CHAPTER II ^^'VM0%^ 



THE ORGANISM AND THE CELL ^~ 



The bodies of animals and plants are composed of protoplasm and its 

 products; an organism is, in fact, an organized mass of protoplasm 

 interacting with its environment. The protoplasm constituting the 

 protoplast, or cell, is almost universality differentiated into two unlike 

 portions, cytoplasm and nucleus, each of these being bounded by a 

 definite membrane. In this chapter we shall examine the various ways 

 in which this dual organization is manifested in the bodies of organisms. 

 As we do so we shall point out certain general inferences which it is well 

 to have in mind before narrowing the scope of our topics in subsequent 

 chapters. 



The Typical Protoplast. — The smallest complete example or expression 

 of the fundamental organization of most living things — nucleated cyto- 

 plasm — is seen in the cell. This cell may exist singly or appear as one of 

 a multitude of such units in a large body. Cells differ greatly in their 

 internal organization and in the envelopes that may surround them, 

 but for the present we shall limit our attention mainly to certain com- 

 ponents common to nearly all of them, leaving further details to be dealt 

 with in the succeeding chapter. 



The cytoplasm is a mass of colorless protoplasm bounded externally 

 by its specialized plasma membrane. It may appear as a clear viscous 

 fluid, but often its many small inclusions give it a granular or an alveolar 

 appearance. In plant cells it commonly encloses a considerable mass of 

 cell sap against which it forms a limiting vacuolar membrane, and also 

 one or more plastids in which carbohydrates are formed. 



The nucleus is enclosed within the cytoplasm and is seldom normally 

 in direct contact with any other component of the cell. It is bounded by 

 a membrane. Usually it appears as a globule of clear matter in which 

 numerous fine threads and one or more nucleoli are embedded. 



The foregoing provisional description, very meager though it is, 

 will serve the purposes of this chapter. The importance of structural 

 and functional organization within the typical protoplast should be 

 stressed at once. The cytoplasm, the nucleus, and the plastids are 

 regions in which the protoplasm differs structurally and carries on dif- 

 ferent functional activities: they are to be regarded therefore as organs 

 of the protoplast. The membranes, too, are such specialized regions and 

 should be thought of not merely as barriers, but as organs of interaction 



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