136 PLANTS AND MAN 



almost entirely from the seeds of the cereals (corn, wheat, oats, 

 rice) and of the legumes (peas and beans). 



There are many different types of proteins, occurring in 

 different forms. Some exist as part of the protoplasm of the cell, 

 others are in solution in the cell sap, and still others are solid 

 particles known as aleurone grains. Proteins in solution are 

 sensitive to extreme heat or cold and, when exposed to either, 

 frequently coagulate. Proteins can not pass through cell mem- 

 branes; therefore transfer of proteins from one part of the plant 

 to another necessitates a change to their amino acid constituents, 

 and in this form the translocation is possible. 



One group of simple proteins includes the albumins, glutelins 

 and gliadins. Albumins are soluble in water, and are widely 

 distributed in both plant and animal tissues, but especially 

 abundant in cereals and legumes. Glutelins are insoluble in 

 water, and differ from most proteins in being unaffected by heat; 

 the commonest form exists as glutenin in wheat seeds. The 

 gliadins are insoluble in water also; they are found chiefly in 

 cereal seeds. The zein already referred to is a gliadin protein. In 

 wheat flour it is the gliadin which combines with the water to 

 form the sticky substance which holds the glutenin particles 

 together. The resulting paste (glutenin and gliadin) is known as 

 gluten, and gives to dough its elastic rising properties. 



Pigments and Other Plant Products 



Most of the activity of a green plant is concerned with the 

 manufacture, utilization and storage of food substances; i.e., 

 carbohydrates, fats and proteins. However, the basic carbohy- 

 drate foods are also important as the sources of numerous other 

 complex products some of which are closely related to the sugars, 

 others to the proteins. The most interesting of these are the ones 

 which determine the color of plants. 



Plant pigments result in a greater variety of colors than is 

 found in the animal world; most common is chlorophyll, 

 which gives the characteristic green hue to vegetation. Chloro- 

 phyll is an organic compound elaborated by plants, but its 

 exact origin is not known. Chemically it is a combination of an 



