116 GENERAL FLUID COMPONENTS. [LECT. III. 



which it is exposed to the action of the air, and 

 light, the great quantity of water it contains be- 

 comes no longer necessary, and is thrown off by 

 perspiration ; whilst the succus proprius, or pecu- 

 liar juice of the plant, from which all its secretions 

 are formed, is produced by the changes resulting 

 chiefly from this exposure. We have, therefore, 

 next to proceed to examine the nature of this pe- 

 culiar juice, as one of the general components of 

 plants. 



GENERAL FLUID COMPONENTS THE PROPER JUICE. 



I should be anticipating the inquiries we have 

 to make into the physiology of leaves, were I now 

 to attempt to explain to you how the sap is con- 

 veyed into these organs, to be exposed to the action 

 of light and air ; or, by what means those changes 

 are effected in them, by which it is converted into 

 the proper juice, succus proprius : at present we 

 have to examine this fluid merely as a general 

 component. It is, however, necessary to remark, 

 that although we may admit with Malpighi that 

 the proper juice is to the vegetable system, what 

 the blood is to the animal, yet, the functions by 

 which it is prepared from the sap must be modified 

 in different kinds of plants, since it exhibits some 

 peculiar characteristics in each kind. 



When a plant is cut through transversely, the 

 proper juice is seen issuing from both divided sur- 



