14 SCIENCE PRIMERS. [iv. 



of oxygen, hydrogen, and carbon ; protoplasm of 

 these together with nitrogen. 



12. When cells are very young they are smaller 

 in size, the cell- wall is thinner, and they are completely 

 filled with the protoplasm ; a darker rounded portion 

 of this is generally to be noticed in the centre and is 

 called the nucleus. As the cells grow in size their 

 cavity becomes larger than the mass of protoplasm 

 which originally filled it. The cell-wall is always 

 lined by a layer of protoplasm, but in the interior of 

 the cell, cavities are formed in the protoplasm which 

 are filled with a watery fluid called the cell-sap. Later 

 on the protoplasm is reduced to a thin film, in which 

 the nucleus is placed and which lines the cell-wall ; 

 strings of protoplasm pass from the nucleus across the 

 cell-cavity. In old wood and cork cells the proto- 

 plasm has completely disappeared, and the cavity of 

 the cell contains nothing but water or air. 



13. New cells are formed by the protoplasm of 

 some which are still quite young dividing into halves, 

 between which a wall of cellulose is then formed. 

 The original cell-cavity is thus divided into two. It 

 is supposed that this breaking up of the protoplasm 

 commences by the division of the nucleus seen in the 

 protoplasm of most cells, and that the protoplasm 

 collects round each half of the nucleus ; but this is 

 not certain. 



14. The rate at which cells thus multiply is asto- 

 nishing, and is most conspicuous in mushrooms, toad- 

 stools, &c., which are formed wholly of cellular tissue. 

 The giant puff-ball grows in one night from the size of 

 a marble to that of a child's head, by the growth and 

 increase of cells, which individually are but a few 



