Dec. 26, 1878] 



NATURE 



183 



ABSORPTION OF WATER BY THE LEAVES 

 OF PLANTS 



'T*HE experiments of Boussingault, referred to in Nature, 

 "*■ vol. xviii. p. 672, find a fitting sequel in those of the Rev. G. 

 Henslow, detailed in a paper read before the Linnean Society 

 on November 7. Although gardeners universally maintain that 

 growing plants have the power of absorbing w ater through their 

 leaves, both in the liquid and the gaseous form, in addition to 

 the power of suction through the roots, yet the contrary theory 

 has been in favour during recent years among vegetable physio- 

 logists. The first recorded experiments of any value on the 

 subject were about the year 1727, by Hales,^ as described in his 

 "Statical Essays;" the conclusion to which he came being 

 that " it is very probable that rain and dew are imbibed by 

 vegetables, especially in dry seasons." This result was con- 

 firmed by Bonnet in 1753. A century later, however, in 1857, 

 Duchartre, experimenting on;the absorptive power of plants, 

 came, after considerable wavering, to the conclusion that rain and 

 dew are not_ absorbed by the leaves of plants. This opinion 

 has been, with but little exception, held by all physiologists 

 during the last twenty years, notably by De Candolle and Sachs ; 

 the explanation offered of the fact that withered plants revive 

 when placed in moist air or when the leaves are moistened, being 

 that transpiration is thus stopped, or is more than counter- 

 balanced by the root-absorption. In his "Text-book of 

 Botany" (English edition, p. 613), Sachs says: — "When 

 land-plants wither on a hot day, and revive again in the evening, 

 this is the result of diminished transpiration with the decrease of 

 temperature and increase of the moisture in the air in the evening, 

 the activity of the roots continuing ; not of any absorption of 

 aqueous vapour or dew through the leaves. Rain again revives 

 withered plants, not by penetrating the leaves, but by moi-tening 

 them, and thus hindering further transpiration, and conveying 

 water to the rooL<:, which they then conduct to the leaves." 

 McNab has, however, proved that leaves clo transpire, even in a 

 moist atmosphere, pro\'ided thev are exposed to the action of 

 light. 



The results of Mr. Henslow's experiments, extending over 

 several years, are altogether in accordance with those of Boussin- 

 gault, and may be considered to set the question of the ab- 

 sorbent power of the leaves of plants completely at rest. The 

 following are the chief conclusions arrived at : — I. The absorp- 

 tion of water by internodes. The experiment consisted of wrap- 

 ping up one or more internodes of herbaceous plants in saturated 

 blotting-paper and in noticing the effects. As a rule the leaves 

 oa the shoots rapidly perished, showing that transpiration was 

 too great for the supply. The stems, however, kept fresh for 

 different periods up to six weeks. 2. Absorption by leaves, to 

 see how far they could balance transpiration in others on the 

 same shoot. The general result is that as long as the leaves 

 remain green and fi-esh in or on water, they act as absorbents, but 

 that the leaves in air keep fresh or wither according as the supply 

 equals or falls short of the demand. 3. To test how far leaves on 

 a shoot can nourish lower ones on the same shoot. It appears 

 that it is quite immaterial to plants whether they be supplied 

 from water by the absorbing leaves being above or below those 

 transpiring. Water flows in either direction equally well. 4. 

 Leaves floating on water. It was found that one part of a leaf 

 can nourish another part for various periods, though the edges 

 out of water died first. 5. Absorption of dew. A long series of 

 cut leaves and shoots were gathered at 4 p.m., then exposed to 

 sun and wind for three hours, then carefully weighed and expo-ed 

 all night to dew. At 7.30 a.m., after having been dried, they 

 were weighed again, and all had gained weight and quite recovered 

 Lheir freshness, proving that slightly wetted detached portions 

 do absorb dew. 6. Imitation dew. Like residts followed from 

 using the " spray," by which dew could be exactly imitated. 7. 

 riants growing in pots and of which the earth was not watered, 

 were kept alive by the ends of one or more shoots being placed 

 in water; e.g., Mimulus moschaius not only grew vigorously 

 and developed axillary buds into shoots, but even blossomed. 



