NUCLEIC ACID 381 



of biological importance as a protection against herbivorous 

 animals and parasitic fungi. Such teleological explanations 

 would appear to be unwarranted in view of the facts that 

 rabbits of the Belgian dunes consume much Hyoscyamus, that 

 rats are fond of poppy heads, and that snails fed upon Saba- 

 litschka's plants of nux vomica. 



As stated above, some of the purine bases such as xanthine 

 and guanine, although sporadic in their occurrence, have a 

 fairly wide distribution. By far the most important form in 

 which they occur is in combination with phosphoric acid and 

 carbohydrate forming nucleic acid ; this substance, being a 

 universal constituent of the nucleus, would appear to be of 

 great significance, for which reason some account of its 

 chemistry is here given. 



NUCLEIC ACID. 



So far as is known, only one nucleic acid occurs in the plant 

 world and this, presumably, is universally present in the 

 nucleus ; since yeast formed the source from which the material 

 was first produced in quantity, it is frequently referred to a 

 yeast nucleic acid, but the prefix yeast has no special signifi- 

 cance except to distinguish it from the so-called thymus 

 nucleic acid, the prototype of nucleic acids of animal origin. 

 The two nucleic acids of vegetable and animal origin are 

 very closely related in their composition, as may be seen 

 by a comparison of the products of their complete hydrolysis : — 



Hydrolytic Products of Nucleic Acid. 



Of plant origin. Of animal origin. 



Phosphoric acid. Phosphoric acid. 



("Adenine. Adenine. 



Purine bases "^^ Guanine. Guanine. 



„ . .,. , fCytosine. Cytosine. 



Pynmidine basesju^^^ji Thymine. 



Pentose (Ribose) . Laevulinic acid (? Hexose) . 



The two nucleic acids are amorphous substances which are 

 sparingly soluble in water but dissolve readily in alkaUs ; 

 with salts of the heavy metals they form precipitates and with 

 protein solutions they likewise form precipitates of the corre- 

 sponding protein nucleates. 



