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¡¡be¡¡composed¡¡of¡¡a¡¡metal¡¡oxygen¡¡combined¡¡with¡¡the¡¡alleged¡¡element¡¡heat£»¡¡Dr¡£¡¡Priestley¡¡thought¡¡it¡¡a¡¡compound¡¡of¡¡positive¡¡electricity¡¡and¡¡phlogiston£»¡¡and¡¡Humphry¡¡Davy£»¡¡when¡¡he¡¡entered¡¡the¡¡lists¡¡a¡¡little¡¡later£»¡¡supposed¡¡it¡¡to¡¡be¡¡a¡¡compound¡¡of¡¡oxygen¡¡and¡¡light¡£¡¡Such¡¡mistaken¡¡notions¡¡naturally¡¡complicated¡¡matters¡¡and¡¡delayed¡¡a¡¡complete¡¡understanding¡¡of¡¡the¡¡chemical¡¡processes¡¡of¡¡respiration¡£¡¡It¡¡was¡¡some¡¡time£»¡¡too£»¡¡before¡¡the¡¡idea¡¡gained¡¡acceptance¡¡that¡¡the¡¡most¡¡important¡¡chemical¡¡changes¡¡do¡¡not¡¡occur¡¡in¡¡the¡¡lungs¡¡themselves£»¡¡but¡¡in¡¡the¡¡ultimate¡¡tissues¡£¡¡¡¡Indeed£»¡¡the¡¡matter¡¡was¡¡not¡¡clearly¡¡settled¡¡at¡¡the¡¡close¡¡of¡¡the¡¡century¡£¡¡¡¡Nevertheless£»¡¡the¡¡problem¡¡of¡¡respiration¡¡had¡¡been¡¡solved¡¡in¡¡its¡¡essentials¡£¡¡¡¡Moreover£»¡¡the¡¡vastly¡¡important¡¡fact¡¡had¡¡been¡¡established¡¡that¡¡a¡¡process¡¡essentially¡¡identical¡¡with¡¡respiration¡¡is¡¡necessary¡¡to¡¡the¡¡existence¡¡not¡¡only¡¡of¡¡all¡¡creatures¡¡supplied¡¡with¡¡lungs£»¡¡but¡¡to¡¡fishes£»¡¡insects£»¡¡and¡¡even¡¡vegetablesin¡¡short£»¡¡to¡¡every¡¡kind¡¡of¡¡living¡¡organism¡£


ERASMUS¡¡DARWIN¡¡AND¡¡VEGETABLE¡¡PHYSIOLOGY

Some¡¡interesting¡¡experiments¡¡regarding¡¡vegetable¡¡respiration¡¡were¡¡made¡¡just¡¡at¡¡the¡¡close¡¡of¡¡the¡¡century¡¡by¡¡Erasmus¡¡Darwin£»¡¡and¡¡recorded¡¡in¡¡his¡¡Botanic¡¡Garden¡¡as¡¡a¡¡foot¡­note¡¡to¡¡the¡¡verse£º

¡¨While¡¡spread¡¡in¡¡air¡¡the¡¡leaves¡¡respiring¡¡play¡£¡¨


These¡¡notes¡¡are¡¡worth¡¡quoting¡¡at¡¡some¡¡length£»¡¡as¡¡they¡¡give¡¡a¡¡clear¡¡idea¡¡of¡¡the¡¡physiological¡¡doctrines¡¡of¡¡the¡¡time¡¡£¨1799£©£»¡¡while¡¡taking¡¡advance¡¡ground¡¡as¡¡to¡¡the¡¡specific¡¡matter¡¡in¡¡question£º


