THE WALLFLOWER 31 



sufficiently high power of the microscope, to represent 

 extremely minute threads or fibrils The nucleus also 

 contains one or more comparatively large rounded 

 masses distinct from the fibrils, and called the nucleoli 

 (see Fig. 17, p. 39). The nucleus has a definite outline, 

 which probably represents a denser layer of the sur- 

 rounding protoplasm. The substance of the nucleus 

 consists of various chemical bodies which are similar 

 to, but not altogether identical with, those of which 

 the protoplasm is built up ; a compound containing 

 phosphorus (nuclein) appears to be constantly present 

 in the nucleus. 



Most cells contain only one nucleus, but in very 

 long cells a number of nuclei are often found. 



The protoplasm, nucleus, and cell-wall are the most 

 constant constituents of a vegetable cell. The proto- 

 plasm, as we have seen, is absolutely constant in all 

 living cells without exception ; the nucleus is equally 

 general in the higher plants, though in some of the 

 lower ones its presence has not been proved ; the cell- 

 wall is formed sooner or later in all cells of the higher 

 plants, but is very frequently absent, at any rate for 

 long periods, from certain cells of the simpler plants. 1 



If we examine rather older cells, such as those 

 which we find at a little distance below the growing- 

 point, we shall see that the whole cell is larger, and 

 that its cavity is no longer so nearly filled by the 

 protoplasm (Fig. 14, B). Clear spaces, which had 

 already appeared in the protoplasm at an earlier 

 stage (v in the figures), have now increased in number ; 

 they are filled with water containing various organic 



1 See Part II., Flowerless Plants. 



