ii ASCENT OF SAP IN STEMS 29 



lift the water columns in trees. Such a clear statement 

 is, however, not possible in the case of Bohm's final 

 theory to account for the ascent of sap. Owing to 

 his contradictory expressions and obscurity in description 

 it will always remain impossible clearly to understand 

 what his hypothesis was. He assigned a part to the 

 capillary forces of the tracheae and to atmospheric 

 pressure. The latter he conceived as driving the water 

 from the tracheae into the leaf-cells, and also he saw no 

 difficulty in the height of the water columns owing to the 

 cohesion of water ; but without doubt he was quite astray 

 as to the conditions under which cohesion could act. 



Function of the tracheal walls. With Sach's 

 imbibition hypothesis, a new factor was introduced into 

 the discussion on the ascent of sap. The assumed 

 mobility of water in the tracheal walls was directly 

 negatived by the experiments of Elfving and Vesque, 

 who showed that only negligible amounts of water 

 passed upwards when the lumina of the tracheae were 

 plugged with cacaobutter. It was objected to Elfving's 

 experiments that cacaobutter was of too greasy a nature, 

 and might enter the wall, so gelatine was substituted by 

 Errera and Strasburger. It might fairly be urged, how- 

 ever, that there is less danger of a greasy substance enter- 

 ing the water-saturated wall than of a substance miscible 

 in water such as gelatine, which, even if entering the wall 

 in minute quantities, might be very injurious to its trans- 

 mitting properties. 



Penetration of gelatine into walls. In this 

 connection experiments were made to test the possibility 

 of gelatine permeating the cell walls. A few of these 

 may be quoted. They all agreed in showing the passage 

 of warm dilute gelatine through the cell-walls possibly 

 in some cases through the closing membranes of the 

 pits only and consequently suggest the probability that 

 it would alter the capability of the wall for transmitting 



