NUCLEUS. 31 



flowering plants. In many instances the layer of protoplasm 

 lining the cell-wall is so thin and closely applied to the 

 latter as to be almost indistinguishable; its existence, 

 however, can be clearly demonstrated when the cell is 

 treated with some water-extracting substance, such as a 

 solution of salt, sugar, or glycerine, which causes the proto- 

 plasm to leave the cell-wall and become contracted. This 

 is known a.s plasmolysis. 



The nucleus is a product of the differentiation of proto- 

 plasm, in which it always remains enclosed. It is at first 

 a roundish mass ; eventually its surface becomes firmer 

 without developing a distinct wall, as in the case of the 

 cell, and in its interior there usually appear one, two, or 

 several granule-like portions or nucleoli. When fully formed, 

 the resting nucleus appears finely granular under a moderate 

 power of the microscope ; but when sufficiently magnified 

 it is seen that the granules are not irregularly scattered, 

 but arranged in rows forming exceedingly fine threads, 

 which are very much contorted and interwoven, and enclosed 

 in a delicate membrane. During the division of the nucleus 

 the coiled threads undergo various complex arrangements. 

 A nucleus is present — at least during the young condition 

 of every cell belonging to the higher plants ; but was at 

 one time considered to be absent from the cells of the 

 simpler algs and fungi. Recent researches, however, have 

 demonstrated the presence of nuclei in many of these simple 

 forms of plant life, and in all probability nuclei in a 

 primitive state of differentiation are present in all plant 

 cells. The exact function of the nucleus is not known, 

 but judging from its almost universal occurrence, and its 

 behaviour in connection with the formation of new cells, 

 it must be supposed to perform some important function. 



