76 THE CELL- WALL, ETC. [CH. 



empty spaces : the liquid they contain is called Cell-sap. 

 As the vacuoles increase they coalesce and drive the 

 protoplasm to the side-walls of the cell, the volume of 

 which increases at the same time, until at length there is 

 one large vacuole occupying by far the greater portion of 

 the cavity, surrounded by the cytoplasm, now thinned 

 out to a mere lining on the cell-wall, in which lining the 

 nucleus, chlorophyll-corpuscles, plastids, &c. still lie. 



The second event is that the cell-wall becomes 

 thickened by additions on its inside during this enlarge- 

 ment : this means that new substance has been added to 

 it, and the important point for the moment is that the 

 substance of the cell- wall comes from the protoplasm, and 

 has been made by it. 



If we follow the fate of such a cell as the above still 

 further we may find that as the cell-wall becomes thicker, 

 the protoplasmic contents and cell-sap disappear alto- 

 gether, and nothing but an empty cell-cavity surrounded 

 by its cell- wall remains i.e. a cell in Hooke's sense filled 

 with air. This is evidently not a complete cell, but only 

 the skeleton of cell-wall. 



The proof that the cell-wall is formed by the proto- 

 plasm is direct ; for not only does every plant known 

 originate from a spheroidal mass of protoplasm which is 

 for a longer or shorter period devoid of a cell-wall, and 

 surrounds itself with one later, but in many Algae and 

 Fungi, as well as in other plants, the protoplasm escapes 

 from certain of the cells, and lives an independent exist- 

 ence for a longer or shorter period, eventually clothing 

 itself with a cell-wall, the substance of which can only 

 have been formed and secreted by the protoplasm. This 

 occurs, for instance, with the zoospores of Algae and many 

 Fungi, and the Myxamcebae of the Myxomycetes. In the 

 case of the antherozoids of many Algae and Mosses, Ferns 



