iv.] VARIOUS THEORIES, &c. 71 



liquid rushes up into the vessels under a pressure 

 which must be regarded as considerable : this is due 

 to the fact that the air in the tracheides, vessels, &c., 

 is rarefied, and the pressure just referred to is the 

 difference between that of the contained air and that 

 of the external atmosphere. Of course Sachs viewed 

 all such cases as only going to show that when the 

 leaves are taking water rapidly from the cell -walls, 

 the latter supply themselves from whatever surplus 

 water may exist in the cavities, and, since air cannot 

 pass through the wet membranes or only to a very 

 slight extent, the contained air-bubbles expanded, and 

 so on. Sachs attributes to these air-bubbles, however, 

 some influence in drawing water into the cavities. 1 



An old experiment of Theod. Hartig's should be 

 mentioned here. If a piece of the stem of a Conifer, a 

 yard or more long and with the ends cut clean across, 

 is held vertically, and one drop of water is placed on 

 the upper section, a similar drop appears at the lower 

 end in a few seconds, enlarging in proportion as the 

 upper one sinks into the wood : this is regarded by 

 Sachs as a proof generally of the easy permeability of 

 wood. I shall have occasion to refer to this experi- 

 ment of Theod. Hartig's several times in the course of 

 the discussion, as some important matters turn on its 



1 Vorlesimgen iiber Pflanzen-physiologie, p. 322 (Eng. Ed. p. 269). 



