124 PHYSIOLOGICAL BOTANY. 



are sources of all the other organic substances of plants, 

 and even the tissues of animals are either directly or in- 

 directly derived from them. 



9. DESTRUCTIVE METABOLISM. 



Daring the construction of new tissues several com- 

 pounds are formed as by-products. These are Tannin, 

 Organic Acids, Colouring Matters, Alkaloids, Volatile 

 Oils, etc. 



Old tissues are aften changed into such substances as 

 Gum Arabic, Cherry Gum, etc. These are the products 

 of degradation which can undergo no further modification 

 in plants. 



In the processes of all vital functions, decomposition is an 

 inevitable occurrence, and the evolution of carbonic acid is 

 the result. 



10. EXCRETION. 



In plants, even waste products which are entirely useless 

 to them are mostly retained in their tissues, and only small 

 portions are excreted. The chief excreta of common green 

 plants are all given off in the gaseous form : they are 

 oxygen, carbonic acid, and watery vapour. 



Although the majority of waste products are retained in 

 plants, they are gradually got rid of by the detachment of 

 the bark, fruits, etc., in which they are abundantly 

 deposited. 



Section 2. EXTERNAL FORCES. 

 1. TEMPERATURE. 



If an herbaceous plant as the Kabura be boiled in water, 

 the plant body will become gradually softened and will be 

 finally killed. 



