IV] VARIOUS THEORIES, &c. 119 



mesophyll cells which give off so much water, to 

 the lacunse communicating with the air through 

 stomata. 



(3) The excretion of liquid water is a special case, 

 and need not be considered here, as it receives full 

 treatment further on. 



(4) This depends on the same assumption as (2), 

 and falls with it 



(5) Very few woods are, like that of the Papayacem, 

 composed largely of parenchyma, and we are not 

 driven to compare such wood forthwith with meso- 

 phyll. 



Boehm comes in for criticism no less severe with 

 respect to other matters, for his views demand that 

 the epidermis cells and mesophyll, on the one hand, 

 and the root-hairs and parenchyma of the root on the 

 other, must have their contents under less pressure than 

 the atmosphere — an assumption of course opposed to 

 all we know of the cell, turgescence, and the properties 

 of protoplasm and cell-sap. Moreover, to suppose 

 that the pressure in the epidermis and leaf cells, could 

 ever be less than that in the tracheal elements surely 

 ignores the negative pressure which exists in the 

 wood at times of active transpiration; besides we 

 know from actual observations that the cells of 

 the leaf and root show strong turgescence, at just 