¡¨There¡¡have¡¡been¡¡various¡¡opinions£»¡¨¡¡Darwin¡¡says£»¡¡¡¨concerning¡¡the¡¡use¡¡of¡¡the¡¡leaves¡¡of¡¡plants¡¡in¡¡the¡¡vegetable¡¡economy¡£¡¡¡¡Some¡¡have¡¡contended¡¡that¡¡they¡¡are¡¡perspiratory¡¡organs¡£¡¡¡¡This¡¡does¡¡not¡¡seem¡¡probable¡¡from¡¡an¡¡experiment¡¡of¡¡Dr¡£¡¡Hales£»¡¡Vegetable¡¡Statics£»¡¡p¡£¡¡¡¡30¡£¡¡¡¡He£»¡¡found£»¡¡by¡¡cutting¡¡off¡¡branches¡¡of¡¡trees¡¡with¡¡apples¡¡on¡¡them¡¡and¡¡taking¡¡off¡¡the¡¡leaves£»¡¡that¡¡an¡¡apple¡¡exhaled¡¡about¡¡as¡¡much¡¡as¡¡two¡¡leaves¡¡the¡¡surfaces¡¡of¡¡which¡¡were¡¡nearly¡¡equal¡¡to¡¡the¡¡apple£»¡¡whence¡¡it¡¡would¡¡appear¡¡that¡¡apples¡¡have¡¡as¡¡good¡¡a¡¡claim¡¡to¡¡be¡¡termed¡¡perspiratory¡¡organs¡¡as¡¡leaves¡£¡¡Others¡¡have¡¡believed¡¡them¡¡excretory¡¡organs¡¡of¡¡excrementitious¡¡juices£»¡¡but¡¡as¡¡the¡¡vapor¡¡exhaled¡¡from¡¡vegetables¡¡has¡¡no¡¡taste£»¡¡this¡¡idea¡¡is¡¡no¡¡more¡¡probable¡¡than¡¡the¡¡other£»¡¡add¡¡to¡¡this¡¡that¡¡in¡¡most¡¡weathers¡¡they¡¡do¡¡not¡¡appear¡¡to¡¡perspire¡¡or¡¡exhale¡¡at¡¡all¡£

¡¨The¡¡internal¡¡surface¡¡of¡¡the¡¡lungs¡¡or¡¡air¡­vessels¡¡in¡¡men¡¡is¡¡said¡¡to¡¡be¡¡equal¡¡to¡¡the¡¡external¡¡surface¡¡of¡¡the¡¡whole¡¡body£»¡¡or¡¡almost¡¡fifteen¡¡square¡¡feet£»¡¡on¡¡this¡¡surface¡¡the¡¡blood¡¡is¡¡exposed¡¡to¡¡the¡¡influence¡¡of¡¡the¡¡respired¡¡air¡¡through¡¡the¡¡medium£»¡¡however£»¡¡of¡¡a¡¡thin¡¡pellicle£»¡¡by¡¡this¡¡exposure¡¡to¡¡the¡¡air¡¡it¡¡has¡¡its¡¡color¡¡changed¡¡from¡¡deep¡¡red¡¡to¡¡bright¡¡scarlet£»¡¡and¡¡acquires¡¡something¡¡so¡¡necessary¡¡to¡¡the¡¡existence¡¡of¡¡life¡¡that¡¡we¡¡can¡¡live¡¡scarcely¡¡a¡¡minute¡¡without¡¡this¡¡wonderful¡¡process¡£

¡¨The¡¡analogy¡¡between¡¡the¡¡leaves¡¡of¡¡plants¡¡and¡¡the¡¡lungs¡¡or¡¡gills¡¡of¡¡animals¡¡seems¡¡to¡¡embrace¡¡so¡¡many¡¡circumstances¡¡that¡¡we¡¡can¡¡scarcely¡¡withhold¡¡our¡¡consent¡¡to¡¡their¡¡performing¡¡similar¡¡offices¡£

