CHAPTER II. 



OCCURRENCE OF PROTEINS IN DIFFERENT PARTS OF PLANTS, AND 

 THEIR GENERAL CHARACTERISTICS. 



PROTEINS are found in the living parts of all plants. They occur in 

 the dissolved state in the circulating fluids and in the solutions of the 

 cell vacuoles, that is in the cell sap. In a semi-dissolved state they 

 occur in the protoplasm, and in the undissolved state as reserve protein 

 in the cells of seeds, tubers, bulbs, buds and roots. 



In many of the cells of these parts of the plant the undissolved 

 protein is found in the form of well-developed crystals of various forms, 

 formerly called crystalloids ; in irregular, semi-crystalline forms with 

 faces and angles on a part of their surface, and as regular or distorted 

 spheres, all of which several forms are found in aleurone grains ; and in 

 an amorphous, finely granular form, generally designated aleurone. The 

 reserve protein occurs in the cells together with the non-nitrogenous 

 reserve food materials, starch, oil, etc., which several substances fill the 

 cells, leaving a thin layer of dried protoplasm between them and the 

 cell wall \cf. Hartig (148, 149); Radlkofer (388); Nageli (286); 

 Schimper (453); Schulz (462)]. 



In most monocotyledonous plants the cells of the endosperm and 

 embryo occupy distinct parts of the seed. The tissues of the endosperm 

 of such seeds when fully ripe are, therefore, made up of cells which are 

 almost entirely filled with the reserve food substances, since the thin layer 

 of protoplasm next to the cell wall forms a very small part of the con- 

 tents of the cell. 



The tissues of the embryo contain protein associated with a greater 

 variety of substances than are present in the cells of the endosperm, and 

 are also rich in nucleated cells, in which much of the protein apparently 

 exists in the chromatin substance of the nuclei in special forms of com- 

 bination with nucleic acid which are generally known as nucleoproteins 

 and nuclein. In this part of the seed the chemical conditions are 

 therefore more complicated than in the cells of the endosperm, since 

 the metabolic processes of the embryo apparently require a greater 

 variety of substances than exist in the cells of the endosperm of the 

 fully ripe seed, whose chief office is to supply food to the subsequently 



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