346 ELEMENTARY ORGANS. CHAP. III. 



chink of a rock will expand very perceptibly if 

 wetted with water. The water must consequently 

 have been absorbed. Spirits of wine will evaporate 

 out of a wooden box however well it may be closed ; 

 and mercury may be forced even through a piece of 

 hard wood by means of the air pump. Hales made 

 water pass through a branch of an Apple-tree cut in 

 autumn, merely by putting the one end of it in a 

 glass tube which he filled with water. It may be 

 said, indeed, that these facts are no proof of the ex- 

 istence of tubular fibres ; and that the passage of 

 the fluid is to be accounted for upon the principle 

 of filtration. It is to be recollected, however, that the 

 principle of filtration will not account for the as- 

 cent of the sap to the summit of the vegetating 

 plant, particularly with the force by which it is 

 known to ascend. 



Analogy also authorizes us in supposing that 

 woody plants are furnished with tubular fibres as 

 well as herbaceous plants, since they are equally 

 necessary to the conveyance of the fluids passing 

 through them. And though the tubular fibres of 

 woody plants do not, like those of many herbaceous 

 plants, readily part with their juice by means of 

 cutting or pressure, yet they will readily part 

 with it if exposed to the action of a strong heat. 



But the tubular structure of the longitudinal 

 fibres of woody plants is cognizable also by the eye. 

 On the horizontal section of a piece of wood that 

 has been long exposed to the action of the atmos- 



