236 



NATURE 



[February 23, 1922 



Letters to the Editor. 



[T/ie Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Transport of Organic Substances in Plants. 



The older writers and modern text-books affirm that 

 the organic materials (carbohydrates, etc.) manufac- 

 tured in the leaves of plants are transported down- 

 wards by means of the bast through their organs to 

 places of consumption and storage. This belief seems 

 to be based entirely on ringing experiments. A priori 

 the bast appears to be very unsuitable for carrying 

 out this function. Even in the most rapidly assimilat- 

 ing plants its cross-section is small. It is formed of 

 short cells and comparatively short, narrow tubes, so 

 that many cross-partitions must be traversed by the 

 stream carrying these organic substances if they use 

 it as a conduit. Furthermore, its resistance must be 

 greatly increased by the fact that a large proportion 

 of its cross-section is occupied by viscid contents — 

 protoplasm and proteins. Evidently, in such a con- 

 duit we could only expect that velocities of transport 

 comparable with diffusion velocities could be attained. 

 Assuming that a lo per cent, solution of sucrose were 

 supplied by the leaves and that this was completely 

 converted into an insoluble carbohydrate in a storage 

 organ 50 cm. distant, then we might expect, after a 

 steady state had been attained, a rate of transport, 

 from diffusion alone, of about 2 milligrams per 

 sq. cm. per diem. This would be equivalent to a 

 10 per cent, solution moving at the rate of 02 mm. 

 per diem. Although this diffusion rate of transport 

 might be somewhat accelerated by protoplasmic 

 streaming, it is quite evident that diffusion in the bast 

 is inadequate to account for the observed rate of trans- 

 port of carbohydrates in plants. The insufficiency of 

 diffusion to transport carbohydrates is strikingly borne 

 out by those experiments in which cut floating leaves 

 exposed to conditions suitable for photosynthesis 

 accumulate carbohydrates, while only negligible quanti- 

 ties find their way into the water. 



Close approximation to the velocity of transport in 

 the bast, if that channel alone is used, may be 

 obtained. 



For example, a potato weighing 210 grams was 

 attached to a stem by a slender branch about i-6 mm. 

 in diameter. In this branch the bast had a total cross- 

 section of 0422 sq. mm. This figure is a maximum, 

 as no allowance was made for cell-walls or any non- 

 functional element in the bast. Through this conduit, 

 ex hypothesi, all the organic substance has passed 

 during the growth of the tuber, viz. in about 100 

 davs. According to analvses, more than 24 per cent, 

 of the tuber is combustible; therefore we may assume 

 »that approximately i:;o grams of dissolved carbohvdrate 

 has passed a conduit 0422 sq. mm. in cross-section in 

 100 davs. The concentration of this solution was 

 probably not more than 10 per cent. Thus 500 c.c, of 

 solution must have passed in 100 days, and the aver- 

 age rate of flow must have been 5/000422 cm. per 

 diem, i.e. more than 1000 cm. per diem, or about 

 40 cm. per hour. This is evidently a much greater 

 velocity than could be attained by diffusion in the bast, 

 even when assisted bv protoplasmic streaming. 



Another way of arriving at the velocity of transport 



in the bast demanded by this view mav be obtained 



from such recorded results as those of Brown and 



Morris on the depletion of leaves. When these results 



NO. 2730, VOL. 109] 



are combined with actual measurements of the total 

 cross-section of the bast strands in the petiole we 

 arrive at similar figures for the velocity of transport, 

 i.e. if carbohydrate moves as a 10 per cent, solution 

 the velocity of flow must be approximately 50 cm. per 

 hour. 



These considerations irresistibly force one to con- 

 clude that the cross-section of the bast is not adequate 

 to transmit the amounts of carbohydrates actually 

 known to travel downwards in the stems and petioles 

 of plants. The same arguments seem to apply in 

 ruling out the cortex as the conduit for the general 

 transport of carbohydrates. The greater cross-section 

 available would still be insufficient to allow the quan- 

 tity transmitted by diffusion alone to account for the 

 quantities observed. 



In this connection a fact pointed out to us by Prof. 

 Seward is of peculiar interest. In several species of 

 tree-like Lepidodendra there is no tissue in the stem 

 which presents the structural characteristics of bast, 

 yet we cannot possibly assume that no transport of 

 organic substances back from the photo synthetic 

 organs took place in Lepidodendron. 



Many observations indicate that the wood is the 

 tissue in which this transport is effected. Hales, in 

 1727, published accounts of experiments showing a 

 reversed or downward current in the stem of trees. 

 One of us and Dr. Joly experimented with inarched 

 branches and demonstrat-ed a reverse current, and 

 quite recently Ricca's brilliant work on the transport 

 of th^ hormone in Mimosa renders the same 

 phenomenon obvious (Boll. della Soc. bot. 

 Ital.,N Ott., 1915, "Soluzione d'un Problema di 

 Fisiologia," Firenze, 1916). Many observers have 

 proved the presence of carbohydrates in the water of 

 the tracheae during spring, and one of us, with 

 Dr. W. R. G. Atkins, has shown that these substances 

 are present in a greater or less degree during the 

 entire vear (H. H. Dixon and W. R. G. Atkins, 

 Notes from the Botanical School of Trinity College, 

 Dublin, vol. 2, pp. 275 et seq.). It is only reasonable 

 to assume that they will travel with the water current 

 whether it moves in an upward or downward direc- 

 tion.' 



Some very striking evidence for the existence of this 

 reversed current may be obtained with plants of 

 Solanum tuberosum. Thus a large potato plant was 

 dug up from the soil with as little injury as possible 

 to its underground stems and roots. After a short 

 exposure to the air, but before any visible wilting had 

 taken place, the apex of one of the leaves was cut 

 off under a solution of eosin by means of a pair of 

 scissors. In an hour the veins of all the leaves, the 

 stems, and the roots were tinged with eosin. Even the 

 roots on the far side of the tuber showed this colora- 

 tion. When sections of the tuber were examined next 

 day the strands of tracheae in the bundles showed out 

 with great clearness owing to their injection with 

 eosin. This experiment was made in September. 



A similar result was obtained with a specimen of 

 Chrysanthemum macrophyllum left undisturbed as it 

 grew. The tip of one of the leaves was cut off and 

 the cut surface immersed in eosin solution at 4 p.rn. 

 on an October afternoon. Next morning the eosin 

 was apparent in all the veins of the leaf and could be 

 traced in the bundles of the petiole. 



The transmission of clogging and poisonous sub- 

 stances by a reversed transpiration current has been 



1 Lately Curtis has criticised this view, basing his attack on the results 

 of ringing experiments. He does not, however, seem to have taken into 

 account the blocking of the tracheaa which results from morbid changes 

 spreading inwards tlirough the wood parenchyma and medullary rays 

 from the injured region. These effects have been discussed at length by 

 Strasburger, " Leitungsbahnen in den Pflanzen." 



