CELLS AND TISSUES 3 



is restricted to a narrow peripheral layer within the cell wall. In 

 cells where the nucleus is centrally located, strands of cytoplasm 

 may extend to it from the periphery. 



As Sharp has pointed out: "It cannot be maintained that any- 

 thing like an adequate chemical picture of living protoplasm has 

 been obtained." The protein content is high, consisting of 

 peptones, albumins, and globulins; but relatively little phosphorus 

 is present, except in the nucleoproteins of the nucleus. The fatty 

 substances most commonly found include true fats and other fatty 

 compounds such as cholesterol and lecithin. Carbohydrates occur 

 most frequently as pentoses, and inorganic salts have been recog- 

 nized in increasing numbers. The amount of water present is large, 

 varying with the type of tissue and its degree of maturity. 



The Nucleus. — The nucleus is conspicuous in meristematic 

 cells; but, as demonstrated by Bailey (3) in cambial initials, there 

 is no constant relationship between the volume of the nucleus 

 and that of the cell as suggested by Strasburger and others. It 

 usually occupies a more or less central position in the cytoplasm, 

 but this may depend upon the degree of vacuolization of the cell, 

 since it always lies embedded in the cytoplasm. It is commonly 

 spherical or ellipsoidal and is bounded by a thin nuclear membrane 

 within which is a transparent substance known as the karyolymph or 

 nuclear sap. In this sap is a reticulum of threads which are associated 

 with or constituent parts of the filamentous portions of the chromo- 

 somes. The existence of the reticulum as a valid nuclear structure 

 has been questioned; but, on the basis of critical comparisons 

 between living nuclei and those to which various fixatives have 

 been applied, it seems to be established that it is in no sense an 

 artifact. The reticulum has been regarded as a dual structure con- 

 sisting of a framework of linin supporting granules of chromatin., 

 but the more recent and probably more generally accepted view is 

 that it consists of a single complex substance, karyotin. One to 

 several nucleates may occur in the resting nucleus which appear as 

 viscous droplets and are composed of proteins and lipins. 



No complete statement regarding the function of the nucleus 

 can be made, but it is associated with the process of cell division 

 and directly or indirectly with wall formation. It undoubtedly 

 plays an important role in the later development and differentiation 

 of the cell, and is intimately concerned in the cellular metabolism. 

 From the genetic standpoint, it may be regarded as carrying and. 



