543 



NA TURE 



[October 21, 1922 



it with a watery solution of chloroform, greatlj delayed 

 tin' disappearance oi starch. 



Czapek formed no definite theory as to how organic 

 substances were moved in the bast. Me was sure that 

 the transport depends on living protoplasm. He did 

 not consider that the streaming of protoplasm con- 

 tributed materially to the motion, seeing that streaming 

 does iint occur in mature sieve-tubes. He regarded 

 e tubes as the most important elements in the 

 transmission of these substances, because the deposition 

 of callus in the sieve-plates synchronises with the 

 stoppage of transport. The transport, according to 

 him, is not simply due to diffusion. He supposed the 

 protoplasm to take up the organic substances and pass 

 them on. If diffusion does not account for the passage 

 from one particle of protoplasm to the next, it would 

 seem that we must suppose the organic substance to 

 be projected from one to the other. 



These observations and their interpretation by 

 Czapek have strengthened the opinion thai the basl 1^ 

 the channel for the downward transport of organic 

 substances. It is remarkable how little weight has 

 been attached to the damaging criticism of Czapek's 

 views by Deleano, especially- as those views are so 

 unsatisfactory from a physical point of view. 



The latter author showed that it is inadmissible to 

 compare externally similar leaves, which often behave, 

 so far as depletion is concerned, very dissimilarly. He 

 also pointed out that without any export a leaf may 

 be depleted of all its starch within thirty-five hours, 

 and partially anticipated an extremely interesting 

 recent observation of Molisch — namely, that transpiring 

 leaves lose their carbohydrates much more rapidly than 

 those the transpiration from which is checked by being 

 surrounded with a saturated atmosphere. Neglect of 

 these facts led Czapek into error. Deleano also showed 

 that organic substances continue to leave the blades 

 even after the petioles have been killed by heat or by 

 chloroform-vapour. The rate of depletion is reduced 

 by the former agent to about one-third, and by the 

 latter to one-half. If this observation is substantiated 

 it would show that the intervention of living elements 

 is not essential for the transport. He further found 

 that the blades attached to petioles which were sur- 

 rounded by chloroform-water lost their starch more 

 quickly than those immersed in water. 



The contradictory conclusions of Czapek and Deleano 

 urgently call for a reinvestigation of the points at issue. 

 If Czapek's work holds good, we shall have to regard 

 the bast, and especially the sieve-tubes, as the channels 

 for the transport of organic substances back from the 

 leaf-blades where they are manufactured, and we must 

 look for some hitherto undreamed-of method of trans- 

 mission through these most unlikely-looking conduits. 

 On the other hand, if Deleano's conclusions are borne 

 out, we should admit that protoplasm is not necessary 

 for the transport, and we would turn to a dead tissue 

 as furnishing this channel. 



So far as I am aware none of the earlier investigatoi 5 

 made any estimate either of the actual quantities of 

 organic material which are transported or of the 

 velocities of flow in the channels which are necessary 

 to effect this transport. 



nay approach this problem from two opposite 

 directions — (1) by dealing with the amount of organic 



NO. 2764, VOL. I to] 



substance accumulated in a given time in a storage 

 organ, or (2) by using the amount exported from an 

 assimilating organ. The cross-section of the supposed 

 channels of transport and the volume of the solution 

 containing the substances in each case will give us the 

 other necessary data. 



For' the first method a potato-tuber will furnish an 

 example. One weighing 210 g. was found attached 

 to the base of a plant by a slender branch about 0-16 cm. 

 in diameter. In this branch the bast had a total 

 cross-section of 0-0042 cm. 2 . This figure is a maximum ; 

 no allowance was made for the cross-section of the 

 cell-walls, or for any non-functional elements in the 

 bast. The cell-walls would occupy probably one-fifth 

 of the cross-section of the bast. Now if the bast 

 exclusively furnished the channel of downward trans- 

 port, all the organic substance in the potato must have 

 passed this cross-section during the time occupied in 

 the growth of the potato. One hundred davs would 

 be a liberal allowance. According to analyses more 

 than 24 per cent, by weight of the potato is combustible. 

 Therefore we must assume that during this time more 

 than 50 g. of carbohydrate has passed down a conduit 

 having a cross-section of no more than 0-0042 cm. 2 . 

 The average concentration of the solution carrying 

 this substance could scarcely have been as much as 

 10 per cent. (2-5-5 P er cent - would be more probable ; 

 the concentration of sugar in bleeding sap is much 

 below this figure, and seems never to reach 4 per cent.). 

 Assuming, however, this concentration, the volume of 

 liquid conveying 50 g. must have been 500 cm. 3 , and 

 this quantity must have passed in 100 days. There- 

 fore the average velocity of flow through this conduit, 

 having a cross-section of 0-0042 cm. 2 , must have been 



5°° ■ , , 



— . i.e. nearly 50 cm. per hour. 

 0-0042 x 100 x 24- - J r 



By the second method we arrive at a different figure. 

 Various investigators, from Sachs onwards, have 

 measured the rate of photo-synthesis per square metre 

 of leaf per hour. Under the most favourable conditions 

 the amount may approach 2 g., and it has been esti- 

 mated as low as 0-5 g. Taking Brown and Morris's 

 determination for Tropceolum majus, namely, 1 g. per 

 square metre per hour, and assuming one-third of the 

 carbohydrate formed is used in respiration in the leaf, 

 we find that a leaf of 46 cm. 2 may form during ten hours' 

 sunshine 0-46 g. ; during the twenty-four hours one- 

 third of this will be respired, leaving 0-31 g. to be 

 transported from the leaf. The volume of the solution 

 (again assuming a concentration of 10 per cent.) will 

 be 3.10 cm. 3 . The cross-section of the bast of the 

 bundles in the petiole was 0-0009 cm. 2 ; therefore the 

 velocity of flow, if the bast was used as the channel of 



i'io 

 transport, must have been — or 140 cm. per 



r 0-0009 x 2 4 



hour. 



Similar figures to these were derived using measure- 

 ments obtained from a number of potato-tubers and 

 from various leaves. The velocities indicated, even 

 assuming a concentration of 10 per cent., lay in all 

 cases between 20 cm. and 140 cm. per hour. These 

 figures are in agreement with those arrived at by Luise 

 Birch-Hirschfeld, as to the weight of organic material 

 transported from leaves. 



A flow of this rate through the bast seems quite 



