a history of science-4-第17节

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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：

〃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：


〃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：


〃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：　　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