26 THE CELL 



exhaustively with these divergent opinions. It may, however, be 

 remarked that a compromise can be effected between the two attitudes, 

 if it be admitted that plasmatic membranes may exhibit varying 

 degrees of specialisation. When such a membrane merely represents 

 a protective layer and an instrument for regulating interchange of 

 material, it can scarcely be regarded as an autonomous organ. As its 

 structure becomes more complex and as the principle of division of 

 labour which extends even to the individual protoplast makes itself 

 felt in connection with its activities, the plasmatic membrane acquires 

 greater independence, and it may thus in certain circumstances even 

 attain to a state of complete autonomy. The autonomous nuclei and 

 chromatophores, which are discussed below (cf. Sections 3 and 4), must 

 after all have been evolved in a similar fashion ; at any rate, it is 

 difficult to imagine how they can have originated otherwise. 



3. The Nucleus. 



Every typical vegetable protoplast is provided at any rate with 

 one highly individualised cell-organ, namely, the nucleus. The nucleus 

 is characterised as an organ of the first importance, not only by the 

 constancy of its occurrence, but also by the uniformity of its structure. 



The principal component of the nucleus is a meshwork of delicate 

 threads, which are intertwined and connected to one another by 

 numerous anastomoses. In properly fixed and stained preparations 

 this nuclear reticulum can be investigated in detail. It is then found 

 that the threads themselves are at most very slightly stained, but that 

 they have embedded in them granules of a deeply staining material, 

 the so-called chromatin. It is not improbable that these granules 

 contain the nucleins, which are the characteristic chemical components 

 of nuclear substance. Somewhere among the convolutions of the 

 nuclear reticulum are situated one or more highly refractive, usually 

 spherical nucleoli, which are readily distinguishable from chromatin 

 granules by their staining properties. The meshes of the nuclear 

 reticulum are filled with a homogeneous nuclear sap. From the 

 surrounding cytoplasm the nucleus appears to be separated by a 

 protoplasmic nuclear membrane. 10 



The shape of the nucleus is to some extent correlated with that 

 of the cell to which it belongs. Thus it is usually rounded, and 

 either spherical or disc-shaped, in isodiametric cells, while in elongated 

 elements it often assumes a spindle- or rod-like form. There are, how- 

 ever, not a few exceptions to this rule. Many fibres (for instance, 

 those of Linum) contain small spherical nuclei. The nuclei of the 

 guard-cells of Omithogalum vmbcllatum are crescentic in outline, while 

 those of the corresponding cells of various grasses are dumbbell-shaped 





