OF LIVING MATTER 33 



attraction by food and the attraction by air might as well be 

 called trophotropism as chemotaxis, the latter being shown in its 

 purest form when solutions of salt are employed. For instance, a 

 1-9 per cent, solution of KCl has a powerful effect, and draws the 

 bacteria into the tube just as peptone does, being still feebly 

 attractive even in a dilution of 0-019 P^r cent. Among the 

 alkalies, potassium is chemotactically the most powerful, then 

 sodium and rubidium. The alkaline earths are less effective. 

 . . . Among organic substances with a high nutritive value, 

 asparagin and peptone may be mentioned as being strongly chemo- 

 tactic, whilst sugar, one of the best food-stuffs and richest sources 

 of energy has but little attractive power. Glycerine is in all cases, 

 as far as is known, inactive. . . . Contrasting with the phenomena 

 just described is the power which certain substances have of 

 repelling bacteria. This is known as negative chemotaxis. Free 

 acids and alkalies have this effect, and capillaries filled with their 

 solutions invariably remain empty. ... In the case of negatively 

 chemotactic compounds, the poisonousness of the substance is no 

 more a criterion of its repellent power than the food value is 

 an index of the attractive power in positively chemotactic bodies. 

 For instance, a solution of o'oig per cent, potassium chloride plus 

 o'oi per cent, mercuric chloride attracts bacteria by reason of the 

 potash it contains, but they rush into the tube only to meet their 

 death from the mercury salt." 



(2) In many cases undoubtedly these chemotactic processes are 

 intimately associated with the nutrition of the cytodes and unicel- 

 lular organisms whose movements are thus incited, and to the 

 processes included under this head we now turn. They are 

 Assimilation alone, and Digestion followed by Assimilation — under- 

 standing ' assimilation ' to be the process by means of which matter 

 from without, of some kind, is converted within the living unit into 

 the special kind of protoplasm of which such unit is composed. 



It was, as Fischer points out, formerly an axiom of general 

 physiology, "that of all organisms the green chlorophyll-bearing 

 plants alone are able with the aid of sunlight, to assimilate the 

 carbon dioxide of the atmosphere and from it to build up carbo- 

 hydrates. All other living things, that is to say, plants devoid of 

 chlorophyll, bacteria, and all forms of animal life, were supposed 

 to be dependent for their carbon, indirectly or directly, upon the 

 carbon compounds already formed by green plants." But then he 



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