32 STRUCTURAL BOTANY 



and inorganic substances (salts, sugars, acids, etc.) in 

 solution, and called the cell-sap. The spaces filled with 

 cell-sap are known as the vacuoles, because they look 

 as if they were empty. 



The nucleus increases little in size, and now bears 

 a much smaller proportion to the whole cell than it 

 did at first The cell-wall will have become some- 

 what thicker (see Fig. 14, B and C). 



A full-grown but still living cell, such as we might 

 find in the older parts of the Wallflower, has its proto- 

 plasm reduced to a thin layer, lining the inside of the 

 wall. The whole interior of the cell appears empty- 

 that is, it is occupied by one large vacuole containing 

 cell-sap. This has been formed by the smaller 

 vacuoles running together. The layer of protoplasm 

 lining the cell-wall is often called the primordial 

 utricle. The nucleus has undergone little change, but 

 is now placed close against the wall, embedded in the 

 protoplasmic layer which lines it. The cell-wall itself 

 has been further thickened by the deposit of new 

 cellulose from the protoplasm. 



This is the structure of most mature living cells of 

 plants. The protoplasm does not keep pace with the 

 growth of the whole cell; it may even actually 

 diminish in amount, owing to its being used up to 

 make new cell-wall and for other purposes, quicker 

 than it is renewed by taking in food. Some cells lose 

 their protoplasm altogether ; then they are dead, and 

 incapable of any further growth, though they may 

 still be of use to the plant as passages for sap or as a 

 mechanical support. We shall find plenty of examples 



