IV] VARIOUS THEORIES, &c. 73 



it in the direction of the long axes of the elements. 

 (3) Cuttings of willows &c., will; when transpiring, 

 exert a pull (so to speak) on mercury, to such an extent 

 as to raise a column sixty mm. in height. (4) Fairly 

 thick longitudinal sections of fresh branches can be 

 so arranged under the microscope as to show that air- 

 bubbles, under feeble pressuie, exist in the vessels and 

 tracheides. Placed in water, the bubbles contract — ie, 

 water is forced through the damp walls, which are 

 impervious to air. It may be mentioned, by the way, 

 that a rough illustration of the imperviousness of a 

 w^et membrane to air is to be seen in any wash-tub, 

 where the imprisoned air drives up the wet linen into 

 rounded hummocks as the laundiess pushes various 

 parts deeper into the water. (5) Boehm rightly lays 

 stress on the importance of Von Hohnel's discovery^ 

 that the pressure in the vessels in summer may be so 

 low, that it does not exceed ten cm. of mercury. 

 (6) He then points out the bearing of his previous 

 papers on the whole subject (of which the present 

 is practically a summary), and his own numerous 

 observations,^ and among others notes the following. 



^ Habeilandt's Wtss. ptaU, UnUrs anf den Geb d pflanzenhaues^ 

 t xi , 1877. 



2 Of course it is impossible to quote here all that bears on this 

 auestion. but the chief of these laaneis are in Latzdw, Versuchs, Siai. 



