22 METABOLISM 



A. Members of the aliphatic series which pass through the protoplasm in 

 so far as they are soluble in water : — 



I. Easily diosmotic : — 



1. Univalent alcohols (methyl alcohol, ethyl alcohol, allyl alcohol, 



ethyl ether). 



2. Aldehydes (formaldehyde, chloralhydrate). 



3. Ketones (acetone, sulphonal). 



4. Halogen-hydrocarbons (chloroform). 



5. Neutral esters of inorganic and organic acids, provided with one 



0-H group. 



II. Not readily diosmotic : — bivalent alcohols (glycol) and the amides of 

 univalent acids. 



III. Diosmotic with difficulty : — trivalent (glycerine) and quadrivalent alco- 

 hols (erythrite), urea. 



IV. Scarcely diosmotic : — hexivalent alcohols, hexoses, amido-acids, neutral 

 salts of organic acids. 



B. Among substances which do not belong to the aliphatic series, 



and which readily enter the protoplasm, the following are known : — 



Benzol, xylol — anilin, formanilide, acetanilide — phenol, resorcin, orcin, 

 phloroglucin — antipyrine — free alkaloids, but not their salts — basic aniline dyes, 

 but not their sulphur containing salts. 



The ready sohibility of all these substances in ether, fatty oils, and similar 

 media is, according to Overton, characteristic, and this authority has assumed 

 that the limiting layer of the protoplasm is impregnated with a substance 

 with a similar power of solution, and thus that only those bodies which are 

 capable of solution in the limiting layer can enter the cell, that is to say, the 

 osmotic peculiarities of the plasma depend on phenomena of selective solubility 

 (compare Tamann, 1892). Overton gives many examples of the similarity 

 existing between solutions in oils and in the plasmatic layer ; he shows 

 especially how by certain substitutions many bodies can be made to enter the 

 plasma by dissolving them in oils. 



Certain poisons also, such as corrosive sublimate, iodine, picric acid, and 

 osmic acid which, owing to the rapidity with which they penetrate the proto- 

 plasm, have been considered as good fixing materials (compare p. 9), are shown 

 by Overton to be readily soluble in oil. Most salts, however, are insoluble 

 in oil, and appear to be incapable of penetrating the protoplasm. Further 

 research on this subject is urgently needed, for it will appear later that many 

 of these inorganic salts are indispensable and must be absorbed from without. 

 Further, accurate research is still required on the mode of absorption of the 

 ordinary gases of the atmosphere, viz. oxygen, nitrogen, and carbon-dioxide. 

 That these gases do penetrate the protoplasm cannot be doubted, as our ex- 

 position of gaseous exchange in the plant will show. 



It is, however, by no means probable that the plasmatic layer consists of a 

 fatty oil, since Algae, for example, are capable of living for whole days without 

 suffering injury in a 2 per cent, solution of sodium carbonate, a substance 

 which would emulsify, and therefore destroy the oily layer. Overton there- 

 fore came to the conclusion that the plasmatic membrane derived its osmotic 

 characters from the presence of a large percentage of cholesterin and lecithin, 

 and this hypothesis he has elaborated in his more recent researches (1900), where 

 he investigates the solubility of various substances in cholesterin. He finds 

 that the solubility of various bodies, more especially anihne dyes, in cholesterin 

 corresponds much more closely with the absorption of such substances by 

 the protoplasm than with their solubility in oil. The presence of cholesterin 

 in the plasmatic layer would thus explain the absorption of oils (Lecture XIII) 



