94 Mr G. F. Scott Elliot on the [sess. li. 



being filled and the fat hardened, he cut a thin section from 

 the cut surface, to take away the ends of the cell- walls which 

 might be affected by the fat, and he then attempted to force 

 water through. This was in every case impossible. On 

 Sachs' \dew there is no reason why the water should not 

 travel by the walls. His experiments have been repeated, 

 always confirmed, and made absolutely free from all objection 

 by Scheit (4), Vesque (13), and specially by Errera (21), who 

 used a mixture of 40 per cent, gelatine instead of coco- 

 butter. It seems to me one must conclude from this evidence 

 that the crude sap travels in the cavities of the vessels. 



Now, with regard to the forces employed, Sachs, followed 

 by Pfefter (22), considers that it is the force of imhibition, 

 a force quite difierent from capillarity, which causes the 

 water to mount in the cell- walls. We have scarcely the right 

 to assume that imbibition is different from capillarity ; cf. 

 Haberlandt (23). Moreover, I fail to see how the water can 

 easily move upwards through imbibition, and at the same time 

 be strongly held in the walls through the same force. Also, 

 the water does not travel in the walls. The theories held by 

 other observers are almost as numerous as the observers. 



It is best to classify them according to the forces supposed 

 to be effective. The first force, which certainly has great 

 influence, is osmosis. There are two different ways in which 

 this acts. Namely, the osmosis of the cells of the root, or root 

 'pressure, and the osmosis of all the cells of the stem and leaf 

 which a vessel supplies. Eirst, as to root-pressure. Von 

 Hohnel and Scheit (4) consider that it is the only force 

 in action. This is, however, probably doubtful, as most ob- 

 servers do not consider that the root-pressure could lift the 

 water to a greater height than six metres. There is, however, 

 so much variation in the measurements given for the root- 

 pressure that one must take all such figures with great reserve. 

 Leclerc du Sablon (25), for instance, found a difference of 180 

 mm. in manometers only 8 or 10 cm. apart on the same 

 tree. There is no doubt, however, that it has some influence. 



Tlie importance of the osmosis of the cells of the leaf and 

 stem was first pointed out by Westermaier (24). The 

 following experiment is worth giving to show the eff'ect : — A 

 circular disc of 6 mm. radius was cut out of a leaf of 

 Peperomia. This was placed in a salt-solution, and weights 



