The Substance Which Is AHve in Plants 



151 



parts in their natural relations and fully-developed condition; 

 and such a cell is represented herewith (figure 48) . 



We should now examine a bit further these parts of the cell 

 and their meaning. 



The wall is composed of a firm-elastic transparent substance 

 called cellulose, whose chemistry is treated in the chapter on 

 Metabolism. It is built by the cytoplasm, which, in suitable 

 places, is supposed to lay down within itself tiny masses (bricks, 

 as it were) called micellae, of cellulose, and continues to add to 



,j;m^^m^^mm^ — wan 



car '^^ 



^^%r plastid 



mV- sap-cavity 



nucleus 

 nucleolus 

 /'it?/ cytoplasm 



Fig. 48. — An optical section through a conventionalized complete plant cell. 



their number until they accumulate to nearly a solid mass. I 

 say "nearlj^," because apparently there always are left between 

 these micellae thin sheets of protoplasm, like the mortar between 

 bricks, so long as the cells are alive, though they are withdrawn 

 when the cell has reached full maturity. It is these thin sheets 

 of cytoplasm, too thin to be visible even to the strongest micro- 

 scope, which keep the wall alive, as it were, so that it can become 

 enlarged, spht, chemically changed, absorbed in places, and in 

 other ways altered, a good while after its formation. But except 

 for such subsequent alterations, the walls of contiguous cells re- 