¡¨1¡£¡¡¡¡The¡¡great¡¡surface¡¡of¡¡leaves¡¡compared¡¡to¡¡that¡¡of¡¡the¡¡trunk¡¡and¡¡branches¡¡of¡¡trees¡¡is¡¡such¡¡that¡¡it¡¡would¡¡seem¡¡to¡¡be¡¡an¡¡organ¡¡well¡¡adapted¡¡for¡¡the¡¡purpose¡¡of¡¡exposing¡¡the¡¡vegetable¡¡juices¡¡to¡¡the¡¡influence¡¡of¡¡the¡¡air£»¡¡this£»¡¡however£»¡¡we¡¡shall¡¡see¡¡afterwards¡¡is¡¡probably¡¡performed¡¡only¡¡by¡¡their¡¡upper¡¡surfaces£»¡¡yet¡¡even¡¡in¡¡this¡¡case¡¡the¡¡surface¡¡of¡¡the¡¡leaves¡¡in¡¡general¡¡bear¡¡a¡¡greater¡¡proportion¡¡to¡¡the¡¡surface¡¡of¡¡the¡¡tree¡¡than¡¡the¡¡lungs¡¡of¡¡animals¡¡to¡¡their¡¡external¡¡surfaces¡£

¡¨2¡£¡¡¡¡In¡¡the¡¡lung¡¡of¡¡animals¡¡the¡¡blood£»¡¡after¡¡having¡¡been¡¡exposed¡¡to¡¡the¡¡air¡¡in¡¡the¡¡extremities¡¡of¡¡the¡¡pulmonary¡¡artery£»¡¡is¡¡changed¡¡in¡¡color¡¡from¡¡deep¡¡red¡¡to¡¡bright¡¡scarlet£»¡¡and¡¡certainly¡¡in¡¡some¡¡of¡¡its¡¡essential¡¡properties¡¡it¡¡is¡¡then¡¡collected¡¡by¡¡the¡¡pulmonary¡¡vein¡¡and¡¡returned¡¡to¡¡the¡¡heart¡£¡¡To¡¡show¡¡a¡¡similarity¡¡of¡¡circumstances¡¡in¡¡the¡¡leaves¡¡of¡¡plants£»¡¡the¡¡following¡¡experiment¡¡was¡¡made£»¡¡June¡¡24£»¡¡1781¡£¡¡¡¡A¡¡stalk¡¡with¡¡leaves¡¡and¡¡seed¡­vessels¡¡of¡¡large¡¡spurge¡¡£¨Euphorbia¡¡helioscopia£©¡¡had¡¡been¡¡several¡¡days¡¡placed¡¡in¡¡a¡¡decoction¡¡of¡¡madder¡¡£¨Rubia¡¡tinctorum£©¡¡so¡¡that¡¡the¡¡lower¡¡part¡¡of¡¡the¡¡stem¡¡and¡¡two¡¡of¡¡the¡¡undermost¡¡leaves¡¡were¡¡immersed¡¡in¡¡it¡£¡¡After¡¡having¡¡washed¡¡the¡¡immersed¡¡leaves¡¡in¡¡clear¡¡water¡¡I¡¡could¡¡readily¡¡discover¡¡the¡¡color¡¡of¡¡the¡¡madder¡¡passing¡¡along¡¡the¡¡middle¡¡rib¡¡of¡¡each¡¡leaf¡£¡¡¡¡The¡¡red¡¡artery¡¡was¡¡beautifully¡¡visible¡¡on¡¡the¡¡under¡¡and¡¡on¡¡the¡¡upper¡¡surface¡¡of¡¡the¡¡leaf£»¡¡but¡¡on¡¡the¡¡upper¡¡side¡¡many¡¡red¡¡branches¡¡were¡¡seen¡¡going¡¡from¡¡it¡¡to¡¡the¡¡extremities¡¡of¡¡the¡¡leaf£»¡¡which¡¡on¡¡the¡¡other¡¡side¡¡were¡¡not¡¡visible¡¡except¡¡by¡¡looking¡¡through¡¡it¡¡against¡¡the¡¡light¡£¡¡On¡¡this¡¡under¡¡side¡¡a¡¡system¡¡of¡¡branching¡¡vessels¡¡carrying¡¡a¡¡pale¡¡milky¡¡fluid¡¡were¡¡seen¡¡coming¡¡from¡¡the¡¡extremities¡¡of¡¡the¡¡leaf£»¡¡and¡¡covering¡¡the¡¡whole¡¡under¡¡side¡¡of¡¡it£»¡¡and¡¡joining¡¡two¡¡large¡¡veins£»¡¡one¡¡on¡¡each¡¡side¡¡of¡¡the¡¡red¡¡artery¡¡in¡¡the¡¡middle¡¡rib¡¡of¡¡the¡¡leaf£»¡¡and¡¡along¡¡with¡¡it¡¡descending¡¡to¡¡the¡¡foot¡­stalk¡¡or¡¡petiole¡£¡¡On¡¡slitting¡¡one¡¡of¡¡these¡¡leaves¡¡with¡¡scissors£»¡¡and¡¡having¡¡a¡¡magnifying¡­glass¡¡ready£»¡¡the¡¡milky¡¡blood¡¡was¡¡seen¡¡oozing¡¡out¡¡of¡¡the¡¡returning¡¡veins¡¡on¡¡each¡¡side¡¡of¡¡the¡¡red¡¡artery¡¡in¡¡the¡¡middle¡¡rib£»¡¡but¡¡none¡¡of¡¡the¡¡red¡¡fluid¡¡from¡¡the¡¡artery¡£

