THE HISTOLOGY OF THE CELL 



75 



Formation and Growth of the Cell- Wall. The cell- 

 wall is very strong and elastic, and is formed from and by the proto- 

 plasm, and its increase in thickness and area depends upon the 

 vital activity of such protoplasm. The cell-wall may grow in area 

 owing to the stretching caused by the pressure set up inside 

 the cell, but throughout this increase -in size new material is 

 deposited to strengthen it. In 

 some cases the stretched cell- 

 wall breaks, and the ruptured 

 edges separate. The break is 

 commonly repaired by the 

 deposition of a plug of new 

 material which connects the 

 disconnected surfaces. 



The cell-wall grows in thick- 

 ness by the deposition of suc- 

 cessive layers on the internal 

 surface of the first-formed layer. 

 This kind of growth is termed 

 growth by apposition. The 

 subsequent growth of the cell- 

 wall by the deposition of new 

 material between the old is 

 called growth by intussuscep- 

 tion. As a general rule growth 

 in thickness does not take place 



until after a cell has reached its full size. When such a cell-wall 

 is examined "by the high power of the microscope, it exhibits a 

 stratified appearance; this is owing to the constituent layers 

 acting on the light differently. All the markings which are 

 found on cell-walls are due to the unequal deposition of the new 

 material during growth in thickness. The spiral, annular, and 

 pitted walls, for example, which are found inThe wood of most 

 plants, are caused by this unequal growth. 



The Chemical Composition of the Cell Wall. If a 

 few cells be treated with iodine-stain 1 the cell-wall will assume 

 a yellow colour, and the further addition of a single drop of 



1 Iodine-stain is made by dissolving crystals of potassium iodide in distilled water 

 until a strong solution is made and then adding crystals of iodine. If this is diluted 

 with distilled water to the colour of brown sherry it is ready for use. The alcoholic 

 solution is made in the same way, only alcohol is used instead of water. 



FIG. 102. Cell with thickened wall. 

 in, middle lamella ; t, pit ; 7f , 

 pitted transverse wall. ( X 300.) (S.) 



