October 21, 1922] 



NA TURE 



547 



the other types are to some extent involved, 6 but 

 for them the disentanglement of local system and 

 galactic field is more difficult. 



Quite analogous to the phenomenon of the Milky 

 Way, the projection on the sky of the faint stars along 

 the central plane of this local cloud gives rise to a sort 

 of secondary Galaxy, 7 the brighter stars of which 

 coincide roughly with the Herschel-Gould belt. The 

 distribution of the B stars indicates that the dimensions 

 of the local system are large compared with those of a 

 globular cluster ; the local system is also more oblate. 

 I believe it can be better compared in dimensions, and 

 possibly in form, with the Magellanic Clouds or with 

 the distinctly delimited small star clouds of the Milky 



\\ ay. The various phenomena of star streaming are 

 undoubtedly connected with the motions of and within 

 the local system. Probably a number of our brighter 

 " moving " clusters should be considered sub-systems 

 in the local cloud, rather than independent systems 

 which for the time being are near at hand. 

 (To be continued.) 



REFERENCES. 



1. Cf. Astroph.Jonr., Proc. Nat. Acad. Sci., Pub. Ast. Soc. Pac, 1915-1921, 

 and Scientia, 1919-1920. 



2. Pub. Ast. Soc. Pac, February 1918 . Mi II . I tmtr., 157. 



3. Mon. Not. R.A.S., 76, p. 563. 



4. Scientia, January 1922. 



5. Harv. Cite, 229. 



6. Van de Linde, Thesis, Rotterdam, 1921. 



7. Scientia, March 1920. 



Transport of Organic Substances in Plants. 1 

 By Prof. H. H. Dixon, Sc.D., F.R.S. 



A MONG physiologists the usually accepted view is 

 JTV that organic substances are distributed through- 

 out the plant by means of the bast. The wood also 

 acts as a channel of distribution for these substances 

 to opening buds and developing leaves, especially in 

 spring when root-pressure is active. The sap of 

 bleeding contains appreciable quantities of these sub- 

 stances, and their distribution to the developing buds 

 in spring by means of the wood was recognised by 

 Hartig and Sachs. 



This upward transport of carbohydrates in the 

 tracheae seems to be accompanied with smaller amounts 

 of proteins. Thus Schroeder showed that the quantity 

 of proteins in the bleeding sap rises and falls with the 

 quantity of sugar. 



This view that the rising current in the tracheae carries 

 organic substances in it and distributes them to the 

 growing regions has lately been impugned. It was 

 pointed out that in.many cases, ringing close below the 

 terminal bud prevents the development of that bud 

 because the wood is unable to transmit sufficient 

 supplies of organic substance. As Strasburger has 

 already shown, this interpretation rests upon the 

 fallacy of supposing that the removal of the bark as 

 far as the cambium leaves the wood uninjured. As a 

 matter of fact, microscopic examination of the wood, 

 from which the outer tissues have been stripped, shows 

 that its trachese soon become blocked with air-bubbles 

 and with substances probably exuded into them and 

 their walls during morbid changes in the cells of 

 the cambium, in the cells of the medullary rays, and 

 in those of the wood-parenchyma. The blocking is 

 accompanied with discoloration, and is most apparent 

 in the outer layers of the wood. It is only reasonable 

 to suppose that the efficiency of the tracheae as channels 

 of transmission is seriously impaired even before there 

 is visible evidence of plugging. 



It is evident that this clogging may act differentially 

 on the water and the substances carried in it. In the 

 first place, the whole cross-section of the wood is avail- 

 able for the transport of water, while probably the outer 

 layers are mainly utilised by the organic substances. 

 Further, colloidal deposits in the walls, and especially 

 in the pit-membranes, would obstruct the passage 



1 From the presidential address delivered to Section K (Botany) of the 

 British Association at Hull on Sept. 7. 



NO. 2764, VOL. I IOJ 



of organic substances much more than they would 

 the water which carries them. These considerations 

 readily explain how it is that, while the water-supply 

 to the buds of ringed branches is adequate, the supply 

 of organic substance may be deficient. 



Apart, then, from the very slow movement of organic 

 substances from cell to cell, there is very cogent evidence 

 that their upward motion is effected in the tracheae 

 of the wood. There is no reason to believe that during 

 this transport the walls or pit-membranes of these 

 tracheae oppose the passage of the dissolved carbo- 

 hydrates or of the simpler proteins any more than the 

 water which conveys them. Hence the velocity of 

 transport of these organic substances is that of the 

 transpiration current, and the amount conveyed in a 

 given time depends on the velocity and concentration 

 of the stream. 



The transport of organic substances in an upward 

 direction in plants is secondary, for, as is well known, 

 carbohydrates certainly, and proteins most probably, 

 are manufactured only in the upper green parts of 

 plants — principally in the leaves, and must be trans- 

 ported in the first instance back from these to the stems 

 to be distributed to the growing regions and to the 

 storage organs. 



This view that the channel for the backward and 

 downward movement of organic substances is afforded 

 by the bast received great support from Czapek's work 

 published in 1897. By section of the conducting 

 tracts in one half of the petiole he showed that depletion 

 of the corresponding half of the blade was delayed. 

 He also showed that only where vertical bridges con- 

 nected the upper and lower portions of bark in ringed 

 stems were the effects of ringing nullified. Oblique 

 and zigzag bridges are ineffective. Thus transverse 

 conveyance in the stem is negligible. The parallel 

 and longitudinal arrangement of the elongated elements 

 in the bast seemed to him to provide adequately for 

 the observed longitudinal passage. Their narrowness 

 and large colloid content did not present themselves 

 as difficulties. Czapek also recorded the observation 

 that the blades of leaves, the petioles of which had been 

 killed by jacketing them with steam, did not become 

 emptied of starch. Similarly, when the petioles were 

 killed with chloroform-vapour, depletion was arrested. 

 Again, anaesthetisation of the petiole, by surrounding 