¡¨All¡¡these¡¡appearances¡¡were¡¡more¡¡easily¡¡seen¡¡in¡¡a¡¡leaf¡¡of¡¡Picris¡¡treated¡¡in¡¡the¡¡same¡¡manner£»¡¡for¡¡in¡¡this¡¡milky¡¡plant¡¡the¡¡stems¡¡and¡¡middle¡¡rib¡¡of¡¡the¡¡leaves¡¡are¡¡sometimes¡¡naturally¡¡colored¡¡reddish£»¡¡and¡¡hence¡¡the¡¡color¡¡of¡¡the¡¡madder¡¡seemed¡¡to¡¡pass¡¡farther¡¡into¡¡the¡¡ramifications¡¡of¡¡their¡¡leaf¡­arteries£»¡¡and¡¡was¡¡there¡¡beautifully¡¡visible¡¡with¡¡the¡¡returning¡¡branches¡¡of¡¡milky¡¡veins¡¡on¡¡each¡¡side¡£¡¨


Darwin¡¡now¡¡goes¡¡on¡¡to¡¡draw¡¡an¡¡incorrect¡¡inference¡¡from¡¡his¡¡observations£º


¡¨3¡£¡¡¡¡From¡¡these¡¡experiments£»¡¨¡¡he¡¡says£»¡¡¡¨the¡¡upper¡¡surface¡¡of¡¡the¡¡leaf¡¡appeared¡¡to¡¡be¡¡the¡¡immediate¡¡organ¡¡of¡¡respiration£»¡¡because¡¡the¡¡colored¡¡fluid¡¡was¡¡carried¡¡to¡¡the¡¡extremities¡¡of¡¡the¡¡leaf¡¡by¡¡vessels¡¡most¡¡conspicuous¡¡on¡¡the¡¡upper¡¡surface£»¡¡and¡¡there¡¡changed¡¡into¡¡a¡¡milky¡¡fluid£»¡¡which¡¡is¡¡the¡¡blood¡¡of¡¡the¡¡plant£»¡¡and¡¡then¡¡returned¡¡by¡¡concomitant¡¡veins¡¡on¡¡the¡¡under¡¡surface£»¡¡which¡¡were¡¡seen¡¡to¡¡ooze¡¡when¡¡divided¡¡with¡¡scissors£»¡¡and¡¡which£»¡¡in¡¡Picris£»¡¡particularly£»¡¡render¡¡the¡¡under¡¡surface¡¡of¡¡the¡¡leaves¡¡greatly¡¡whiter¡¡than¡¡the¡¡upper¡¡one¡£¡¨


