IV.] VARIOUS THEORIES, &c. 75 



and drive water up. In this way a suction-pump- 

 action was supposed to be transmitted from above 

 downwards, from leaves to roots, and here the neces- 

 sary water passes in from the soil, under atmospheric 

 pressure. 



Boehm points out that in any vessel there is a series 

 of air-bubbles at more or less regular distances, and 

 separated by capillary columns of water. In fact 

 the vessel constitutes a veritable chapelet de Jamin^ 

 where the surface actions are so powerful that even 

 enormous pressures will not move the column as a 

 whole, though there is no difficulty in supposing parts 

 of the capillary columns of water to pass through the 

 permeable cell-walls if the neighbouring air-bubbles 

 undergo alterations of pressure. 



A point on which Boehm laid some stress, by the 

 way, is the blocking up of the passages by means of 

 tyloses^ ingrowths of surrounding parenchyma-cells 

 which push through the bordered pits of the vessels, 

 and fill their cavities with a spurious tissue. It is (says 

 Boehm) these tyloses which render the heart-wood 

 impervious or nearly so, and it is they also which 

 gradually block up the vessels of cut branches. 



A few words as to Boehm's views regarding the 

 origin of the air-bubbles and their reduced pressure. 

 The air enters in solution at the root-hairs, at the 



