IV.] VARIOUS THEORIES, &c. 109 



patent vis a tergo in the plant, and the capillaries 

 are not narrow enough for the heights required. 

 Boehm, Hartig and others have tried to show that 

 the water columns are held supported and im- 

 movable in the lumina, and hence exert no down- 

 ward pressure which would cause them to fall : Sachs, 

 and those with him, reply that the capillary forces 

 which hold these columns so fast as to prevent their 

 falling, will, with equal obstinacy, prevent their being 

 moved upwards. 



Elfving then examines the well-known and oft- 

 quoted experiment of Th. Hartig. The vertical 

 piece of branch say yew consists of series of 

 tracheides each containing water and air, and closed 

 by pit-membranes which are very permeable to water ; 

 but this constitutes a chapelet de Jamin, the only 

 peculiarity being the intercalation of the extremely 

 permeable membranes at more or less regular heights 

 between the columns of water. In Hartig's experi- 

 ment then, the vertical rows of tracheides form so 

 many immovable diapelets de Jamin> but with liquid 

 communications at the permeable pits : the move- 

 ment of the water, caused by the weight of a drop 

 placed above, must therefore be in a sinuous course, 

 through the lateral pits, and not confined to the one 

 column. If we take this sinuous course, and examine 



