ON SAP. 77 



Emily. Aquatic plants which live wholly under water, 

 you told us, were not provided with stomas ; but now that 

 I comprehend the nature of their functions, I do not un- 

 derstand why the plants should not derive benefit from 

 them : for while the roots absorbed the water holding in- 

 gredients in solution, the stomas would evaporate it in a 

 pure state, leaving all its riches behind. 



Mrs. B. The plant has not power to exhale water into 

 water : it requires the assistance of the air to dissolve it 

 and carry it off. Those aquatic plants which rise to the 

 surface, are abundantly furnished with stomas to disbur- 

 den themselves of their excessive supply of water. 



Let us now turn our attention to the nature of the sap 

 which remains in the leaf, after having disengaged its su- 

 perfluous moisture. It consists of "about one-third of the 

 water originally absorbed by the roots, but augmented and 

 enriched by the acquisition of all the nutritive particles 

 which the evaporated water has deposited. 



Caroline. In this state it is certainly better calculated 

 to nourish the plant ; and from this ample store I suppose 

 the various organs select and assimilate the food they each 

 require. 



Mrs. B. It is true that every organ performs a chem- 

 ical change on that part of the sap which it assimilates to 

 its own substance ; but the sap previously undergoes a 

 general change, in some measure analogous to that which 

 the blood undergoes in the lungs, to prepare it for assim- 

 ilation. This operation is also performed in the leaves, 

 which may be considered as the laboratory in which the 

 sap is submitted to a regular chemical process. 



Emily. This, indeed, bears a very striking resemblance 

 to the chyle, which is the sap of animals, and which is 

 converted into blood, fitted to go through the general cir- 

 culation, and nourish the several parts of the body.* 



* See Conversations on Chemistry Conversation xxv. 



420. What does Mrs. B. say of aquatic plants as to their power of 

 exhaling waterl 421. And what does she say of the sap which re- 

 mains in the leaf after perspiration has taken place'? 422. What 

 change does the sap undergo previous to its becoming assimilated to the 

 several parts of the plant 1 ? 423. Where is the operation perform- 

 ed 1 ? 424. To what does Emily say this process of the sap bears a 

 strong resemblance. 



7* 



