1886-87.] Movement of Water in Plants. 91 



on the surface of the water, only smking when the contained 

 air was driven out by water. He does not, however, take into 

 consideration the possible entrance of atmospheric air at the 

 section surface. I tried the experiment with the addition of 

 cutting in every case two pieces of wood ; — one of these 

 was cut imder water to exclude atmospheric air, the other 

 cut in the usual way and then dropped into water. I found 

 in many cases that both twigs sank. In two cases, how- 

 ever, I found that the twig cut under water never rose to 

 the surface, while the one cut in the air floated. The 

 difference in these cases was therefore clearly due to the 

 entrance of atmospheric air. This experiment, however, is 

 far too rough to be worth working out, for, as Sanio (8) 

 points out, air in the intercellular spaces would account for 

 the floating. Sachs' experiments cannot therefore be re- 

 garded as conclusive. 



Direct experiments by cutting sections of wood under oil 

 or glycerine, — Hartig (9), Elfving (10) — particularly if con- 

 ducted with specially constructed apparatus, — Yolkens (11), 

 Schwendener (12), — certainly appear to show that there are 

 bubbles of air in the vessels and tracheids. Scheit (4) 

 maintains that these bubbles are not air-bubbles, but simply 

 empty spaces, because in glycerine these cavities quickly 

 disappeared. This last view requires confirmation, but the 

 presence of air does not show that there is no water. 

 Hartig (9) gives the proportion of air and water as varying 

 from one volume air to three volumes of water to from nine 

 volumes to ten volumes water ; Schwendener (12) gives air 

 0"33 mm., water 019 mm, in the vessels. 



Most observers — Hartig (9), Von Hohnel (3), Boehm (5), 

 Volkens (11) — seem to think that the vessels and tracheids 

 are full of water at night, and that it is only during the day, 

 at the time of most active transpiration, that air enters 

 (according to Scheit (4) the vessels are emptied). The air is 

 also at an extremely low tension. Boehm (5) found that if 

 a twig of Bobinia is cut under mercury, the mercury rises in 

 the vessels to a height of 116 cmm. ; cf. also Von Hohnel (3). 

 Vesque (13) found in one or two cases that the contained 

 air was at a pressure equal to that of the atmosphere ; but in 

 his experiments it is not easy to see how the air in the 

 intercellular spaces can be distinguished from that in the 



