ON CAMBIUM. 91 



into the sap ; when this undergoes evaporation, a large 

 portion of these salts is deposited in the leaves, the rest 

 remains in solution in the cambium, incorporates with 

 the plant, and, after combustion, may be discovered in 

 its ashes. 



The silicious particles, contained in the plant being, on 

 the other hand, nearly insoluble, enter very little into the 

 composition of the cambium, the greater part remaining 

 in the leaves, where it has been deposited by the evapo- 

 rating sap ; and the fall of the leaf is attributed to the ac- 

 cumulation of this hard earthy matter, which in the course 

 of time clogs and indurates their vessels, so as to render 

 them impervious to the juices requisite to their vegeta- 

 tion. The vessels composing the petiole, in which they 

 are so closely bound together, are more especially liable 

 to suffer from these obstructions : unable any longer to 

 transmit nourishment to the leaf, the petiole dries, with- 

 ers, and falls off; and the plant is thus disburdened of a 

 useless substance, the accumulation of which would be 

 prejudicial to its growth. 



Caroline. It must be confessed, that it is rather a se- 

 rious remedy to destroy the organ, in order to get rid of 

 the inconvenience with which it is afflicted. 



Mrs. B. You must consider, that when Nature con- 

 structed these organs in so frail and delicate a manner, 

 it was with the intention that they should be annually re- 

 newed : it becomes expedient, therefore, to get rid of the 

 old leaves, in order to make way for the new ones. 



Azote, an ingredient chiefly of the animal kingdom, is 

 to be found also, in very small quantities, in vegetables : 

 they obtain it both from the atmosphere, of which it forms 

 the chief constituent part, and from the animal matter of 

 manure. 



From the cambium, with all the component parts of 

 which you are now acquainted, a great number of differ- 

 ent juices are secreted, such as oil, resins, gum, &/c. 



Caroline. Just as tears and saliva are, in the animal 

 economy, secreted from the blood. 



495. What is said of the potash and soda in wood after combustion 

 takes place 1 ? 496. To what is the fall of leaves attributed 1 ? 497. 

 What is Baid of the vessels composing the petiole in relation to this 

 Bubject 1 ? 498. What does Mrs. B. say of the annual destruction of 

 leaves'? 499. What is said of azote in vegetables'? 500. What 

 juices are secreted from cambium 1 ? 



