CHAPTER XII 

 EXCRETION OF WASTE PRODUCTS 



DURING metabolism various waste products are formed. 

 Plants do not possess any special excretory mechanism 

 such as that found in animals. Carbon dioxide can be 

 excreted from the plant by the leaves or used for photo- 

 synthesis. In the latter case oxygen is excreted in its place. 

 Some nitrogenous substances may be resynthesised into 

 proteins. 



In plants the waste products are frequently formed into 

 insoluble substances which are deposited either inside the cell 

 or in the cell wall. Excretion of waste products by the roots 

 has been said to occur (De Candolle), but the evidence of such 

 excretion is not conclusive.* Grass grown round trees has, 

 however, an injurious effect on fruit trees, f The cell walls 

 are required for support, so although they may contain sub- 

 stances from the cells we cannot always be certain that they 

 are waste products. 



Thus it may be that waste substances may still be used for 

 supporting the structure of the plant. We cannot attempt 

 any discussion of excretion in plants as there are not any 

 general points to build upon, the processes being mainly 

 local and dependent on the activity of individual cells, 

 diffusion and precipitation being sufficient in most cases for the 

 removal of the waste products. 



Shedding of dead leaves and loss of dead branches represent 

 excretory processes. 



In multicellular animals excretion may occur through the 

 lungs (or gills), alimentary canal, skin, or special excretory 

 organs. 



From the lungs volatile or gaseous substances are removed. 

 The excretion of carbon dioxide from the blood is a process of 

 diffusion described in Chapter XI. Volatile substances can 

 pass into the lungs until the vapour pressure in the lung 



* C. Daubeny, Phil. Trans., 1845, p. 179. 



t The Duke of Bedford and S. U. Pickering, i3th Report of 

 Woburn Experimental Fruit Farm, 1911. 



175 



