iinsii (iAin)i:M\c; 



th.- s.Tii.-. 1.. .-xi-hiiii til- r.-.il ii.ilm r this iras 



as an oxi.l.- of tarl»oii. II.- sli..\V4-.l (!iat w li.ii 

 |.m-.- larlx.n in anv of its f..i ins is li.init in ..\yi:i-p 

 llu- rai-»...ii .lisai.]..-ars. a r,-Haiu amu.iiit >>f 

 ..wu.-ii ln'in^' Jis.-.l ii|. in tli.- i)r.M .-ss. an.l dial 

 rarhonif acid u'a-s is t..iiiM-tl. l-'niihi-r. In- piommI 

 that thr w.-ijri.t of .aihunic a.id s.. 'niin-.l was 

 ,.,|iia! ti. tlif .unihin.-tl w.-iKhis « f tlir ..hIi.mi .ind 

 (iwiit'n usfil ni> in hnrnini:. TlHUiiih ' iisitl \\]<. 

 h..\v.-v.T. h.- i.t.ints out Imw tli.-sr .•l.iiirnls w.iv 

 not lost .If .Ifslrov.-tl. Siniilariy In- sluiwci whm 

 anv siil>stanr.' c.iitaininLr <ailM.n is linrnt in ti;.- 

 Mi.linai-v wav. tin- saint- n'as is vi-utlvn.-d.. atui thus 

 th.- air u.-ts • injni-i-(l." as rri.-sl!«-y pnl it. \\ h.n 

 th.- v.-n«-nla^'.- of lh«' K'as incn-ascd alxiv.- a 

 ..-rtaiii voint liis <an(llt' w.-iit ont . thont;h tin- 

 sni.plv of oxv^'i'U was nncxhanstftl. Willi this 

 a.l.lilional kii..\vh-<lir«' to hand. ln^'(•nh.Mls7. was 

 n..w in a positi.ni lo cxidain tin- pi-oct'ss hy which 

 plants i.nrilifd tin- air -namely, that carlMUii.- 

 a.i.l was ahsorh.-d hv tin- It-av.-s and split up l.y 

 th.- aid of siiidiirht. the carhon l>.-ini; r.-tain.-d l>y 

 th'- plant and ilu- oxygen si-t fr.-.-. 



Tin- ••h-ai-.'st .-xi.ositii.n of th.- wh.il.- pro.-.-ss 

 was. how«-v.-i-. furnish. ■.! !>> I':.' work ol Ih- 

 Sanssiir.- (1S(I|). 11.- show.-.l tli:;1 wat.-i' .■n1.-r.-d 

 into <•..inhinati.ni witli farh..n to loiin th.- dry 

 >nhstan.-.- of plants. thoni,di. ..win-to th.- ha.k- 

 ward statt- <»f ..r^Mnic clu-niistry li.- was unalil.- to 

 d.-lint- tin- i-oini)onnd.s thus formed. lie fnrlher 

 sh..we.l thai. th.nn;h hy far the Ki'»'i»ter part <.f the 

 l-lant substance is formed in this way. yet a 

 .-.-rtaiu .small amount of mineral inatter is 

 n.'ce.ssary in the formation of certain other 

 sul>stan<-es essentia! to the life of the iilant. lie 

 point. -.1 .mt how all this was in keejiin^? with the 

 ■ .■nvir..nment " (to use a later term) of the 

 plant tl-.e s.dl. owin'>: to its s'isht solulnlity, bein^ 

 nnal)le to furnish any considera})le part of the 

 v.-^etable materia!, whereas tin- sui)i)ly of water 

 is .•.mstantiy i-.-ph-nish.-d by tlie rain and a never- 

 failinu suj.ply ..f carbon exists in t!ie air. 



Th.- amount .>f cjiibouic acid in the air, however, 

 all bcinii said and done, is only ;ibout '.'> vohinu-s 

 in 1(1.0(1(1. or .():i per cent., a very minnte Iraction 

 when ..lie comes to t'link tliat a plant has t.i 

 ol)tain all its carbon from this source. 



1I.)W largely carbon does enter into the eom- 

 positi.tu of veK^'table matter may easily be seen 

 bv lieatinji a twij? or bit of (-hi]) in a test tube. 

 The moisture Jjeinj? driven off as steam, and some 

 heavv combustible j^ases rich in carbon being 

 also driven .MT. the greater part of the carbon 

 remains in th.- test tube unburnt for want of air. 

 Houuhlv alM.ut half of tin- drv weight of a plant 

 is .-arrH.n. 11. -n..- tin- dilTlculty at tir.st of 

 believing that all this carbon comes from a gas 

 pr.-sent in tlie air in such small (piantity. The 

 id. -a seemi'd far-fetche.l. A mort- natural source 

 w..uld aj.i.ear at lirst sight to i-xist in the plant 

 remains or humus present in every fertile soil. 

 Opinion veered round to this ])oint of view, 

 and 1)(- Saussures' inv(-stigations were set aside, 

 and his interpretation fell into neglect. The 

 hunuis theory, as it was called, held sway for 

 the lirst few* decades of the century, and. even 

 Berzelius, the great Swedish chemist, gave the 

 theory the weight of his support. 



