SECRETIONS. 57 



a singular phenomenon presented by a plant named Fraxinellii, 

 which in hot days exhales an essential oil in such abundance, 

 that if it be approached with a light, the vapour with which the 

 plant is surrounded takes fire and burns, like that we force out 

 of an orange or lemon skin by pressure, into the flame of a can- 

 dle. Other plants secrete a caustic juice, which is frequently 

 poured out through hollow hairs, and thus produces a lively irri- 

 tation at the bottom of punctures made by these hairs. The 

 nettle is an example of this kind. Again we have wax secreted 

 by the leaves or epidermis of young branches and afterwards 

 expelled ; and we have also produced in this way gluey, acid, 

 saline, sugary, and other secretions. 



56. These excretions* are formed by the roots as well as by 

 the leaves ; and as the matters thus expelled are of a nature that 

 is injurious to the plants which produce them, we understand 

 through the knowledge of,this fact why plants of the same species 

 do not flourish when kept for a long time in the same soil ; foi 

 the matters expelled by the roots are deposited in the earth sur- 

 rounding them, and are again absorbed by the plants growing in 

 it. But the matters expelled by one plant may often be suitable 

 nourishment fora plant of another species, and it is for this reason 

 that the ground often becomes fitted for certain culture when it 

 has been previously made to produce plants in which the excretion 

 by the roots is abundant. The art of assolement or succession 

 of crops, so important in agriculture, is chiefly based upon the 

 results depending on this excretion by the roots. We give the 

 name of assolement to the succession in the same soil of different 

 crops, combined in such a manner as to produce as largely as 

 possible ; and we say triennial, quatrennial assolement, &c. 

 according as the cultivation of the same plant recurs every three 

 every four years, &c. 



57. The liquids secreted by plants and designed to remain in 

 the interior of their organs are designated under the name of 

 proper juices ; if they escape externally, it is altogether by acci- 

 dent, and their production appears to be useful to the health of 

 the plant that forms them. These juices are sometimes milky, 



* [Excretion : from the Latin ezcer'nere, to separate from. The throwing 

 oft' those matters which are supposed to be useless or injurious to organic 

 life, as the perspiration in animals. An excretion is a secretion that is 

 hrown out of a plant or animal because useless to its internal well-being. J 



56. Do other parts than the leaves of plants form excretions ? Why is 

 it that farmers do not plant the same plant in the same field, year after 

 year ? What is meant by excretion ? 



57. What are proper juices ? What are then characters ? 



