THE CELL-SAP 



109 



increase in size and become variously modified in shape and 

 structure in accordance with the special functions which they 

 have to perform in the fully-developed organs of the plant. 



If during growth the cell-wall increases in all directions alike, 

 the original cubical or polyhedral form is maintained; most 

 frequently, however, growth is irregular and the cells assume a 

 great variety of shapes, the chief of which will be mentioned 

 when dealing with the organs of the plants in which they occur. 



A great many cells after a time lose their protoplasmic contents 

 and nothing then remains except the cell-wall and the cell-cavity 



Fig. 50. — Diagrammatic illustration of thickened cell-wall; A, uniformly thickened 

 wall ; B, wall with simple pits; C, wall with bordered pits. 



generally filled with air. To these empty shells the term cell is 

 commonly applied although some other term would be more 

 suitable. Sometimes the cell-walls remain thin, but very often 

 they become greatly thickened before the cell completely loses 

 its protoplasm ; such thickened cell-walls give firmness and 

 strength to the structures which contain them and act as 

 mechanical supports for the deHcate parts of the plant. 



The thickening consists in the deposition of successive layers 

 of some form of cellulose on the inner surface of the cell-wall. 



