23.] CHAPTER I. THE CELL. 97 



simply in the constriction of the nucleus into two halves (Fig. 

 48). * 



Indirect or mitotic division of the nucleus is generally associated 

 with cell-division ; in coenocytes, however, the indirect nuclear 

 division is not followed by cell-division, but each division results 

 simply in a corresponding increase in the number of protoplasmic 

 units present (energids, see p. 90). The description of this 

 mode of nuclear division is included in the account of cell- 

 division (p. 118). 



c. The Centrosphere, though a perfectly independent body, is 

 always closely associated with the nucleus. It is spherical in 

 form, and consists of a central particle (the centrosome), surrounded 

 by a considerable hyaline area, limited by a granular outline. It 

 undergoes division in connection 



with the division of the nucleus, as C 



described in the section dealing with 

 that subject. It appears that there 

 are normally two of these bodies 

 associated with the nucleus, lying 

 close together (Fig. 49) in the kino- 

 plasm. 



d. The Plastids are differentiated 

 portions of the protoplasm which, 

 like the nucleus, are not formed de 



novo, but multiply by division. Their FlG 49 ._ Yo ung pollen-grain of 



form varies widely. Structurally, Lilium Ma rtagon, showing, c two cen- 

 ,1 f -, trospheres. and n the resting nucleus. 



they seem to consist of a ground- (Affc ^ r Guignard : x750 .) 

 substance with imbedded fibrils, 



denser at the surface. Chemically, they have been described as 

 consisting of two proteid substances ; the one, termed chloroplastin, 

 constitutes the ground-substance, and seems to be similar to the 

 cytoplastin of the cytoplasm; the other, termed metaxin, is the 

 material of the fibrils, and appears to be altogether peculiar to the 

 plastids. 



The plastids may either be colourless, when they are termed 

 leucoplastids ; or coloured, when they are termed cliromatopliores. 

 The chromatophores are distinguishable as chloroplastids, when they 

 contain the green colouring-matter chlorophyll; or as chromoplastids 

 when they contain no chlorophyll, but some other colouring- 

 matter. Plastids are not found in the Fungi, nor, apparently, in 

 the Cyanophyceae among the Algae. 



