1839] 



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more rapidly than soma species of llingiis, nnd yel 

 these are cellular plants, without one sin<fl8 tube 

 in their whole slructurc. In tliese iiistance>', ca- 

 pillary altraciion is manifestly out of the quesiion. 

 Another objection is, that al'iertlic death oi'a plant, 

 alihoutrh the tubular vessels are ail [jeriect, as is 

 manilested by the rise of water in a piece of dry 

 wood which has one end placed in a cup of water, 

 the circulation of the sap entirely ceases. For 

 these reasons we must (five up the idea thai ca- 

 pillary attraction is the cause of the rise of the 

 sap, or if we admit thtit it has any part in produ- 

 cing this elfect, that part must be a very incon- 

 siderable one. 



There are objections equally insuperable to the 

 second part of this theory, viz. that the sap re- 

 turns by gravii}-. One ol' these objections is, that 

 in returning towards the root, the sap has very 

 frequently to move ufiwards, iii direct oppnsi'ion 

 to gravit\', and not downwards, in conformity with 

 ill at force. This is manifestly the case in twigs 

 which put out from the lower side ot" limbs, and, 

 for at least a part of their course, grow downward. 

 On almost every tree, a greater or less number ol 

 such twigs may be found. The tvvio-s of the weep- 

 ing willow, tiiose parts oi'the tree which are grow- 

 ing most rapidly, and in which the sap is circula- \ 

 ling most abundantly, always bend downward.! 

 Another objection is, that we can assicrn no reason 

 why, when two tubular vessels precisely similar 

 in their struclitre, and placed in similar circum- 

 stances, the sap should ascend in one by cafiillary 

 attraction, and descend by gravity in theotiier; 

 and if we attempt to accomil for any phenomenon 

 upon mechanical principles, we must assume 

 nothing f^r which we can not assign a reason. 

 Besides, the ficl that a whole class of plants are 

 entirely destitute of continuous tubular vessels, is 

 as fatal to the idea that tiie sap descends by gravity, 

 as that it ascends bycajjiHary attraction. I shoidd 

 not have thousjht it worth the while to have been 

 thus particular in stating the objections to this 

 theory, had [ not known that it has been lor a 

 lonij time the commonly received theory, and one 

 wiih which most vvh.o have ever spent much 

 thotiyht on the subject have become acquainted 

 with in some lorm or another. 



2nd. Eadofiniosis. Perhaps tiie simplrst dcfi- 

 nilion which can be^ivon of liiis term is, that en- 

 dosmosi^^ is that inlTllralion of one liuid info an- 

 other, which takes [ilace ;hroui?h bnrlies possessf'd 

 of no visible pores. Thin animal and veixetable 

 nienibranes, which in ordinary circumstances are 

 impermeablo, lo fiuids, when made to separate 

 lliiids of difrcrent densities, are traversed by tiiose 

 huids with greater or less rapiditv. An e\'am])le 

 ortu'owill make ihr^ matter plainer. '-Tal;", a 

 conunon half-ounce vial, the boitnni of which has 

 been irroimd off, and tie a piece of bladder tiirhiiy 

 over the opening thus elieded. Fill u with a so- 

 lution of iTuiii arabic in water, and place it so thai 

 its lower end is beneath the surlace of some v%"ater 

 in a glass. The denser fluid will slowly attract 

 the Wafer ihroLigh the membrane, and the. con- 

 tents of the vial will run over. Reverse the ex- 

 periment by (iiliuir such a vial with water, and 

 plunije the lower end into a solution of gum, and 

 'he water will pass nut of the viai, through the 

 bladder, as will be manilt?sied by the siiddng of 

 the water in the vial, and the risinir "f 'hat in the 

 glasjj."' In l>oth instances, the r.u'cr will tiass 



into the denser fluid by endosmosis. Again, if 

 we blow a soap bubble with atmospheric air, and 

 then place it in ajar of amnioniacal gas, the bub- 

 ble will be seen at once lo enlarge, and display a 

 beautilid play of colors upon its surface; and on 

 examinino' the gas which it contains, we will 

 find it a mixture of almosphericair and ammonia. 

 In this instance the ammonia must liave [)assed 

 through the sides of the soap bubble, although 

 the film which Jbrmed the bubble is in ordinary 

 circumstances impervious lo gases. It passed 

 by the process termed endosmosis. 



Many experiments have been performed to prove 

 that the cellular tissue of plants is capable of act- 

 ing by endosmosis. Of these the follovvitig one 

 ol Dutrochet may be taken as an example. One 

 of the valves of the capsule ol' the balsam was 

 placed in water ; the effect was, that it curved 

 rapiiily inwaids and soon rolled itself up; it was 

 afterwaids placed in thick gum water, and it then 

 curved outwardly, in a direction just the opposite 

 of that which it had at first taken. In order to 

 understand this, we must bear in mind the fact 

 that the outside of a valve of the balsam is com- 

 posed of larger and thinner cellules than the inside, 

 and of course endosmosis would go on more rap- 

 idly througli the sides of the outer cellules, than 

 through those of the inner. When the valve 

 was placed in water, the sap which the cellules of 

 that valve contained, being the denser liquid, the 

 water passed into the cellules ; and inconsequence 

 of the nature of the cellules, it passed more rapiiily 

 on the outside than on the inside of ttie valve. 

 The necessary effect of this was, to distend the 

 cellules on the outside more rapidly than those on 

 the inside, and thus to curve the valve inwards. 

 When the valve was placed in a thick solution of 

 gum, the solution was the denser fluid, and the 

 effect of endosmosis was to draw the sap out from 

 the cellules of the plant. This efl:ect from the 

 nature of the cellules would take place more rapidly 

 on the outside than on the inside of the valve. In 

 this case the inner cellules would soon be the most 

 distended, and the valve would of course curve 

 outwards. This experiment Vv-as frequently re- 

 peiited and always with similar results. Ey such 

 experiments, the fact is established beyond a doubt, 

 that the vegetable tissues when filled with sap are 

 capable ot" endosmomelric action. 



In order to get a clear idea of the part which 

 endosmosis is supposed to act in the circulation 

 of the sap, let us suppose that there is a line o 

 cellules, stretching from the root to the leaf of a 

 plant, each cellule in contact with the ones imme- 

 diately above and below it, and all filled with a 

 fluid of uniibrm density. The uppermost cellule 

 of \b.i'. series heiiiirm contact u'ith tiie atmosphere, 

 and exposed to the action of ihe sun and nf the 

 air, the sap which it contains will part with a por- 

 tion of its water by evaporation, and will thus be 

 rendered more den^e than that of the one below it. 

 !n accordance with the principle already stated, 

 endosmosis immediately coinnipnces, tending to 

 render the sap contained in the two cellules of 

 equal density, in part bv the passage of the denser 

 fluid downwards, and" in part by the passage of 

 the rarer fluid upwards. In this way the fluid 

 which the second cellule of the series contains 

 will he rendered more tiense than that of the one 

 immediately be'ow it, and endosmosis commences 

 , between these loo, tending to render ilieir fluida 



