380 PHYSIOLOGY 
protein synthesis. Like CO 2 and H 2 O, they have been called " foods "; 
but it is far better to look upon them as raw materials out of which, with 
others, food can be made. 
Given carbohydrates (finished and partly torn up again, or " in the 
making ") plus nitrates, sulfates, and phosphates, most plants can make 
proteins. There is no set of plants to which protein synthesis is re- 
stricted, as is photosynthesis to the green plants. Yet there are plants 
(certain bacteria for example) which require their nitrogen supplied in 
other forms than nitrate, and some even which can use nothing less 
complex than proteins. Here we may properly speak of assimilation 
rather than of synthesis. 
No special organs. In the larger plants protein synthesis is not re- 
stricted to a particular organ. Neither chlorophyll nor light is essential 
to it, for it is carried on freely by fungi which have no chlorophyll, and 
it is doubtful, in spite of much experimenting, whether light has any in- 
fluence upon its rate. Since carbohydrates are usually the basis of pro- 
tein synthesis, the leaves, in green plants, are the chief seat of this pro- 
cess; for in the leaves carbohydrates are being made, and to them stream 
the dilute watery solutions of salts, brought via the xylem bundles by 
evaporation. 
Process. So long as the constitution of proteins remains unknown 
it will be impossible to describe the process by which they are made. 
Inasmuch as all proteins on decomposition yield amides (amino-acids), 
and the simpler ones are certainly formed from them by condensation, 
it is supposed that carbohydrates are converted into amides first, by the 
introduction of NH,-groups here and there, and that these amides link 
themselves together, some becoming modified by the incorporation of 
sulfur and phosphorus molecules, and so form proteins of various kinds. 
But the details are all uncertain and only the vaguest statements can be 
made. 
4. OTHER WAYS OF GETTING FOOD 
Dependent plants. The green plants are sometimes distinguished 
from others by the term aulotrophic, meaning that they nourish them- 
selves by their ability to make in their own bodies the most important 
foods, the carbohydrates. All others are heterotrophic plants, signifying 
that they secure food in a different way. (But see p. 362.) The more 
important ways are now to be described. 
