LIVING SUBSTANCE 21 



the cytoplasm. Nucleus plus cytoplasm together 

 make up the cell. 



Bounding the cytoplasm there may be a definite, 

 often thickened cell-wall. This is especially charac- 

 teristic of plant tissues, in which the cell- wall becomes 

 very thick and rigid through the deposit of various 

 carbohydrates (pectin, cellulose) . It is the latter (or 

 iti derivative, lignin) that gives its characteristic 

 rigidity and hardness to wood. In animals, on the 

 other hand, in only one rather obscure group J does 

 cellulose occur, and it is the exception rather than 

 the rule for the cell-wall to attain any considerable 

 thickness or prominence. In many animal cells there 

 is no cell- wall at all, the viscosity and surface ten- 

 sion of the mass of protoplasm holding it together. 



The nucleus and cytoplasm are found by delicate 

 tests to be chemically different, the former combining 

 more readily with basic and the latter with acid sub- 

 stances. The cytoplasm often contains vacuoles 

 filled 'with a watery fluid or with different sorts of 

 non-living substances, such as crystals of silica 

 (that give the knife-edge to certain kinds of grass), 

 chlorophyll bodies (that give the green color to 

 plant leaves), starch grains (as in the potato), 

 yolk granules (such as make up the bulk of the 

 yellow of a hen's egg), and various other substances. 

 Such substances, being non-living material, manu- 

 factured by the cell, are called metaplasm in contra- 

 distinction to the living protoplasm. 



In the cytoplasm is also found the centrosome, 



1 The Ascidians or "Sea Squirts." 



