250 COLLOIDS IN BIOLOGY AND MEDICINE 



substance relatively was extracted from them by " fibrous clay" or 

 filter paper. 



It appeared furthermore, that the rising sap of the birch contains 

 only a small quantity, from 3.5 to 8.4 per cent of dry colloid substance. 

 From this fact WISLICENUS concluded that in the rising sap (until 

 about the opening of the leaves end of April) there is no new for- 

 mation of colloids, but there occurs a dissolving of everything that 

 is soluble (partial reversal of wood formation). 



The cambial juice on the other hand contains, at the time of most 

 active wood formation (end of May to end of July) large quantities 

 of adsorbable colloids (24 to 37 per cent). Towards the end of July 

 or beginning of August, when wood formation quickly ceases, the 

 colloid content of cambial sap also decreases rapidly. 



Pine i Beginning of July 31 ! P er cent adsorbed colloids. 



( beginning of August 6. 41 per cent " " 



Gra ash i be S innin g of July 24. 19 per cent " " 



( beginning of August 8.04 per cent " " 



Thus H. WISLICENUS has convincingly demonstrated that wood 

 formation is a colloid-chemical process. 



Enzymes without doubt play a most important part in the de- 

 velopment of organs. We know that enzymes serve the body by 

 changing colloids into crystalloids. They split albumin into poly- 

 peptids and amino acids, starch into saccharids, etc. Construction 

 or synthesis is also brought about by enzymes. Reactions which are 

 hastened by enzymes are reversible and it depends entirely upon sur- 

 rounding conditions, whether the balance of the process weighs more 

 in one direction than in another. Thus, for instance, A. CROFT HILL 

 was the first to show that the same ferment which splits maltose into 

 glucose actually forms maltose in a concentrated solution of glucose. 

 Since then, similar reversals have been frequently observed: POTTE 

 VIN split fats by means of pancreatic lipase and with the same 

 enzyme he also prepared fats from glycerin and oleic acid. The well- 

 known cleavage of fats with the enzyme of the castor bean was so 

 successfully reversed by WELTER that he obtained synthetically 

 almost 30 per cent of neutral fat with the same enzyme. 



In general the cleavage process proceeds best in the presence of 

 much water, whereas synthesis is most favored by the absence of 

 water. By the swelling or shrinking of the colloids present during 

 the reaction, the organism is able to permit the process to proceed 

 in one or the other direction. Swelling and shrinking are in turn de- 



