2 PLANT PHYSIOLOGY " 



the characteristic shape of each of the cells. Vital functions 

 belong only to the contents of the cell, specifically to that part 

 of it termed the ''protoplast'' and composed of the cytoplasm 

 with its plastids, mitochondria, vacuoles, etc., and the nucleus 

 with its chromosomes, etc. The cell sap comprising the major 

 part of the volume of the cell and various cell inclusions (such 

 as crystals and starch grains) often present in cells are not living 

 parts. They are products of the vital activity of the protoplasm 

 not actively undergoing metabolic change. 



Certain properties of the protoplast are closely connected with 

 its physicochemical organization and chemical composition. 

 A more detailed acquaintance with the chemical composition 

 of the protoplasm will be obtained in Chap. II, where the chem- 

 ical transformations occurring in the cell will be discussed. At 

 present, it is sufficient to indicate that the principal substances 

 composing the protoplasm are the proteins, which represent the 

 basic carriers of vital phenomena. But pure isolated proteins 

 never reveal any symptoms of vital activity. When extracted 

 from the organism and deprived of their peculiar structure, they 

 prove to be fifeless organic compounds, similar to fats, carbo- 

 hydrates, and other substances that are always present in cells 

 of plants. In order to serve as a substratum for life processes, 

 proteins must be closely combined with many other compounds, 

 first of all with lipoids, which are substances closely related to 

 fats. Only in the presence of lipoids and other substances, 

 however slight in quantity in comparison to the proteins, is it 

 possible to maintain the structure of protoplasm, so essential for 

 its functions. 



Proteins are extremely complicated organic substances. 

 Elementary analysis always reveals in them at least five chemical 

 elements: carbon, hydrogen, oxygen, nitrogen, sulphur, and 

 sometimes phosphorus and several other elements. Of these 

 elements, carbon constitutes about 55 to 56 per cent; nitrogen, 

 15 to 18 per cent; hydrogen, 6.5 to 7.5 per cent; sulphur, 0.3 to 

 2.5 per cent; and oxygen 20 to 25 per cent. The protein molecule 

 is complicated in structure and enormous in size. It contains 

 both NH2 and COOH groups, thus possessing the properties 

 of both acids and alkalies. Under the influence of dilute acids 

 and other agents, proteins disintegrate into a number of amino 

 acids, which therefore are assumed to be the chief constituents 



