NUTRITION OP VEGETABLES. 



31 



hladdor, wlien the |)Ositive wivo of a giilvanic 

 trough was iinmcrseil into the full compartment, 

 and the negative wire into the otlier. M. Du- 

 trochet, im])roving on this experiment, attached 

 an empty coecum round tlio end of a glass tube, 

 the aperture of which was stopped up l)y a cork, 

 and through this cork were passed into the 

 bladder tlie negative wire of a galvanic pile, and 

 a fine capillary tube, to allow of the escape of 

 the hydrogen gas, formed by the decomposition 

 of the water. The bladder being now immereed 

 in water, into which the positive wire of the 

 pile was plunged, the galvanic circle was no 

 sooner thus completed, than the bag began to 

 swell, and in a few minutes the water rose in 

 the tube, and flowed over its upper orifice. 

 Here endosmose was produced without the aid 

 of a fluid differing either in density or in chemi- 

 cal nature from the transmitted fluid. Reversing 

 the wires, the phenomena of exosmose was pro- 

 duced, and in half an hour the coecum was 

 emptied. 



Dutrochet then points out the numerous re- 

 semblances between the common electric actions, 

 and those of endosmose, concluding that in every 

 essential circumstance these phenomena are ex- 

 actly similar, so that very little doubt can be 

 entertained that the passage of fluids by endos- 

 mose and exosmose, depends on electric influ- 

 ences; in short, that on the internal and external 

 surfaces of the membranous bags being brought 

 into opposite states of electricity by the action 

 of fluids, differing in density, or chemical com- 

 position, and applied, the one internally, and the 

 other externally, the phenomenon takes place. 

 One very striking point of argument confinna- 

 tory of this conclusion seems to be that an 

 elevation of temperature powerfully increases 

 the electric action, as well as the ascent of 

 vegetable juices; the great difficulty hitherto 

 encountered in the theories, explanatory of the 

 circulation of the juices of plants, was to ex- 

 plain by what means the sap both ascended 

 from the roots upwards, and descended by the 

 leaves and bark. This difficulty can, however, 

 be satisfactorily overcome by the ingenious dis- 

 coveries of Dutrochet. It may be remarked 

 also how appropriately the spongioles of the 

 radicles are constructed to produce endosmose. 

 They have a delicate, thin, and jienneable epider- 

 mis, covering a series of cellular bodies, which 

 contain a fluid denser than water. Turgidity 

 must therefore take place in them, when water 

 is supplied in sufficient quantity, and as there is 

 no other outlet by which the turgidity can be i-e- 

 Icased, under the elastic reaction of the walls of 

 the spongioles, the absorbed fluid ascends. We 

 have mentioned that endosmose ceases when the 

 permeable membrane, through which it is exer- 

 cised, loses its regularly organised texture by 

 decay. Accordingly the sap cannot be made to 



rise at all, if the roots arc allowed to dry so far 

 as to undergo decomposition. The action also is 

 comparati vely feeble when the temperature is low ; 

 hence when the sap is rising in the spring it is 

 found to attain different elevations, according to 

 the degree of heat. The stagnation of the flow 

 of sap in autumn and winter, and of its return 

 in spring, are all referable, also, to the state of 

 the temperature, and to the condition of the 

 spongioles at those periods. For towards the 

 close of autumn, the epidermis of the rootlets 

 becomes coarse and impermeable, and it is not 

 till new spongioles with a tender skin are formed 

 in spring, that absorption can again take place. The 

 leaves also are of such a structure, as to possess 

 the power of exciting endosmose; hence they have 

 a constant tendency to absorb moisture whenever 

 it is presented to them of less density than the 

 juices contained in their cells, and consequently 

 a power of suction must be exerted upon the sap 

 ascending in the lymphatic vessels. This ac- 

 tion has been termed the afflux, and seems inde- 

 pendent of any action of the vessels of the stem, 

 but is due solely to the endosmose of the mem- 

 brane of the leaves. Thus if a twig of any 

 plant is immereed in water acidulated with 

 sulphuric acid, this fluid will be continued to 

 be drawn up, although every part of the vege- 

 table with which it comes in contact is immed- 

 iately killed, so that the cellules and tubes may 

 be considered as so many inorganic capillary 

 passages. Endosmose may also be a prime agent 

 in the ordinary transpiration of matter by the 

 leaves. For as has already been stated, wherever 

 endosmose takes place, there is also the recipro- 

 cal action of exosmose. The discharge of fluid 

 by the leaves cannot be accounted for by the 

 laws of evaporation alone. For the extent of 

 the discharge is under the influence of agents 

 which cannot affect evaporation, such as diffiise 

 light, and it appears to take place to some extent 

 even under water. Nor is this power generated in 

 the leaves, all expanded in the function of expira- 

 tion ; part of it seems to be spent in effecting the 

 passage of the elaborated sap downwards through 

 the ample tubes ; at least, this fluid does not des- 

 cend from the mere influence of gravity, for 

 when a branch having a ring of bark removed 

 from it is bent downwards, the bark is removed 

 as usual only at the edge of the incision next 

 the leaves, although the elaborated sap must in 

 this position ascend to form it. 



Transpiration in the leaves. We have just 

 seen by what force and by what organs the sap 

 is raised fi'om the roots to the extremities of all 

 the branfches of the plant. Here other phenom- 

 ena are produced, and a new circulation com- 

 mences. When the sap has arrived at the ex- 

 tremities of the branches, it spreads out into 

 their leaves, where it loses part of the principles 

 which it contained, and acquires new ones. 



