i8 A TEXTBOOK OF THEORETICAL BOTANY 



less spherical. Inside the membrane is the karyolymph or nuclear sap, 

 resembling the cell sap, except that it is very viscous and sometimes almost 

 solid. In some respects therefore the nucleus may be looked upon as a 

 special kind of vacuole. Where it differs from other vacuoles is in containing 

 the substance chromatin, which plays an entirely special role in the cell. 

 In it are located those factors which control the development of the cell 

 and hence the development of the whole organism, and serve, moreover, to 

 transmit the same characteristics to the following generation. In short, the 

 chromatin is the seat of the mechanism of heredity which maintains the 

 continuity of the species. 



The chromatin is visible as a spherical network of fine threads inside the 

 nuclear membrane and in contact with it. This is called the reticulum of 

 the nucleus, and it consists of simple proteins of the protamine and histone 

 types. These are basic substances and they therefore stain easily with acid 

 dyes. The nuclear surface is probably the seat of protein synthesis in the 

 cell so that these proteins are formed /;/ situ. At many points of the reticulum 

 are seen lumps or knots of material which stain differently, chiefly with 

 basic dyes. This is due to the fact that they contain thymo-nucleic acid, 

 and they are spoken of as heterochromatin, while the rest of the reticulum 

 is called euchromatin. 



H— C=0 H— C\ 



! I \ 



H— C— OH H— C— H\ 



I I O 



H— C— OH 0=C— H/ 



I - I / 



H— C— OH U—C/ 



CH2OH CH2OH 



d-ribose C5H10O5. desoxy-ribose CgHgOg. 



Thymo-nucleic acid differs from the ribo-nucleic acid that we spoke of 

 in connection with mitochondria. The chief difference lies in the sugar 

 which forms part of the molecule. In ribo-nucleic acid there is, as the 

 name implies, ribose, a stable sugar which can be isolated as a pure substance. 

 In thvmo-nucleic acid, on the other hand, ribose is replaced by a molecular 

 group, desoxy-ribose, which only exists in combination with other molecules, 

 and which has a strong tendency towards the linking of its molecules to one 

 another in long chains to produce complexes or " polymers " of as many 

 as 6000 molecules, arranged in a similar manner to the molecules in the 

 protamine proteins. This substance can be identified microscopically by a 

 staining reaction known as the Feulgen Test,* and its presence is taken as 

 indicative of the existence of chromatin in a cell. 



In addition to the reticulum each nucleus contains one or two rounded bodies 

 called nucleoli, which are attached to part of the reticulum. The nucleolus 



* Thymo-nucleic acid, set free by hydrolysis of chromatin, gives a purple colour when 

 treated with Basic magenta decolorized by Sodium sulphite. 



