26 STRUCTURE AND PROPERTIES 



in organized bodies, being limited by their na- 

 ture ; thus hair, whalebone, &c. though capable 

 of being employed to indicate the state of com- 

 parative dryness or dampness of the air, from 

 their power of attracting moisture to a certain 

 extent, are nevertheless, under ordinary circum- 

 stances, insoluble in water. It is the same with 

 several vegetable productions, which can, conse- 

 quently, be similarly employed. Vegetable tissue 

 is in general more hygrometric in proportion as 

 it is less loaded with extraneous substances : the 

 woody fibre is, in this respect, very different from 

 the bark ; this latter being scarcely hygrometric, 

 while the woody fibre imbibes moisture with great 

 facility. This absorption of water occasions an 

 enlargement of the woody portion, which thus 

 presses itself, as it were, against the bark, and it 

 is in consequence of this pressure, that the gums 

 contained in and under the bark of certain trees 

 are forced outwards, as in the cherry, plum, &c. 

 Senebeir has greatly exaggerated the effects of 

 this power in attempting to account by its agency 

 for the ascent of the sap, and for some of the 

 most important phenomena of vegetation. The 

 fact, that the sap ascends in plants which live 

 in water, and that it does not rise in dead plants, 

 might alone prove his theory to be erroneous. 



