362 PHYSIOLOGY 
The most familiar physical characteristic 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 