The s(-ience of chemistry was, however, making 

 headway all the time, and. in the forties the work 

 of Liebig greatly extended its sco])e, more 

 especially in its bearing on industry and agri- 

 culture. " He studied the feeding of plants and 

 aninials fron^ tlie chemical ])oint of view, and, 



m.ir.- th.in au\ .m.- .-'s.-. p.-ihaps. ni.i\ h.- sai.l to 

 havi- lai.l th.'- l..un<lalion of sci.ntiil.- farming. 

 In .arrviii',' .ml his r.-si-ir.lies .m tin- f.-.-din-,' id" 

 plants ii.- p .t I'm- humus 1 h.-oi> to tlir t.-st ..f 

 cxi>.-iiiri.ii1 in :i -iinpli- w .i v . Two plants 

 similaiK |i.ilt.-.l in -.nl li. h in hnin'i> wi-re 

 pla.T.I iiiul.T Kla>N. pioxivi,,,, hriui made for 

 n.-c. -ssary intiik.- .d' air in i-,m h . .is.-. a'l I'.indit i.ms 

 being the same i-xcept (h.il tin' .arh.mii- acid gas 

 was r.-mi>vcd h\- suil.ihl.' a l)>orli.iit s fiMin the air 

 supplic.l to .uij- ..r th.' plant-. It w.is fouii.l 

 th,-iv w.i> 11.. new gr..\\t!i in I!.- ialt.'i- .as.-, 

 licit h.-r was anv slar.-h foiiii.-d in th.- h-av.-s. 

 thus showing liiat i. hints do ik.I obtain lli.-ir 

 carli..n ir.un organic- (-om|).iiiii.ls in the soil. 



L.-aving the realm of exp.-iimeiilal sciem-.- f..r 

 lliat <.f math. -mat i.-s. IJehig sh..w.-d bv ca'culali..n 

 that tlie perc.-ntage ..f carb.mic acid gas in th.- air. 

 lh..m,di small, was v.-t aiiiph- f..r t In- n.-.-ds <.f the 

 iu..st luxuriant v."-g.-t at i..n. Hut it might b.- 

 ask.-d h..w .an 1 h.- plants u'.-l th.- gas in suJli.-i.-nl 

 .piantilN .linusr.i ,i- it is llii-ough..ut t h.- wh.de 

 atniospiieic. Thi,- .lillinill > .lisa pp.-ai-s. h.-wever. 

 when wi- eoiisidei- ,,l' llie iritne of uases and 

 th.- teii.leiKV towards .iilliision w iiicli is one o| 



liieir essentia! propeili 's. If .-.pn oi f the 



gases pivs.-nl in tin- air shoul.i happ.'i. to he 

 remove.! ..I' .liininished at .iiu poinl. tin- loss is 

 mail.- g.M.d by an inlL.w ' ..f Ih.- gas in 

 ((ueslion to the phu-e where, its " level "' or 

 prc\ssui-e has tlius been reduced. Wh.-n. for 

 instance, carbonic ac-icl gas is removed fr.uu th.- 

 air-spaces within the- leaf ot a tree, through bein;;- 

 decomiiosed in sunMghl, there is set up an inflow 

 of the gas fr..m outside to take its place, and the 

 tree thus, as it were, automat ical I y ab.sorbs the gas 

 from the air to oljtain its carbon supitly. 



I>iebig, in short, sc4-t!ed t)ie matter onc-e for all 

 that a plant does not feed on organic- substances 

 as animals do, but that it buil.ls up its tls.sues 

 from inorganic materials alone. Tims the Initmis 

 theory went overboard and De Saussures" 

 interpretation c-ame once more into its own. 



The ])rocess of assimilation within the leaf 

 was studied in detail bv Sachs. Professor of 

 Botany (1S()7-1S!)()) in the Iniversity of VVhirz- 

 burg ill Bavajia. Hi- found tliat .starc-h was the 

 lirst .irganic c.imii..und formed from the raw 

 materials, carboni.- acid and water — or at least it 

 was the lirst to l)e recognised in the leaf — and that 

 it was formed on'y in sun'ight. The starch was 

 c-onveyed, away, usually at night, in the form of 

 sugar, to the growing ])oints or wherever else 

 required in the ])lant. More c-omplicated c-on\- 

 pounds. su(-h as proleids. wherein nitrogen, 

 sulphur, iV:.-., are assimilated, may be formed 

 away from light, hut .uily in the lu-esenc-e of, 

 and at the exjiense of. the energy stored up in. 

 the starc-h or sugar. 



Sachs made nmc-h use of water and sand 

 c-ultui-es to lincl out the e.s.sential elements of 

 the mineral food of plants, but we are not now 

 concerned with these; researc-hes, excejit in so far 

 as they show* that a ]dant has no need, of carbon 

 as a food in the soil. As we have already seen, it 

 gets that element elsewhere, but these ])ot culture 

 experiments confirm our belief in the fact and, 

 as it were, complete the chain of evidence in the 

 ])roof. 



As a teacher of plant i)hysiology, .Sachs was 

 pre-eminent in his time, and his fame, s])reading 

 beyond the confines of Bavaria, attracted 

 .students from all ])arts of the world. Under the 

 niaster's guidance and insjiiration they learnt 



