5G 



KNOWLEDGE 



[Maech 1, 1900. 



however, lost to it, but sooner or later finds its way back 

 again in the form of Carbon dioxide from the lungs of 

 animals and from burning or decaying organic sub- 

 stances. From every coal fire Carbon returns into the 

 air from whence it was taken by the vast forests which 

 flourished untold ages ago. There is thus maintained 

 a continuous circulation of Carbon, from its gaseous 

 form (Carbon dioxide) to the more or less solid state in 

 animals and plants ; here it is presented in an organic 

 form, sooner or later to be destroyed, once more setting 

 the Carbon free to rejoin the air as Carbon dioxide. 



The knowledge which we at present possess of Carbon 

 dioxide in its relation to green plants is the result of 

 the patient labours of manv investigators extending ove/ 

 a long period. As an example of the methods which 

 have been used to throw light upon this problem, it will 

 be interesting to notice one historic experiment. In 

 1844 a French chemist tried to prove that Carbon 

 dioxide is actually taken from the air bj- green leaves. 

 He placed a leaf-bearing branch of a vine in a large 

 glass vessel, which was afterwards closed so that no air 

 could enter or leave it except through two tubes. He 

 then passed a very slow stream of air through it by 

 way of these tubes. It was found that the air which 

 left the vessel during the daytime contained less Carbon 

 dioxide than that which entered it ; from this he rightly 

 concluded that some had been removed by the leaves 

 of the vine. It is now well known that all green 

 leaves remove Carbon dioxide from the air during the 

 day. Although it is impossible to see this actually 

 taking place, it is not difficult to observe two other 

 phenomena which almost always go on at the same 

 time. As Carbon dioxide enters the leaf Oxygen gas 

 leaves it and solid starch is fomied within its cells. 

 By means of a little simple manipulation it is quits 

 easy to observe the appearance of the one and thj 

 escape of the other. 



Let us first consider the escape of Oxygen. A small 

 green water-plant[| should be placed in a glass flask 

 or test-tube, which is quite filled^y with water. The 

 glass vessel is then turned upside down and made to 

 stand upon its open end in a tray of water. Sunlight 

 is now allowed to fall upon the plant, and almost 

 immediately minute bubbles are seen to form upon 

 various parts of it; these rise and collect at the top of 

 the vessel. By using the proper chemical tests this 

 gas can be shown to be almost pui-e Oxvgen. The same 

 thing happens in green leaves gi'owing in the air; but 

 in this case the chem^al action evades observation. 



To show the presence of starch in a gi-een leaf requires 

 a little more time. It is well known that if a drop of a 

 solution of Iodine be placed upon a few granules of 

 starch, a blue stain is produced. Upon this fact is 

 founded the method by which we find out whether a 

 leaf contains starch or not. A thin leaf (such as the 

 common Nasturtium or Indian cress, Tropceolum majus) 

 is boiled for about one minute in water, and then placed 

 in spirits of wine until all the chlorophyll is dissolved 

 out, and the leaf becomes colourless. It should thon 

 be placed in a weak solution of Iodine.** If the leaf 

 contains starch it will be stained blue or black ; if, how- 



|l The so-called " CsDadian " water-weed. Elodea canadensis, is 

 ciiiivenient for the purpose. All natural waters contain dissolved 

 t'arbon dioxide, from which submerged green plants obtain their 

 supplies. 



•^ '' Quite filled," so that when turned upside down no bubble of 

 air should be present in the water. 



*» " Tincture of Iodine " diluted with water until it has the colour 

 of dark beer. 



e\er, no starch was present, the Iodine would only 

 colour it light vellow or brown. 



Inside the cells of the leaf the Carbon dioxide is 

 decomposed — that is, separated into its elements, 

 Carbon and Oxygen. Oxygen escapes and the Carbon 

 enters into combination with Hydrogen and Oxygen 

 (which have come from the roots in the form of water 

 and mineral salts), and an organic substance is formed. 

 This series of changes — in the coxu'se of which the 

 Cai-bon of Carbon dioxide becomes Carbon of an or 

 ganic compound — is spoken of as the " Assimilation of 

 Carbon dioxide," or, more briefly, " Assimilation." 

 Of the intemiediate steps of this series of changes very 

 little is known. We do not know, for instance, what 

 is the first organic substance to be formed, but it is 

 nearly always the case that starch appears in the leaf 

 very soon after it commences to assimilate. This fact 

 provides a means of studying the conditions under whicli 

 a green leaf will assimilate, for, as we have seen, it is 

 qtiite easy to find out whether starch is present. If, 

 for example, we put a growing plant into a dark cellar 

 for several hours and then test its leaves with Iodine, 

 we find no starch. Here is a proof that leaves do not 

 assimilate in the dark, from which it follows that, in 

 Nature, assimilation goes on only during the daytime. 

 A leaf contains more starch at sunset than at anv other 

 time of the day, as would be expected; in the night 

 the starch which was formed during the day is removed, 

 and in the early morning a leaf contains very little 

 or none at ail. 



That the formation of starch takes place in the 

 light and not in darkness can be shown in a very 

 striking manner by means of a photogi'aphic negative.! f 

 A leaf is emptied of its starch by keeping the plant on 

 which it grows in the dark for several hours. It is 

 then laid flat on some support and covered by a 

 negative and exposed to sunlight for some hours. On 

 testing it for starch with Iodine, a print of the negative 

 will be obtained ; those parts which were beneath tho 

 dark portions of the negative being uncoloured, for 

 tbey contain no starch; the light parts of the negative 

 will be represented by starch stained black by Iodine. 



Assimilation only goes on in the gi-een parts of a 

 plant. The leaves of many ornamental plants ari 

 variegated — that is, they are not green all over, but 

 " dashed ' with white or some other colour. If such 

 a leaf — e.g., variegated Ivy — is pulled at the end of 

 a summer's day and tested for starch, the white parts 

 will not be coloiu-ed blue by Iodine, but all the stai'ch 

 will be found in the green parts. It is therefore only 

 the green parts of the leaf which are able to assimilate 

 Carbon dioxide; in other words, the power of assimi- 

 lation resides in the chlorophyll, which posseess this 

 power only when the leaf containing it is in the light. 



We have now to consider how aii- and the Carbon 

 dioxide which it contains find their way into the leaf. 

 Simple as this problem appears on paper, it is only 

 since 1895 that it has been at all clearly understood. 

 The cells which form the upper suiface of a leaf are 

 in most cases brick-shaped, and fit together like tiles in 

 a pavement, leaving no openings between them. On 

 the lower side, however, numerous very minute pores 

 between the cells lead into the interior of the leaf. 

 These openings are called 'Stomata" (from the Greek 

 word. Stoma, a mouth) ; each stoma is surrounded 

 by two special guard-cells, which under some circum- 

 stances alter their shape so much as to close or nearly 



•H- W. Gardiner, in Nature, Vol. XLI. (1890), p. 16. 



