22] 



CHAPTER I. THE CELL. 



69 



a semi-fluid clear ground-substance, the nucleohyaloplasm. In 

 the nucleo-hyaloplasm lies a fibrillar network in which are dis- 

 tributed a number of granules of a substance termed chromatin. 

 One or more small granules, termed nucleoli, are to be seen lying 

 in the ground-substance. On treating the nucleus with staining 

 reagents, the fibrillar network becomes stained on account of the 

 absorption of the colouring-matter by the chromatin-granules, as 

 also do the nucleoli. Its^form is most commonly spherical, but it 

 may be lenticular, or elongated, and straight or curved. 



A formation of a nucleus de novo does not take place under any 

 circumstances ; hence all 

 the nuclei in a plant 

 have been derived by re- 

 peated division from the 

 nucleus of the spore from 

 which the plant was de- 

 veloped. The nucleus 

 divides into not more 

 than two parts, which 

 are similar to each other 

 in all respects. 



c. The Plastids are 

 differentiated portions of 

 the protoplasm which, 

 like the nucleus, are not 

 formed de novo, but 

 multiply by division. 

 Their form varies widely. 

 Structurally, they seem 

 to consist of a ground- 

 substance, denser at the 

 surface, with imbedded 

 fibrils. 



The .plastids may either be_colourless, when they are termed 

 leiLcoplastids ; or coloured, when they are termed chromatopliores. 

 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 theJFungi, nor, ap- 

 parently, in the Cyanophycese among the Algse. 



The Leucoplastids may be spheroidal, fusiform, or cylindrical in 



PIG. 38. Chloroplastids in the cytoplasm of the 

 cells of the prothallium of a Fern. A Optical section 

 of the cells ; B part of a cell seen from the surface- 

 Some of the plastids have begun to divide. ( x 400.) 



