362 PHYSIOLOGY 



The most familiar physical * haracleristic of many proteins is that they 

 coagulate; heat, prolonged shaking, the action of acids, alcohol, salts, 

 etc., cause the protein to change from a liquid or semi-liquid form to 

 a firmer " clot," which by pressure can be separated into a fluid and a 

 more solid portion. The coagulation of white of egg by heat, of milk on 

 souring, and of the fibrin of blood on contact with a vessel are familiar 

 examples. Ordinarily the coagulum is insoluble in water. But the neu- 

 tral salts act differently, producing a soluble clot. Advantage is taken 

 of this fact to separate various mixed proteins and purify them partially 

 for analysis by " salting out." Other physical peculiarities are their 

 high resistance to the electric current, their large molecular weight (prob- 

 ably 15,000 and more in many cases) and hence slow diffusibility, so 

 slow usually as to be negligible. 



Some proteins crystallize, but most do not. When first discovered 

 such crystals were called " crystalloids," because it was not believed 

 that true crystals could be formed by organic matter. They are regularly 

 present in the protein grains of the Brazil nut, castor bean, etc. (fig. 664). 



Plant foods again. — Plant foods, then, are specifically these complex 

 organic compounds — not the simple inorganic substances out of which 

 green plants alone can make food. This is practically implied in the 

 terms proposed by authors who reject this use of the term food, and 

 used frequently to distinguish plants as to their mode of nutrition, viz. 

 autotrophic, or self-nourishing, plants, and heterotrophic plants. The ob- 

 vious objection to these two terms, if they are anything more than con- 

 venient and figurative ones, is that only some parts of most so-called 

 autotrophic plants are strictly self-nourishing. Only the plants whose 

 every cell contains chlorophyll are actually autotrophic. If the term 

 be used in the wide sense, green plants are not merely self-nourishing 

 — they nourish all living things. 



Kinds of food needed. — However, there is a wide difference among 

 plants as to the kind of food that they require. The known variety is 

 so great that it is impracticable to state it in detail here, and only a 

 small number of plants, chiefly fungi, have been carefully studied in this 

 respect. Some thrive best on comparatively simple compounds; others 

 require the most complex proteins. Some flourish on material which is 

 useless or even highly injurious to others. The proverb " what is one 

 man's meat is another man's poison," is quite applicable to plants. 

 Among the lowest and simplest plants, the bacteria, there are some which 

 live upon substances almost as simple as the food materials of higher 



