74 METABOLISM 



bility improbable. Only the first method remains, viz. that the water must 

 move between the air-bubbles and the wall of the vessel. This view was first 

 put forward by Vesque (1883) and subsequently maintained by Strasburger 

 (1891). It must first of all be noted in this connexion that owing to the 

 sectional outline, which is far from being a circle, and owing to the frequent 

 occurrence of local thickenings on the wall — especially if these be of the spiral 

 variety — a close adhesion of the air-bubbles to the wall of the vessel is prevented, 

 and quite large spaces may exist between the bubble and the wall, in which 

 a movement of water would appear possible, and this would still be the case if 

 only the so-called adhesive water film between the air-bubble and the wall 

 were present. Vesque has, as a matter of fact, seen a stream of water flow- 

 ing past the stationary air-bubbles, both in plant vessels and in capillary glass 

 tubes, and Copeland has more recently (1902) confirmed Vesque's observa- 

 tions. Although there can be scarcely any doubt of this fact still it is not 

 possible at present to explain physically the ascent of sap in this way. By 

 this arrangement we have continuous columns of water which, every here and 

 there — at each air-bubble — have a diminished diameter ; this periodic lessen- 

 ing of the diameter decreases the tendency to sink on the part of the water 

 column, on account of the additional friction, and must, for the same reason, 

 hinder its ascent. Sinking of the column appears to be inevitable as soon as its 

 weight exceeds the resistance to filtration offered by the root-cells and the 

 friction in the vessels. If, however, these resistances are sufficiently great 

 to prevent this sinking, how a force arises which is capable of effecting 

 a lifting of the water is not apparent. One cannot resist the impression that 

 the more intimate physical conditions of the ascent of sap in the plant have 

 not as yet been clearly made out, and we must still wait for further elucidation 

 of these phenomena by experimenting with apparatus more nearly resembling 

 vessels than the glass tubes which we have hitherto employed to represent the 

 vessels. An apparatus of this sort has been devised by Copeland (1902). 

 He has shown that if a tube, over 12 m. high, be filled with plaster of Paris, 

 water, and air, evaporation at the upper end is followed by an ascent of the 

 water. He has observed, further, that the pull of transpiration is active much 

 further down than Schwendener thought, but he is unable to explain 

 physically the way in which his own apparatus works. It seems to us most 

 important to elucidate, first of all, the distribution of negative pressure in the 

 tree. According to Strasburger' s (1891) observations, and also according to 

 those of Pappenheim (1892), the negative tension at the tips of the branches 

 does not appear to differ from that at the base, and a negative pressure ap- 

 parently exists at the bottom of the tree, as is proved by the sucking up of 

 water through the cut surface. Whether this is dependent on the suction 

 exerted by the leaves, or arises from other causes, it should not be difficult to 

 determine. If it be due, as appears to us likely, to the activity of the leaves, 

 then the osmotic suction must certainly reach down to the root (in opposition 

 to what Schwendener found), and then one might truly say that transpiration 

 provides the force which causes the ascent of sap. But even then physical research 

 would require to show how it was that the air in the vessels at the base of the 

 stem was not compressed by the superincumbent water columns. 



In addition to purely physical forces the activity of the living cells is fre- 

 quently brought forward as a cause of the ascent of sap. In a certain sense 

 their co-operation cannot be doubted, in so far, that is to say, as the Uving cells 

 build up the vascular system and develop into forms suitable for performing 

 this function in the plant. The vessels, when they begin to act as water 

 conduits are already filled with water. If the water be withdrawn from the 

 vessels they cannot be again filled by the plant's agency, their capacity is for 

 ever lost, and the plant dies unless water be injected artificially into the vessels. 