But¡¡in¡¡point¡¡of¡¡fact£»¡¡as¡¡studies¡¡of¡¡a¡¡later¡¡generation¡¡were¡¡to¡¡show£»¡¡it¡¡is¡¡the¡¡under¡¡surface¡¡of¡¡the¡¡leaf¡¡that¡¡is¡¡most¡¡abundantly¡¡provided¡¡with¡¡stomata£»¡¡or¡¡¡¨breathing¡­pores¡£¡¨¡¡From¡¡the¡¡stand¡­point¡¡of¡¡this¡¡later¡¡knowledge£»¡¡it¡¡is¡¡of¡¡interest¡¡to¡¡follow¡¡our¡¡author¡¡a¡¡little¡¡farther£»¡¡to¡¡illustrate¡¡yet¡¡more¡¡fully¡¡the¡¡possibility¡¡of¡¡combining¡¡correct¡¡observations¡¡with¡¡a¡¡faulty¡¡inference¡£


¡¨4¡£¡¡¡¡As¡¡the¡¡upper¡¡surface¡¡of¡¡leaves¡¡constitutes¡¡the¡¡organ¡¡of¡¡respiration£»¡¡on¡¡which¡¡the¡¡sap¡¡is¡¡exposed¡¡in¡¡the¡¡termination¡¡of¡¡arteries¡¡beneath¡¡a¡¡thin¡¡pellicle¡¡to¡¡the¡¡action¡¡of¡¡the¡¡atmosphere£»¡¡these¡¡surfaces¡¡in¡¡many¡¡plants¡¡strongly¡¡repel¡¡moisture£»¡¡as¡¡cabbage¡¡leaves£»¡¡whence¡¡the¡¡particles¡¡of¡¡rain¡¡lying¡¡over¡¡their¡¡surfaces¡¡without¡¡touching¡¡them£»¡¡as¡¡observed¡¡by¡¡Mr¡£¡¡Melville¡¡£¨Essays¡¡Literary¡¡and¡¡Philosophical£º¡¡¡¡Edinburgh£©£»¡¡have¡¡the¡¡appearance¡¡of¡¡globules¡¡of¡¡quicksilver¡£¡¡¡¡And¡¡hence¡¡leaves¡¡with¡¡the¡¡upper¡¡surfaces¡¡on¡¡water¡¡wither¡¡as¡¡soon¡¡as¡¡in¡¡the¡¡dry¡¡air£»¡¡but¡¡continue¡¡green¡¡for¡¡many¡¡days¡¡if¡¡placed¡¡with¡¡the¡¡under¡¡surface¡¡on¡¡water£»¡¡as¡¡appears¡¡in¡¡the¡¡experiments¡¡of¡¡Monsieur¡¡Bonnet¡¡£¨Usage¡¡des¡¡Feuilles£©¡£¡¡Hence¡¡some¡¡aquatic¡¡plants£»¡¡as¡¡the¡¡water¡­lily¡¡£¨Nymphoea£©£»¡¡have¡¡the¡¡lower¡¡sides¡¡floating¡¡on¡¡the¡¡water£»¡¡while¡¡the¡¡upper¡¡surfaces¡¡remain¡¡dry¡¡in¡¡the¡¡air¡£

¡¨5¡£¡¡¡¡As¡¡those¡¡insects¡¡which¡¡have¡¡many¡¡spiracula£»¡¡or¡¡breathing¡¡apertures£»¡¡as¡¡wasps¡¡and¡¡flies£»¡¡are¡¡immediately¡¡suffocated¡¡by¡¡pouring¡¡oil¡¡upon¡¡them£»¡¡I¡¡carefully¡¡covered¡¡with¡¡oil¡¡the¡¡surfaces¡¡of¡¡several¡¡leaves¡¡of¡¡phlomis£»¡¡of¡¡Portugal¡¡laurel£»¡¡and¡¡balsams£»¡¡and¡¡though¡¡it¡¡would¡¡not¡¡regularly¡¡adhere£»¡¡I¡¡found¡¡them¡¡all¡¡die¡¡in¡¡a¡¡day¡¡or¡¡two¡£

