38 THE VITAL FUNCTIONS. 



ed gold, have the power of destroying its ductility.* In the 

 experiments made by Sir John Herschel on some remarka- 

 ble motions excited in fluid conductors by the transmission 

 of electric currents, it was found that minute portions of 

 calcareous matter, in some instances less than the millionth 

 part of the whole compound, are sufficient to communicate 

 sensible mechanical motions, and. definite properties to the 

 bodies with which they are mixed. t 



As Silica is among the densest and least soluble of the 

 earths, we might naturally expect that any quantity of it 

 taken into the vegetable system in a state of solution, would 

 very early be precipitated from the sap, after the exhalation 

 of the water which held it dissolved; and it is found, ac- 

 cordingly, that the greater portion of this silica is actually 

 deposited in the leaves, and the parts adjacent to them. 

 When once deposited, it seems incapable of being again 

 taken up, and transferred to other parts, or ejected from the 

 system: and hence, in course of time, a considerable accu- 

 mulation of silicious particles takes place, and by clogging 

 up their cells and vessels, tends more and more to obstruct 

 the passage of nourishment into these organs. This change 

 has been assigned as a principal cause of the decay and ulti- 

 mate destruction of the leaves: their foot-stalks, more espe- 

 cially suffering from this obstruction, perish, and occasion 

 the detachment of the leaves, which thus fall off at the end 

 of each season, making way for those that are to succeed 

 them in the next. 



6. Secretion in Vegetables. 



WHILE the powers of the simpler kinds of cells are ade- 

 quate to produce in the returning sap the modifications 

 above described, by which it is converted into gummy, sac- 

 charine, amylaceous, or ligneous products; there are other 

 cellular organs, endowed with more extensive powers of 



* Hatchett. f Philosophical Transactions for 1824, p. 162. 



