64 METABOLISM 



mercury. It must further be specially remembered that this maximum 

 pressure shows itself, in general, in all our native plants only under conditions 

 when the evaporation of water is much reduced. Bleeding may be observed 

 in the birch and vine in springtime, before the appearance of the leaves, but 

 in the middle of summer special precautions have to be taken to retard tran- 

 spiration before the phenomenon can be demonstrated. When transpiration 

 is most vigorous no water exudes from the stump of a felled tree, indeed it is 

 greedily absorbed, and if a manometer be fixed laterally in the wood of a leafy 

 stem, it shows a negative rather than a positive pressure, that is to say, 

 a pressure in the interior of the stem less than that of the atmosphere. 

 This negative pressure, to which we have previously drawn attention, is 

 especially well seen in leafy branches, so that it is obvious that the bleeding- 

 pressure which makes itself apparent in twigs, branches, and in the stem 

 itself, does not exist at all during transpiration, or is so inconspicuous that it 

 is not worth considering as a means whereby water may be supplied to the 

 leaves. This tallies with V. Hohnel's observations (1879) on herbaceous 

 plants, for he observed that grasses gave off water in the form of drops in the 

 morning owing to the repletion of their vessels, and exhibited a conspicuous 

 negative pressure in the afternoon when transpiration became vigorous. For 

 these reasons it is impossible to ascribe to bleeding-pressure any fundamental 

 significance apropos of the conduction of water, although, perhaps, when it 

 does occur, it may to some extent aid in the ascent. 



If, then, water be not forced up by the agency of a pressure acting from 

 below, we are compelled to cast about for a force which must have its seat of 

 activity at the upper end of the plant, and which is capable of sucking the water 

 up. We have already learned of the existence of such a force, viz. transpira- 

 tion ; our experiments with the potometer made this clear to us, but it is 

 possible to demonstrate its existence much more definitely if we fix a transpiring 

 shoot, air-tight, in a long glass tube, fill the tube with water, and stand it verti- 

 cally with its lower end immersed in mercury (Fig. 15). The branch continues to 

 absorb water by its cut surface, and mercury rises to take its place in propor- 

 tion as the water disappears from the tube. The height to which the mercury 

 ascends serves as a direct indication of the force exerted by the suction. This 

 sucking process is, however, a purely physical phenomenon, and for that reason 

 it will be advisable for us to study it in the first instance with the aid of a phy- 

 sical apparatus. We employ for this purpose a thistle tube, closed at the ex- 

 panded end by parchment and filled with water (Fig. 16), and with its lower 

 narrow end plunged into mercury. The parchment loses its imbibed water 

 by evaporation and, in consequence, sucks water out of the tube. This water 

 is in turn replaced by mercury. Since the atmospheric pressure on the outside 

 of the membrane exceeds the atmospheric pressure on the inside by the weight 

 of the column of water and mercury, the air easily passes through the parchment 

 into the interior and occupies the space between the membrane and the water, 

 and any further ascent of mercury ceases. For this reason, Askenasy (1895) 

 replaced the parchment by a block of gypsum, which, when wet, is less per- 

 meable to air. Since the pressure in the interior of the apparatus becomes 

 less and less as the mercury rises higher and higher, the air escapes from 

 the water just as in an air-pump and stops any further ascent. If one uses 

 boiled water, however, very considerable heights are obtained. In Askenasy' s 

 (1896) experiments, for example, the mercury reached a height of 82 cm. with 

 the barometer standing at 76-2 cm., and, in another case, 89 cm., when the 

 atmospheric pressure was 75-3 cm. It is obvious that still higher values could 

 have been obtained had the gypsum cork been able to resist the passage of air 

 entirely. 



These heights reached by the mercury, exceeding as they do that of the 



