PROTOPLASM 107 



In many cells the living protoplasm exhibits a characteristic 

 spontaneous movement ; in some instances it flows in one 

 direction in a continuous stream round and round the cell, in 

 others, currents in several different directions are observed in the 

 protoplasm. 



From Fig. 49 it is seen that the protoplasm of the cell is 

 not homogeneous, but consists of the following parts : — 



(a) A dense more or less spherical or oval portion («), the 

 cell-mideus ; 



(b) A number of smaller bodies (/), termed plastids or 

 chromatophores ; and 



{c) A more liquid and finely granular substance the cell-plasm 

 or cytoplasm {r), in which the nucleus and plastids are always 

 imbedded. 



In very young cells {A, Fig. 49), the protoplasm entirely fills the 

 cell-cavity and it is only after the growth of the cell that vacuoles 

 appear. In the majority of living cells of the higher plants a 

 single nucleus is present in each ; in some long cells, how- 

 ever, several nuclei are frequently found. 



All nuclei arise by the division of previously existing nuclei. 

 Their functions are not completely known, but cells artificially 

 deprived of them soon die. As the essential part of the sexual 

 fertilisation process consists in the union of two nuclei it is 

 thought that the latter are the carriers of the hereditary characters 

 of the parent organisms from which they are derived. Moreover, 

 in cell-division which results in multiplication of cells the nucleus 

 seems to initiate and control the process of division. 



The thin lining of cytoplasm, or the primordial utricle as it is 

 sometimes called, controls the passage of soluble substances into 

 and out of the cell-sap filling the vacuole. 



The plastids are small bodies of protoplasm resembling nuclei 

 in density : three kinds are recognised, namely — 



(a) chloroplasts, {b) chromoplasts, and (c) leucoplasts. 



They always arise from previously existing plastids by division 



