66 METABOLISM 



If in place of the block of gypsum used in Askenasy's experiment we 

 employ a Pfeffer's cell (p. 13), it would appear probable that higher values will 

 be obtained, values which far exceed those of atmospheric pressure. In this 

 form the apparatus would present a greater likeness to the conditions obtaining 

 in the plant, for the Pfeffer's cell would correspond to a parenchyma cell 

 of the leaf and the glass tube to a vessel. Let us assume that the cell borders 

 directly on an intercellular space on the one side, and that the vessel, on which the 

 other side abuts, is filled with water and has its lower end plunged into mercury, 

 and that its wall, as in the case of the glass tube, is impermeable to air. When 

 the cell-sap becomes concentrated owing to transpiration having commenced, 

 the water is withdrawn from the vessel and the mercury rises to take its place. 

 The question then is, how high will it rise ? Let us imagine, for instance, 

 that the mercury rises to a height of 150 cm., then there is no question but 

 that it must be supported by the cell ; but one must not imagine that the cell 

 is not affected by it. It must exert a suction on the cell, just as if we had 

 placed an osmotically active solution near one side of the cell. If the height 

 of the mercury still goes on increasing there comes at last a time when this 

 suction becomes equal to the osmotic pressure in the interior of the cell. 

 When this point is exceeded the mercury acts just like a plasmolysing solution. 

 At the same moment any further absorption of water on the part of the cell 

 comes to an end, the water passes backwards into the vessel, and the ascent 

 of the mercury ceases. 



Such considerations as these compel us to conclude that continuous water 

 columns in the vessels appear unequal to the task of raising water easily, since 

 they are unable to overcome the great cohesion existing between the particles 

 of water. In proportion as the columns become longer the absorption of water 

 by the leaf-cells becomes more difficult, and must, in the long run, become 

 quite impossible. We are further, for many reasons, unable to calculate how 

 long the water columns may actually be. In the first place, we are ignorant 

 as to whether very lofty trees may not perhaps possess an especially high 

 osmotic pressure in their leaves (and in this relation it must be remembered 

 that Dixon's (1896) determinations of osmotic pressures in the leaf are by no 

 means above criticism), and in the second place, the power of suction possessed 

 by the water column depends not only on its length but also on the friction 

 which it meets with, owing to the tendency to collapse on the part of the walls 

 of the vessels, as well as on the opposition to its passage through the living 

 cells of the root. 



Should we desire to conduct a research with the view of finding out the 

 amount of frictional opposition the water suffers (whether in ascent or descent) 

 in the wood of a tree, we must make it our business first of all to obtain a clear 

 conception of the intimate structure of the vessel, and, above all, to note in what 

 respects it differs from the glass tube which has hitherto served as our model ; 

 i.e. we have to study its length and breadth and the nature of its wall. 



The vessel varies greatly in length according to its mode of develop- 

 ment. Two extreme conditions may be noted and designated respectively 

 tracheae and tracheids. The term ' vessel ' is generally taken as the equiva- 

 lent of ' trachea'. We shall follow in this work the suggestions of Rothert 

 (1899) in regard to nomenclature. The short and comprehensive title ' vessel ', 

 to include both tracheae and tracheids, is, we think, the more necessary, for 

 both morphological and physiological reasons, since very often, in individual 

 cases, it is by no means certain whether tracheae or tracheids are concerned 

 in the process. 



Tracheids are nothing more nor less than much elongated cells which 

 remain entirely closed ; tracheae, on the other hand, are cell chains which run 

 through the plant in definite directions, and whose lumina have become united 