By these interesting experiments the physiological botanist is 

 again placed in harmony with the gardener who syringes his 

 plants, not merely for the purpose of washing off dust and in- 

 sects, but in order to facilitate the actual absorption of water 

 by the surface ; and with the field botanist, who sprinkles the 

 plants in his vasculum with water to keep them fresh till he 



' Inhis " Geschlchte der Botanilc," pp. 513-321. Sachs gives an admirable 

 epitome of the great service rendered to the progress of botanical science by 

 m. researches of this eminent botanist and ph>-sic.st. 



reaches home. The fact which now seems established beyond 

 all doubt by the observations of Darwin and others, that certain 

 plants have the power of absorbing through their leaves and 

 digesting the remains of animal substances, also implies, as a 

 necessary corollary, the absorbent power of leaves for certain 

 liquid or gaseous substances. In connection with this subject 

 sufficient attention has perhaps not been attracted to the obser\'a- 

 tions of Prof. Calderon, as detailed in a paper printed at Madrid, 

 in English (1877), entitled, "Considerations on Vegetable Nutri- 

 tion." ^ Calderon's statements — which, however, require at pre- 

 sent to be confirmed by other observers — are to the effect that 

 plants have the p)ower of absorbing the nitrogenous organic 

 matter which is constantly floating in the air, and that, if air be 

 deprived of all organic matter, it is »nable to sustain vegetable 

 life, all the physiological functions of plants being then sus- 

 pended. A. W. B. 



UNDERTONES 



'T'HESE formed the subject of a lecture delivered by Her*" 

 ■*■ Auerbach before the meeting of Natiutilists at Casse^ 

 this year. 



The term "undertones," he pointed out, is an extension of 

 the nomenclature which denotes certain accompanying tones of 

 a given note " overtones." Undertones may be observed in the 

 following way: — If a struck tuning-fork be set on a sound- 

 board, a tone is heard sounding strongly, which before was 

 little perceptible. The stem of the fork makes longitudinal vilwa- 

 tions, which, by action on the sounding-board (a very thin one), 

 generate transverse \-ibrations, and these spread over the large 

 surface of the plate. Should the tone of the board only differ 

 in intensity from that of the fork, the vibrations executed by the 

 stem of the tuning-fork must be small ; it is otherwise, however, 

 when the vibrations exceed a certain amount. 



Herr Auerbach demonstrated his observation with a tuning- 

 fork giNing the A of a violin, and so 435 vibrations per second. 

 When he placed the vibrating fork firmly on a sonud-board, the 

 tone was heard distinctly at a distance. When, however, he 

 brought the tuning-fork, struck very vigorously, into very light 

 contact with the plate, there was heard the lower octave of the 

 fork's tone. With other materials, which were not then at his 

 command, he could produce also the lower fifth of the lower 

 octave, and the lower fourth of this tone, i.e., the double 

 octave of the fork's tone. The \'ibration numbers of these 

 resonance tones are \, J, J, &c., of that of the tuning-fork's 

 tone, i.e., the resonance-tones form the series of the harmonic 

 undertones. 



With regard to the mode of occiurence of these tones, Herr 

 Auerbach said this: "To prove to you, first of all, that the 

 strength of the ^-ibrations is the fundamental condition of the 

 phenomenon, I will once more make the experiment, and con- 

 tinue it longer. You heard first, again, the lower octave ; but 

 then the tone sprang over into the higher, so that it became 

 identical with the proper tone of the tuning-fork. Consider this 

 re.-ult along with the fact that the vibrations of the fork rapidly 

 diminish ; remember, too, that it is only when the fork is 

 vigorously struck and lightly placed on the plate that the under- 

 tones occur, and you will see that the cause of the phenomenon 

 lies in the amplitude of the \-ibrations." 



Herr Auerbach further supposes that the resonance-surface of 

 the plate, being imperfectly elastic, follows the movements of 

 the stem of the fork immediately downwards indeed, but not up- 

 wards ; an interval then occurs which only disappears on the next 

 passage downwards of the stem. If the retardation be a small 

 one, the plate, at the moment of meeting the stem a second time, 

 has nearly completed a vibration. If, however, the retardation 

 be great, the imdertones arise (as the lecturer showed graphically) 

 from the combination of the vibrations of the stem and the plate. 

 The stem-end, i.e., in this latter case, is no longer an uncondi- 

 tionally free end, but its freedom is a periodic fimction of the 

 time, and this period is twice as great as that of the tuning-fork 

 vibrations. " That the undertones arise in consequence of in- 

 ternal friction, was easy to see i priori: what the experiments 

 have shown and explained is the interesting fact, that precisely 

 the harmonic undertones are produced ; that is a consequence of 

 the fact that the resonance is the action of a periodic force, and 

 so, in a certain sense, a discontinuous phenomenon, otherwise 

 the undertones must form a continuous series, which is not the 



case." 



See Xatcre, vol. xv. p. 108. 



