CYTOLOGY 



17 



Whether the plastids are in fact permanent cell structures cannot yet be 

 decided. In some plants, notably Mosses, they can be traced through the 

 reproductive cells from one generation to the next, but in most cases the 

 idea of permanence rests on supposition, as the pro-plastids cannot be 

 adequately distinguished from other cell granules. 



The Nucleus 



Every normal cell has one nucleus, and one only. Exceptions to this 

 rule are rare, except in Fungi and in some reproductive structures. The 

 Bacteria and the Cyanophyceae have no nucleus as it is understood in higher 

 plants, though their cells may contain nuclear substances. In some Thallo- 

 phyta large cells occur containing many small nuclei, and these structures 

 are called coenocytes on the supposition that they may be really compound 



cells. 



The fully developed nucleus, like the chloroplast, is an organ of remark- 

 able uniformity, not only over great groups of the plant kingdom but 

 throughout most of the animal world as well. This means that the structure 

 of the nucleus is fundamental to the organization of the living cell, and that 

 conclusions drawn from its study must have a close bearing on the general 

 problems of vitality. The nucleus is in many ways the centre of the life 

 of the cell. If a cell is deprived of its nucleus it may continue to respire for 

 some time, but it does not grow or reproduce. Whether the nucleus or the 

 cytoplasm is the more ancient, that is to say which came first in evolution, 

 cannot be definitely answered, but as some lowly organisms have no nuclei it 

 seems possible that the nucleus was secondary, at least in its fully organized 

 form. 



Nucleus and cytoplasm are separated by the nuclear membrane (Fig. 4), 

 which seems to be a cytoplasmic structure like the tonoplast, but more 

 delicate and less permanent. The nucleus it surrounds is usually more or 



g.V.-.vX^'^^- ' •■■• NUCLEAR MEMBRANE 

 ■^^<-'^■ - NUCLEOLUS 



RETICULUM 





rMy CHROMOCENTRE 





Fig. 4. — Viciafaha. Diagram of the structure of a nucleus 



in the " metaboHc " or " resting " condition. 



(After Eraser and Snell.) 



