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Show 102 VEGETABLE RESPIRATION. NOTE XXXVII. ower of thefe abforbents of the roots of it. Veget. Stat. P· r_o2. St~ch a power c:m Pno t be produced by capt' II ary atti.a cnu·o n, as that co.u ld only ratfe a flutd nearly to the u er edge of tl:te attracting cylinder, but not enable tt to flow ove_r that edge, and much lcpfsp ton'{ ie 21 cte et a b ove 1·t • What then can this p.o w. er be owt.n g t. o? Douhb tl·e fsfl to t I1 C Il.V ·I ng acnL !·V l· t Y 0 fthe abrIIo rbent veffels I and to th• eu m' creafed vtvaetty. from t e 1. 11 u-ence of the warmth of the fpring fucceeding the wmter s cold, and t~etr :hence ~~eater fufceptibility to irritation from the juices w~ich they abforb, refemblmg m all crrcum! l:anccs the action of the living veffels of anunals. ----- NoTE XXXVII.-VEGETABLE RESPIRATION. Jf/hile .JPr~ad isz air the leaves l'cfpiring play. CANTO IV. 1. 4'2 I. I. THERE have been various opinions concerning the ufe of the l_eaves of plants in the vegetable ceconomy. Some have contended that they are perrp1ratory organs; this does not feem probable from an experiment of Dr. Hales, Veg .. Stat. P· 3°· He found by cutting ofF branches of trees with apples on them, and tak1ng ofF tl~c leaves, that an apple exhaled about as much as two leaves, the furfaccs of whtch _were nearly equal to the apple; whence it would appear that ap_rles have as good a cla1m to be termed perfpiratory organs as leaves. Others have believed them excretory orgm~s of excrementitious juices; but as the vapour exhaled from vegetables has no tafl:e, th1s idea is no more probable than the other; add to this that in moiil: weather, they do not appear to perfpire or exhale at all. . . . The internal furface of the lungs or air-velfcls tn men, IS fa1d to be equal to the external furface of the whole body, or about fifteen fquare feet; on this furface the blood is expofed to the influence of the rerpired air through the medium however of a thin pellicle; by this expofure to the air it has its colour changed f~om deep red to bright fcarlet, and acquires fomething fo necelfary to the exifl:ence of l1f-:::, that we can live fcarcely a minute without this wonderful procefs. The analogy between the leaves of plants and the lungs or gills of animals feems to embrace fo many circumil:ances, that we can fcarcely wirhhold our aLfent to their performing fimilar offices. 1. The great furface of the leaves compared to that of the trunk and branches of trees is fuch, that it would feem to be an organ well adapted for the pmpofe of expofing the vegetable juices to the in.fluence of the air; this however we 01all fee afterwards is probably performed only by their upper furfaces, yet even in this cafe th<! furface of the leaves in general bear a greater proportion to the furface of the tree, than the lungs of animals to their external furfaces. NoTE XXXVII. VEGETABLE RESPIRATION. 10J 2. In the lungs of animals the blood af h . trcmities. of the pttlm onary arte' ry, 1· s c ha ngrdte _r av,ng been expofcd to the air in the ex- 10 c 1 f d certainly in fome of · t n· · 1 . . . ? our rom eep red to bright fcarlet, and 1 S enentta p!OpertleS j It IS then coJJ ct d b h . and returned to the heart. To lhew a IJmilar· . e e ~ t e pulmonary vem the foil · · lty of Clrcumflances m the leaves of plants OWing expenmcnt was made, June 24 I78I An lk . hI ve fi \ l · f I · f (E · ' - · Ha Wit eaves and fecd- t; so a! ge purge 'nphorb!a heliofc~)pi:t) h:tc.l been fcveral d I d . I ct. of madder (Rubia tinctorum) fo that the lower part ofth 11 ayspdace 111 a teco lOll mol1 leaves were immerfed in it. Afrer .havincr watbc~ the~, an ~wlolof the_ nnder-wa te r, I cou ld rea d'1! y dt·f cern the colour of the mba ddc rr e Jmmenec caves In clear 1 . . eac h Ie a f . T Ill·S red artery was beautifully vifible bot!r pau11n g a odn g the m1dd!e nb of ' o f t I1 e leaf; but on the upper fide many red branches 1w one ret Je un er and upper furface r . · f · extremi•t i•e s of the leaf, wh1. ch on the other fide were n t 'f1i~be[n gowg rom 1t to .t he t h roug I1 . . . 0 VI ' e except by look1ng It agaml1 the l1ght. On this under IJde a fyf!em f b 1 · rr . a pale mi· lky flui. d were feen coming from the extremiti0e s orfa nthc 11ng veuels carryu1g J f d . h . ..1.. 1 d r.d · · . . . e ea , an covermg t e \v 110 e un eru e of 1t, and JOintng Into two large veins one 0 1 r:d f h . . . - ' n eac 1 Jl e o t e red artery In the m1ddle nb of the leaf, and along with it delice d' h c n lk petw. le. On fh.t tm. g one of thefe leaves with fcilfars and h n· wg to t e wotna · or . . , avwg a common magnifymg lens ready, t~c m1 lky _blood _was feen oozing out of the returning veins on each wde of the red artery m the mtddle nb, but none of the red fluid from the artery. All thefe ap_pear~nce~ were more eafJly feen in a leaf of Picris treated in the fame manner; for rn th1s milky plant the 11ems anll migdle rib of the leaves are fometimes ~aturall y col_oure~ redditb, ~nd hence the colour of the madder feemed to pafs further Into t~c ram1ficatwns of then leaf-arteries, and was there beautifully vi!lblc with the returnmg branches of milky veins on each fide. 3· From thefc experiments the upper furface of the leaf appeared to be the immediate organ of refpiration, becaufe the coloured fluid was carried to the extremities of the leaf by :eff~ls mofi confpicuous on the upper furface, and there changed into a milky fluid,. wh1ch 1s the blood of the plant, and then returned by concomitant veins on the under furface, which were feen to ooze when divided with fcilfars, and which, in Picris particular] y, render the under furface of the leaves greatly whiter than the upper one. 4· As the upper furface of leaves con11itutes the organ of refpiration, on which the fap is expofed in the terminations of arteries beneath a thin pellicle to the aetion of the atmofphere, thefe furfaces in many plants 11rongly repel moi!turc, as cabbage-leaves, whence the particles of rain lying over their furfac<'s without touching them, as obfervcd by Mr. Melville (Effays Literary and Philofoph. Edinburgh) have the appearance of globules of quickfJ!ver. And hence leaves laid with the upper furfaces on water, wither as foon as in the dry air, but continue green many days, if placed with the under li1rfaces on water, as appears in the experiments of Monf. Bonnet (Ufagedes Fcuilles.J Hence fome aquatic plants, as the water-lily (N ymphrea) have the lower fides of their leaves floating on the water, while the upper furfaces remain dry in the air. S· As thofe infects, which have many fpiracula, or breathing apertures, as wafps .and flies, are immediately fuffocated by pouring oil upon them, I carefully covered with. |