72 



changed by the addition of various chemical substances and 

 is often converted into wood, i.e. is lignified, or into cork, 

 i.e. is suberised. The communication between the living 

 protoplasm of adjacent cells is, however, not cut off by the 

 thickening of the wall, for exceedingly fine connecting threads 

 of protoplasm are found to pass chiefly through the thin 

 membranes of the pits but sometimes also through the thick- 

 ened wall. 



When a cell-wall separating two adjacent cells has become 

 thickened, its central portion which occupies the position of 

 Middle the original primary wall is called the middle lamella, and this 

 Lamella. differs more or less markedly from the rest of the wall. In 

 lignified cell- walls it is, as a rule, more highly lignified than 

 the rest of the wall, and if a piece of wood is treated with a 

 solution, such as nitric acid with chlorate of potash, which is 

 capable of dissolving woody substance (lignin), the individual 

 cells which compose the woody mass will be isolated by the 

 dissolution of the middle lamella and the structure of the 

 wood is consequently destroyed. In this case only the outer 

 thickening layers of the walls will persist which consist mainly 

 of cellulose and contain less lignin than the middle lamella. 

 In some cases the latter dissolves easily in boiling water and 

 some cells can thus be dissociated by being placed in hot 

 water. 



Shape of 67. Young cells such as those at 



Cells - the growing points of plants, are at first more or less cubical, 



or rectangular, in shape, but as they grow older and increase 

 in size, as they do just behind the growing points, they may 

 undergo a great change of form, and may also lose their 

 living protoplasm, in which case they are often termed dead 

 cells, although really they are then only cell cavities bounded 

 by the cell walls. 



