October 21, 1922] 



NA TURE 



549 



impossible. The narrow transverse section of its 

 elements, the frequent occurrence of transverse walls, 

 and the lining of protoplasm and large protein contents 

 practically preclude the mass movement of liquid 

 through this tissue. If we imagine the flow restricted 

 to the sieve-tubes the velocity must be corresponding!}' 

 increased, and the excessively fine sieve-pores, more 

 or less completely occupied by colloidal proteins, must 

 be reckoned with. Simple diffusion, as Czapek recog- 

 nised, cannot account for the transport, and there is 

 no reason to suppose that adsorption on the surfaces of 

 the colloid contents of the sieve-tubes can increase 

 the velocity of diffusion, as Manghan suggests. 



As soon as one realises the volume of the solution 

 which has to be transported, and the velocity of the 

 flow that this necessitates, one naturally turns to 

 consider if the open capillary tubes of the wood may 

 not be utilised as channels of transport. Deleand's 

 results, indicating that the depletion of leaves continues 

 even after the living elements of their petioles have 

 been killed, support this conjecture. 



The emphasis which has been laid on the function 

 of the wood as providing a channel for the upward 

 movement of water usually obscures its function as a 

 downward and backward channel also. Early experi- 

 menters, however, fully recognised that, under certain 

 conditions, the current in the wood may be reversed. 

 There is, of course, recent work also showing this 

 reversed current. 



By means of an eosin solution this reversal of the 

 transpiration current may be very easily demonstrated. 

 If the tip of a leaf of a growing potato-plant is cut under 

 eosin solution, the coloured solution is very quickly 

 drawn back into the tracheae of the conducting tracts 

 of the leaf ; from there it passes into those of the 

 petiole, and makes its way not only into the upper 

 branches and leaves, but also passing down the sup- 

 porting stem may completely inject the tracheae of the 

 tuber, and from thence pass up into the wood of the 

 remaining haulms of the plant. Its passage is entirely 

 in the tracheae of the wood of the conducting tracts. 



Another very striking experiment may be carried 

 out with the imparipinnate leaf of Sambucus nigra. 

 Its petiole is split longitudinally for a few centimetres 

 and half removed. The remaining half is set in a 

 solution of eosin. The solution is rapidly drawn up 

 the wood-capillaries of the intact half-petiole, and soon 

 appears in the veins of the pinnae on the same side of 

 the leaf, beginning with the lowest, and gradually 

 working up into the upper ones. Finally it appears 

 in the terminal pinna. All this while the veins of the 

 pinnae on the other side remain uncoloured. Now, 

 however, the eosin begins to debouch into the base of 

 the uppermost of these pinnae and spreads through its 

 veins ; finally it makes its way down the offside of the 

 rachis to the bases of the lower pinnae, and from thence 

 spreads into their veins. In this case we see very 

 clearly how transpiration actuates an upward current 

 on one side and a downward current on the other. It 

 is interesting to note that if the terminal pinna and its 

 stalk is removed the eosin does not appear in the pinnae 

 of the second side, or only after a considerable time, 

 when the small anastomosing conducting tracts are 

 utilised. 



Luise Birch-Hirschfeld also described recently many 



NO. 2764, VOL. 1 IO] 



experiments with herbaceous and woody plants, tracing 

 the path of the reversed current by means of lithium 

 nitrate and eosin. 



In all these cases the tension of the sap determines 

 the flow from a soun e wherever situated, and transpira- 

 tion from the leaves, or parts of leaves, which are not 

 supplied with liquid water from without, draws the 

 water through the plant along the channels of least 

 resistance. Hence it is that if the cut vein of a lateral 

 pinna provides the point of entry, the solution may 

 pass backwards in some of the conducting tracheae, 

 leaving others quite uncoloured, so that some of the 

 veins only of the pinna are injected. The injected 

 tracts bring the solution down the rachis and petiole 

 into the stem, while a few or many, as the case may be, 

 remain filled with colourless liquid, presumably the 

 sap drawn upward to supply the transpiring surfaces 

 of the leaf. Generally the coloured liquid descends 

 an appreciable distance in the tracheae of the stem 

 before it begins to rise in the ascending current, mount- 

 ing to other transpiring leaves. As a rule after some 

 time — depending on the rate of transpiration and the 

 amount of water supplied by thfj roots — the presence 

 of the coloured liquid may be demonstrated in certain 

 continuous series, or filaments of tracheae in several 

 bundles of the lower parts of the stems. Similarly, if 

 tubers or rhizomes are present, examination of these 

 parts, after a suitable interval, will show that many 

 of their filaments of tracheae are injected. Meanwhile 

 the parts above the supplying leal become coloured, 

 and it will be seen that the distribution of coloured 

 tracheae is decided by the anatomical connexions of 

 those filaments of tracheae which convey the coloured 

 liquid directly from the point of supply through the 

 petiole to the stem. In tracing the path of the solution 

 one is impressed with the fact that the path of least 

 resistance is by no means always the shortest path in 

 the wood. Transverse motion across several tracheae 

 seldom occurs, and the separate linear series of con- 

 ducting tracheae are practically isolated from each other 

 laterally. Here we may recall Strasburger's experi- 

 ments showing the very great resistance offered to the 

 flow of water in a transverse direction in the wood 

 of trees. This isolation of the separate filaments of 

 tracheae in the leaf and in the stem enables the tension 

 developed by the transpiring cells of the leaves, while 

 it raises a column of water in one series of tracheae, to 

 draw down a solution in a neighbouring filament of 

 tracheae terminating above in some local supply. If 

 the anatomical connexion of the two series is located 

 in a subterranean organ the tracheae of the subterranean 

 organ may become filled from that supply. 



So far the evidence of reversed flow in the water- 

 conducting tracts which we have been considering has 

 been derived from plants under artificial conditions — 

 plants the conducting tracts of which have been cut into 

 and otherwise interfered with. Is there any evidence 

 that reversal of the transpiration-current normally 

 occurs in uninjured plants ? 



Some recent work on the transmission of stimuli 

 seems to me to indicate that these reversals are con- 

 tinually occurring in normally growing plants. 



The first piece of work to which I would direct 

 attention is that of Ricca on Mimosa. It has long been 

 known that the stimulus which causes the folding of the 



