants by the action 

 of dilute ammonia or 02 per cent. catTein, such as 

 had been described as " aggregation " by Charles 

 Darwin, in the tentacles of Drosera. Czapek first 

 established that this " myelin-Iike " precipitate is a 

 compound of caffein with the soluble tannin of the 

 living cell, and is produced in all cells that contain 

 tannin, the mesophyll of Echeveria and Sedum being 

 the most suitable material. He then found that if 

 such living cells are immersed in solutions of organic 

 substances for some hours, the power of giving a 

 precipitate with caffein may be entirely lost. This 

 loss was traced to exosmosis of the tannin from the 

 living cell, and it was further found that for each 

 organic substance there was a particular limiting 

 concentration below which no effect was produced 

 and above which the exosmosis became very rapid. 

 On comparing these limiting concentrations for the 

 series of monovalent alcohols it was found that at 

 each step in the homologous series the molecular con- 

 centration required diminished to one-third. Such a 

 relation was, however, exactly what Traube had 

 established for the surface-tension effect of the 

 members of this series. 



1 '* Ueber eine Melhofle ziir clirekten Besttmmung der Oherflachcnspannung 

 derPlasmahaut voti Pflan^enzellen." By F. Czapek. Pp. iv+86. (Jena: 

 Gustav Fischer, 1911). Price 2.60 marks. 



Following up this clue, Czapek measured the 

 surface-tension of a large number of solutions of 

 organic substances, and compared their action in 

 i causing exosmosis from the cell with their activity in 

 j lowering the surface-tension of water. He thus was 

 i able to establish securely the unexpected generalisa- 

 tion that on dissolving in water sufficient of any 

 organic substance whatever to lower the surface- 

 tension to about 0-685, " solution is obtained which 

 just causes the exosmosis of the contents of living 

 cells. The critical concentration may require twelve 

 to twenty-four hours to produce its effect, but stronger 

 solutions with a lower surface-tension work very 

 quickly and thoroughly, so that after a short time 

 treatment of the cells with caffein gives no intra- 

 vital precipitate at all. 



The power of the protoplast to retain its dissolved 

 contents is thus shown to be a matter of physical 

 organisation, depending upon the surface-tension of 

 the cell being below that of the medium in contact 

 with its outSr surface. 



From true solutions Czapek passed to try the effect 

 of emulsion-colloids of a lipoid nature (proteids and 

 carbohydrates do not lower surface-tension enough to 

 give the critical value of 0685). The lipoid emulsions 

 are, however, extremely active, and give exactly the 

 same effects as true solutions. 



The last step in the progress was an attempt at 

 identification of the substance actually present in the 

 plasmatic membrane which causes it to have normally 

 so low a surface-tension as 0-685. Czapek finds that 

 saturated emulsions of neutral fats lower the surface- 

 tension just to this value and no further, so that it 

 seems very probable that these are the effective sub- 

 stances in the living cell. 



We have thus striking support for the view, widely 

 adopted from the work of Overton and Meyer, that 

 the plasmatic membrane is of a lipoid nature. 

 Overton's later view was that lecithin and cholesterin 

 rather than neutral fats were the particular lipoids 

 present, but these give a lower surface-tension down 

 to about 0-5. The present line of work indicates 

 that these may be the effective substances in some 

 cells, not those of the higher plants, for yeast and 

 red-blood corpuscles require a medium of about this 

 lower surface-tension to bring on exosmosis of 

 invertase and haemoglobin respectively. 



Many supporters of the lipoid theory of the con- 

 stitution of the plasmatic membrane have interpreted 

 it to mean that there exists at the surface of the cell 

 a continuous film of a lipoid nature, and this has 

 raised difficulties in understanding the intake of 

 typical nutrient substances which are freely soluble 

 in water, but not in fat. Czapek points out that an 

 emulsion containing only a small percentage of fat is 

 all that is needed to endow the cell with the observed 

 specific properties. 

 j Willard Gibbs showed from thermodynamical con- 

 j siderations that substances in a solution which 

 , strongly reduce surface-tension must accumulate in 

 I the surface-layer until their return by local excess 

 of osmotic pressure produces a state of equilibrium 

 between the surface and the mass. With emulsified 

 I fat particles, however, the osmotic pressure is 

 very slight, and very great accumulation in the 

 surface-laj'er must result. 



This piece of work may serve as a model of scien- 

 tific method on account of the way in which the 

 I mysterious phenomenon of "aggregation," described 

 I by Charles Darwin, has been followed on and on 

 I until it has led to the evaluation of so fundamental a 

 I vital constant as the surface-tension of the living 

 cell. F. F. Blackman. 



NO. 2242, VOL. 90] 



