70 



JOURNAL OF HORTICULTURE AND COTTAGE GARDENER. 



[ July 26, 1877. 



round with a stick moistens the whole. I have never found 

 mice to touch them after this. Petroleum besides is a fertiliser, 

 carbolic acid is not, and likely to destroy the vital power of 

 some delicate seed. — A. Deck, Cambridge. 



EXPEBIMENTS ON THE FLOW OP THE SAP. 



[Read at the Scientific Committee of the Royal Horticultural Sooiety.] 



At the beginning of this session I drew the attention of this 

 Committee to the course of the s dp, being of opinion that recent 

 researches rendered some modification in our views necessary 

 on that subject. 



The proposition that I submitted to the Committee was, pure 

 and simple, that there was no such thing a' descent of the sap at 

 all, but that its course was always upwards. I found the Com- 

 mittee quite in accord with me, so far as regarded anything like 

 circulation. I think most of them, if not all, repudiated any 

 belief in the whole theory of the ascent of the sap by the fibro- 

 vasenlar bundles of the wood and its descent by the cellular 

 layers of the inner bark ; but I found the majority still embued 

 by the theories of Sachs, and holding with him, and on his 

 grounds, that descent by some means was absolutely necessary 

 in respect that assimilation could only take place in the light, 

 and consequently that the whole of that function must be per- 

 formed in the leaf, whence the assimilated matter there pro- 

 duced must be transported in some way or other to the other 

 parts of the plant in which it is found ; and as these are lower 

 down, and some of them even underground, as in the case of 

 tubers, it followed that there must be a descent in some way or 

 other, and the prevailing opinion seemed to be, as was, I think, 

 first suggested by Mr. Herbert Spencer, that this took place by 

 a slow swaying or wandering motion, by means of endosmose 

 and exosmose, through the walls of the cells, which imper- 

 ceptibly and independent of the current of the sap mixed the 

 whole up together, or carried the different ingredients to where 

 they were wanted. 



Since I last spoke on the subject I have endeavoured to see if 

 actual experiment would throw any light upon it. 



I made experiments with the Vine, the Fig, the Horse Chest- 

 nut, and the Hyacinth, but as they all, so far as they went, 

 tended in the same direction, I shall speak principally from the 

 Vine, which was much more manageable and more readily took 

 up my infusions than any of the others. Thanks to the experi- 

 ments of Professor M'Nab and Professor Churcb, I knew of the 

 virtues of lithia as an easily absorbed agent, whose presence 

 could be detected anywhere by the spectroscope in however 

 small a quantity it might be present, and I had the advantage 

 of Professor Church's own kind assistance in determining for 

 me whether it was present or not. As lithia, however, is colour- 

 less, I added to my infusions enough of litmus to colour them 

 deeply, and I am bound to record as the result of my own ex- 

 perience that the lithia told me nothing that the litmus did not 

 equally well. The combination of both, no doubt, adds to the 

 confidence with which I can trust to my experiments, but the 

 litmus had one great advantage over the lithia — that it might 

 be easily handled, and dropped or spilt, without interfering with 

 the experiment ; whereas with lithia we have constantly to be 

 on our guard against any carelesB dispersal of it — aB, for instance, 

 by allowing a drop to spill on the bark, or by using a knife that 

 had been employed in cutting a portion of a branch that has 

 been lithiated to cut one that has not. The form and propor- 

 tion in which I used the lithia were five grains of citrate of lithia 

 to each fluid ounce. To this I added a little glycerine, with the 

 view of equalising the specific gravity of the mixture with that 

 of the sap, and then as much as I found necessary of small lumps 

 of litmus. 



I then passed gutta-percha funnels over the shoots to be ex- 

 perimented on, and secured them as cups, with the shoots grow- 

 ing up the middle, by means of cork and tallow. I tried water- 

 proof cloth, but it did not hold in, but the gutta-percha funnels 

 did perfectly. My experiments were made in April and May, 

 when the leaves were beginning to open. I put one cup on the 

 stem of the Vine. It held perfectly, and no esoape of the liquid 

 took place. After the cup was properly luted to the stem with 

 tallow I cut a nick in the bark a little above the fitting, and then 

 filled the cup with the lithiated litmus mixture, so as to cover 

 the nick. I then allowed it to remain on for six weeks, con- 

 stantly renewing the mixture in the cup as it disappeared. 



After the expiration of six weeks I took np the plant and ex- 

 amined it : and here let me say that the Committee and I are in 

 accord aB to what I should have found had Sachs' theory been 

 well founded. I imagine that upon every principle I should, on 

 the ordinary principles of gravitation, have found the severed 

 vessels below the nick, and on the same side as it, filled with the 

 infusion in consequence of its descent. So far as regarded that 

 part of the plant it was no longer a closed tube, and there could 

 be no ascent, but being, as it were, merely an open tube, what- 

 ever was ponred into it should simply find its way to the bottom. 