¡¨It¡¡must¡¡be¡¡added¡¡that¡¡many¡¡leaves¡¡are¡¡furnished¡¡with¡¡muscles¡¡about¡¡their¡¡foot¡­stalks£»¡¡to¡¡turn¡¡their¡¡surfaces¡¡to¡¡the¡¡air¡¡or¡¡light£»¡¡as¡¡mimosa¡¡or¡¡Hedysarum¡¡gyrans¡£¡¡¡¡From¡¡all¡¡these¡¡analogies¡¡I¡¡think¡¡there¡¡can¡¡be¡¡no¡¡doubt¡¡but¡¡that¡¡leaves¡¡of¡¡trees¡¡are¡¡their¡¡lungs£»¡¡giving¡¡out¡¡a¡¡phlogistic¡¡material¡¡to¡¡the¡¡atmosphere£»¡¡and¡¡absorbing¡¡oxygen£»¡¡or¡¡vital¡¡air¡£

¡¨6¡£¡¡¡¡The¡¡great¡¡use¡¡of¡¡light¡¡to¡¡vegetation¡¡would¡¡appear¡¡from¡¡this¡¡theory¡¡to¡¡be¡¡by¡¡disengaging¡¡vital¡¡air¡¡from¡¡the¡¡water¡¡which¡¡they¡¡perspire£»¡¡and¡¡thence¡¡to¡¡facilitate¡¡its¡¡union¡¡with¡¡their¡¡blood¡¡exposed¡¡beneath¡¡the¡¡thin¡¡surface¡¡of¡¡their¡¡leaves£»¡¡since¡¡when¡¡pure¡¡air¡¡is¡¡thus¡¡applied¡¡it¡¡is¡¡probable¡¡that¡¡it¡¡can¡¡be¡¡more¡¡readily¡¡absorbed¡£¡¡¡¡Hence£»¡¡in¡¡the¡¡curious¡¡experiments¡¡of¡¡Dr¡£¡¡Priestley¡¡and¡¡Mr¡£¡¡Ingenhouz£»¡¡some¡¡plants¡¡purified¡¡less¡¡air¡¡than¡¡othersthat¡¡is£»¡¡they¡¡perspired¡¡less¡¡in¡¡the¡¡sunshine£»¡¡and¡¡Mr¡£¡¡Scheele¡¡found¡¡that¡¡by¡¡putting¡¡peas¡¡into¡¡water¡¡which¡¡about¡¡half¡¡covered¡¡them¡¡they¡¡converted¡¡the¡¡vital¡¡air¡¡into¡¡fixed¡¡air£»¡¡or¡¡carbonic¡­acid¡¡gas£»¡¡in¡¡the¡¡same¡¡manner¡¡as¡¡in¡¡animal¡¡respiration¡£

¡¨7¡£¡¡¡¡The¡¡circulation¡¡in¡¡the¡¡lungs¡¡or¡¡leaves¡¡of¡¡plants¡¡is¡¡very¡¡similar¡¡to¡¡that¡¡of¡¡fish¡£¡¡¡¡In¡¡fish¡¡the¡¡blood£»¡¡after¡¡having¡¡passed¡¡through¡¡their¡¡gills£»¡¡does¡¡not¡¡return¡¡to¡¡the¡¡heart¡¡as¡¡from¡¡the¡¡lungs¡¡of¡¡air¡­breathing¡¡animals£»¡¡but¡¡the¡¡pulmonary¡¡vein¡¡taking¡¡the¡¡structure¡¡of¡¡an¡¡artery¡¡after¡¡having¡¡received¡¡the¡¡blood¡¡from¡¡the¡¡gills£»¡¡which¡¡there¡¡gains¡¡a¡¡more¡¡florid¡¡color£»¡¡distributes¡¡it¡¡to¡¡the¡¡other¡¡parts¡¡of¡¡their¡¡bodies¡£¡¡The¡¡same¡¡structure¡¡occurs¡¡in¡¡the¡¡livers¡¡of¡¡fish£»¡¡whence¡¡we¡¡see¡¡in¡¡those¡¡animals¡¡two¡¡circulations¡¡independent¡¡of¡¡the¡¡power¡¡of¡¡the¡¡heartviz¡££»¡¡that¡¡beginning¡¡at¡¡the¡¡termination¡¡of¡¡the¡¡veins¡¡of¡¡the¡¡gil

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