380 PHYSIOLOGY 



protein synthesis. LikeCOo and H 2 0, 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 whiclv 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 2 -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 autotrophic, 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. 