 '• And so in fact it did — the infusion below the nick descended 

 to the very fibrils of the roots. In like manner the part above 



the nick being a closed tube — closed by the cup at the bottom 

 and by the leaves acting as a sucker at the top — we should expect 

 that the infusion would ascend; and so it did, nearly as com- 

 pletely as it descended below the nick. But how aa regards the 

 parts that were on the opposite side from the nick ? According 

 to Sachs' theory, whether you call it the result of metastasis or 

 of endosmose and exosmose, the infusion should have been 

 found extravasated and UDfiltered through that side, both above 

 the nick and up the ascendiDg branches, and in fact everywhere 

 a little ; but in point of fact there was not the slightest extrava- 

 sation nor a solitary particle of lithia or litmus in any of these 

 places. The depth of the nick was the measure of the extent of 

 the penetration of the infusion, and it was as sharply defined as 

 a line could be ; and this is just what I said should be the case. 



I said that the rapidity of the current would prevent any in- 

 termingling of ingredients by endosmose or exosmose. At night 

 there can be no ascending current, for the force that produces 

 it, the sun, is withdrawn, but the tube is full and in equilibrium. 



To keep strictly within my experiences I must explain that 

 this is the result of all my experiments as regards litmus, and 

 of all but that on the Vine as regards lithia. The experiment 

 as to lithia in the uncut side of the Vine could not be tested, 

 because I sent Professor Church the portions of the plant to be 

 tested, all cut up transversely, aBking him to cut certain specified 

 ones longitudinally, and then test them separately; but he ex- 

 plained that that would be a delusive test after the portions had 

 travelled from London to Cirencester, for the lithia would have 

 had time to pass by endosmose and exosmose from one Bide to 

 the other, after the plant was cut in pieces : which of course it 

 would, for there was no longer any current to prevent its infil- 

 tration ; but the distribution of the litmus when the plant was 

 newly cut showed clearly enough what the result of a search for 

 lithia would have been at that time. 



To my mind this is conclusive on the question. Sachs must 

 be wrong, and we must now re-examine his arguments and see 

 where the flaw lies. His position is thus stated in his "Physi- 

 ologic Vegetale :" — " The absolute necessity (says he) of the 

 intervention of light for assimilation in plants with chlorophyll 

 is proved directly by their mode of development in darkness. 

 "When we cause seeds to germinate in such conditions, roots, 

 internodes, and leaves are developed generally in proportion to 

 the mass of the seed. When all the provision of elaborated 

 principles contained in it are exhausted the development ceases. 

 If up to that period the seed is allowed to germinate in the light 

 and it is then removed into darkness the result is the same— 

 the young leaves, although green, assimilate nothing; but if 

 they are allowed to remain long enough in the light to have 

 assimilated a little, there will be developed in darkne3s leaves 

 and internodes until that new provision be exhausted also." 



But there is one important fact that Sachs omits to keep in 

 view here. If the plant in darkness assimilates nothing, neither 

 does it take any food to assimilate. It is well known that plants 

 do not feed in the dark, and nothing is easier than to prove it 

 by experiment. Let anyone with a Hyacinth growing in water 

 in a glass mark, by a thread or narrow strip of paper glued to 

 the glass, the height at which the water stands at night, he will 

 find it the same height to-morrow morning, but very different 

 to-morrow night. But the fact is not disputed. 



Now on what ground are we to hold that the reason why the 

 plant does not assimilate is the absence of light in preference to 

 the absence of food? Either will account for it, and one will 

 suit Sachs' theory, but the other not. No doubt the food is not 

 taken up because of the absence of light, but it may very well 

 be that if the plant were placed in light so that it could take 

 food generally, and some portions of it were secluded in dark- 

 ness, we should find that assimilation went on as well in those 

 in the dark as in those exposed to light; and this, in fact, is just 

 what nature does with tubers. They are in darkness while the 

 plant is in light, and they contain assimilated matter in as great 

 abundance as any Apple in the blaze of sunshine. Now I object 

 to a question of that kind, or, indeed, any other kind, being 

 answered by giving the thiug to be proved as part of the proof ; 

 but this is what Sachs does. He has to prove that assimilation 

 cannot take place except in light. I offer an instance of its 

 apparently taking place in darkness, and the reply is that it 

 cannot be, because assimilation cannot take place in darkness. 

 During the day the light and heat of the sun draw up the sap 

 to all the terminal parts of the plants, such as the axial extremi- 

 ties of the branches, the buds, the leaves, the tubers (which are 

 only subterranean buds), where it is partly evaporated and partly 

 assimilated, and as it is used up the roots abBorb a corresponding 

 flow to supply the consumption ; but at night, when the motive 

 power is withdrawn, the upward flow of sap ceases, the roots 

 beoome inactive and cease to feed ; at the same time there is 

 nothing to hinder growth going on, it may pile cell upon oell 

 whether the machine is working or not, and it does so, but assi- 

 milation ceases. Sachs has confounded an accessory with a 

 principle, and mistaken an effect for a cause. 



Allow me, however, further to cite a well-known fact in favour 

 of my views, which it reflects no credit upon ns not to have 



