CHAPTER VII 



THE NUCLEUS OF THE PROTISTA AND PLANTS 



A. PROTISTA 



The nucleus of the Protista is not constructed on such a uniform plan 

 as that of the Metazoa or Metaphyta. Certain bacteria indeed are said 

 to exhibit no differentiation into nucleus and cytoplasm, being alterna- 

 tively interpreted as consisting wholly of the one or the other. Dobell 

 (1911), however, who has made a special study of the larger forms of 

 bacteria (on which alone reliable cytological observations of this kind 

 are possible) finds a differentiation between nucleus — or, at least, chromatin 

 — and cytoplasm in all the forms which he examined. 



In many other Protista there is no organized nucleus, the chromatin, 

 etc., being scattered through the cell in the form of chromidia. This 

 may possibly be the permanent condition of the nucleus in some of the 

 more lowly organized forms, but much more often it is a temporary 

 phase, a compact nucleus being formed at other phases of the life cycle, 

 as in certain bacteria (Dobell, loc. cit.) and many Protozoa, of which 

 examples are given below. 



Even when an organized nucleus is present in the Protista, its form 

 is more varied than it is in the higher organisms. 



Two chief types of organized nucleus are commonly distinguished in 

 the Protista, namely, the vesicular and the granular. In the commonest 

 type of the former, which appears to be the less specialised form of 

 nucleus, the greater part or possibly sometimes all the chromatin is 

 aggregated into a single central mass or karyosome, which lies in a 

 vacuole containing fluid, probably of the same nature as the Metazoan 

 karyolymph. The chromatin in the karyosome is probably in the form 

 of granules bound together by linin or plastin (as in the amphinucleolus 

 of Metazoa). In certain Protista the karyosome also contains a body 

 which has been identified as the centrosome, which is therefore intra- 

 nuclear in these forms. 



In the granular type of nucleus the chromatin is distributed in the 

 form of granules or small masses over a linin framework. This type 



201 



