IRISH GARDENING 



25 



The Food of Plants. 



liy CUKMIST. 



Thk keei)ing of a gcirdeu is associated with one of 

 the earliest traditions of the human race ; and 

 no doubt gardens liave been kept and the culture 

 of fruit and vegetables carried on by civilised 

 peoples from time immemorial. Yet very little 

 seems to have been known of the ])rocesses of 

 ])lant nutrition until quite recent times. As a 

 matter of fact, whatever knowledge in this 

 department of plant ])hysiology we now possess 

 has been acquired for the most part during the 

 ccmrse of the nineteenth century. Towards the 

 close of the pre\ious century the work of Lavoisier 

 had laid, the foundations of the infant science of 

 chemistry on a renlly fiiin basis. His interpreta- 

 tion of the |)hejioiueii(>n of burning and the closely 

 allied one of aiiiiua! breathing ])aved the way for 

 a ra]<id advance in genera! chemistry as well as in 

 animal ])hysiology, and from the progress thus 

 attained new light was shed on the ])rob!ems of 

 vegetable ]ihysiology. We certainty know more 

 aboxit the ])rocesses of vegetable growth and 

 nutrition than our ancestors knew, though it 

 must be owned that life itself, which guides and 

 controls these i)rocesses, remains as much as ever 

 a naarvel and a mystery. 



In this article it is proposed to treat from the 

 historical point of view the question of carbon 

 assimilation, the most fundamental of all the 

 ])rocesses concerned in the manufacture of plant 

 food, or indeed animal food for that matter. It 

 may not be out of place therefore to look back on 

 some of the early experiments in this doniain, 

 those reconnaissances, as we may say, of an 

 unknown land, and to study them for a nioment 

 in the light of our present-day knowledge. 



Probably the first experinient in plant i)hysio- 

 ln<i:y in which the balance was called into use, is 

 tlic well-known one of Van He'mont carried out 

 three hundred years ago in the garden attached 

 to his laboratory in his native city Brussels. He 

 ])lanted a willow slip 5 lbs. in weight in a large 

 earthenware pot, which he buried in the soil and 

 covered over with 7jerforated tinfoil to keep out 

 dust, &c. He watered it with either rain or 

 distilled water, and at the end of five years his 

 sli]) had grown into a little tree 169 tt)s. 3 ozs. in 

 weight, not taking any account of the leaves 

 which had come and gone meantime. There had 

 been originally placed in the pot 200 lbs. dry 

 weight of soil, which when removed at the end 

 of the five years and again carefully dried and 

 weighed was found to contain the (»riginal 200 His. 

 all but 2 ozs. Where did his 104 lt>s. of wood, 

 bark and roots come from ? 



Van Helmont concluded the increased weight 

 came from the water, and he thvis came to regard 

 water as the chief primary element in nature. 

 Obviously the air, the other medium with which 

 his willow tree came in contact, did not ai)pear 

 t<i him as a source wherefrom. some of this weight 

 luiglit have been obtained. The conce])tion of 

 luattei' in the gaseous form was not then clearly 

 (leiiiied. Aiis(()1!e, one of the greatest thinkers 

 of auti()uil y, ln'ld the view that air had no weight, 

 having couviiued himself bv actual exj)eriment 

 thai a l)la(l(lei' iuflated with air is no heavier than 

 the same blacbler deflated and empty. Of course, 

 we know now that the iiithiled bladder is l)Uoyed 

 up^to a greater extent than the deflated one, 

 owing to the greater dis](hicemeut of the sur- 



rounding air in one case than in the other. As 

 a matter of fact it is buoyed u]> to a greater 

 extent by a force equal to the weight of the 

 extra volume of air dLs])laced, and so it is the 

 weight of the air within the bladder is just 

 neutralised. Where the volunu^ of the containing 

 vessel remains unatered through the experiment, 

 as, for instance, when a flask came to be used on 

 the invention of the exhavist pum.]), the idea that 

 air has no weight was very soon found to be 

 erroneous. 



But to return to our subject. That plants did 

 affect the coni|)osition of the air was ])roved to be 

 the case about two hundred years after Van 

 Helmont's time, by I'riestley, an English scientist, 

 whose name i.s better known in connection with 

 the discovery of oxygen. He had noticed that 

 air is vitiated, by biirning and breathing. When, 

 for instance, a lighted candle is ]ilaced within a 

 closed vessel the candle soon go?s out, the r?sidual 

 air being no good so far as combustion is con- 

 cerned. And similarly with expired air. Now 

 since burning and respiration continually tend 

 to vitiate the air, there must be, Priestley 

 reasoned, soxne natural reverse jirocess wiiereby 

 the air conies to be revivified. Havhi'i 

 noticed that bubbles of gas were given off 

 by green slime (al^ae) in water, it occurred to 

 hinx to collect and exaniine some of the gas 

 thus set free. He had already hit upon a simple 

 method of collecting gases by the displace- 

 ment of water and other liquids, a method often 

 since made use of and well known to the school- 

 boy of to-day, wlio watches the ])rocess with 

 unusual attention. Priestley found that the air 

 thiis set free by waiter plants behaves very like 

 oxygen towards a glowing chip. Another 

 experiment we shall let him describe in his own 

 words : — " I have been happy as by accident to 

 have hit upon a metliod of restoring air which 

 has been inj\ired by the burning of candles, and 

 to have discovered at least one of the restoratives 

 which na,ture eniploys for the purj^ose. It is 

 vegetatioti. ... I |)ut a s])rig of mint into a 

 quantity of air in a glass jar in which a candle 

 had burnt out and inverted the jar over water, 

 and found after ten days a candle would burn 

 again in the air perfectly well. Several times I 

 divided the quantity of air in which the candle 

 had burnt out into tAvo parts, and putting the 

 plants into one, lelt the other in the same ex- 

 posure contained also in a glass vessel inverted 

 in water, but without any plant, and never failed 

 to find that the candle would burn in the former 

 but not in the latter." 



Another scientist, a Dutch man nanxed Ingen- 

 hoiis'/. living in England, and a contemi)orary of 

 Priestley's, carried the matter a step further: — 

 '■ I observe," he says^ " that ])lants not only have 

 a facility to correct bad air in six or ten flays, as 

 the experiments of Dr. Priestley show, but that 

 they ])erform this im])ortant office in a few hours ; 

 but this wonderful oi)eration is by no means 

 owing to the vegetation of the plant, but to the 

 influence of the sun, and to its illuminating and 

 not to its heating i)ower." 



But neither Ingenhousz nor Priestley could 

 explain the ])roce.ss by which the plants thus- 

 purified the air. At the time (about 1779) the 

 composition of carbonic acid gas was not under- 

 stood, though the gas itself was well known under 

 the name of " fixed air " as a constituent of 

 limestone, chalk, washing soda, ])earlash, Arc. 

 It remained for Lavoisier, who now a.pjieaj*s on 



