172 FOUNDATIONS OF BOTANY 



pound (ligniri) characteristic of wood-fiber, that go to make 

 up the main bulk of a large tree must be carried on in the 

 roots, trunk, and branches of the tree. 



184. Digestive Metabolism. Plant-food in order to be 

 carried to the parts where it is needed must be dissolved, 

 and this dissolving often involves a chemical change and 

 is somewhat similar to digestion as it occurs in animals. 

 The newly made starch in the leaf must be changed to a 

 sugar or other substance soluble in water before it can be 

 carried to the parts of the plant where it is to be stored 

 or to rapidly growing parts where it is to be used for 

 building material. On the other hand, starch, oil, and 

 such insoluble proteids as are deposited in the outer por- 

 tion of the kernel of wheat and other grains are extremely 

 well adapted to serve as stored food, but on account of 

 their insoluble nature are quite unfit to circulate through 

 the tissues of the plant. The various kinds of sugar are 

 not well adapted for storage, since they ferment easily in 

 the presence of warmth and moisture if yeast-cells or 

 suitable kinds of bacteria are present. 



Two important differences between starch-making in 

 the green parts of plants and the non-constructive or the 

 destructive type of metabolism should be carefully noticed. 

 These latter kinds of metabolism go on in the dark as 

 well as in the light and do not add to the total weight 

 of the plant. 



185. Excretion of Water and Respiration. Enough has 

 been said in Sect. 174 concerning the former of these pro- 

 cesses. Respiration, or breathing in oxygen and giving 

 off carbonic acid gas, is an operation which goes on con- 

 stantly in plants, as it does in animals, and is necessary to 



