February 14, 1918] 



NATURE 



465 



as one connected whole, and are impatient of the 

 many individual attacks which have wasted half 

 their effort by failure to keep contact with flanking 

 movements by workers coming- from other direc- 

 tions, who should be regarded as allies, but are 

 often treated as rivals. This report ought to have 

 a valuable effect in unifying research activity. No 

 similar presentation of the subject has appeared 

 before in any language. 



A century of laboratory attack has driven 

 several salients forward, of which perhaps three 

 stand out conspicuously. W'e may briefly consider 

 how far each has progressed, as reported in this 

 pamphlet, and what may be expected of the future. 

 These advances concern (i) the pigments of the 

 leaf (chap, ii.) ; (2) the products of carbon 

 assimilation formed in the leaf (chap, v.); and 

 {3) the influence of external factors on the rate of 

 carbon assimilation (chap. iv.). 



In chap. vii. will be found set out those specula- 

 tions that have any significance as theories of the 

 assimilation process. During the process that 

 takes place in the illuminated green cell, whereby 

 carbon dioxide disappears and sugar appears, it 

 is clear that^ somehow or other, reduction and 

 "synthesis " must take place; but even now it is 

 quite unclear to what system of reactions this 

 result is to be attributed. Many hypotheses have 

 been put forward, and Baeyer's "formaldehyde 

 theory" has been almost canonised as an eternal 

 verity, yet there is really no good evidence for it. 

 Its perennial attraction no doubt is due to its 

 aesthetic simplicity. It appears now that the re- 

 action must be much more complex (unless, as is 

 possible, we are entirely on the wrong tack), and 

 this is our excuse for the slowness of progress. 

 A knowledge of the reacting system at work would 

 be equivalent to storming the citadel of the whole 

 defence, but so far no one has advanced a satis- 

 factory hypothesis that can be put to the proof of 

 experiment. We have still to advance by slow 

 hammering tactics from various directions. 



The advance that has been made in elucidating 

 the nature of the pigments of the green leaf under 

 the guidance of Prof. Willstatter really amounts 

 to a- shock attack, so continuous and rapidly 

 widening has been the progress. 



In 1864 Sir George Stokes stated that he had 

 proved that the green matter of leaves consisted 

 of two green and two yellow pigments, though 

 he never published his evidence. In the last 

 decade this conclusion has been finally established 

 by the monumental research of Prof. Willstatter 

 and his colleagues. Before Prof. Willstatter there 

 was no clue to the real chemical nature of these two 

 green pigments, and it could be hoped that when 

 their chemistry was known the process of reduc- 

 tion of carbon dioxide would be elucidated. 



The curious nature of the green and yellow 

 pigments is now made quite clear; the greens 

 are esters of a big alcohol molecule, phytol, and 

 a tricarboxylic acid based on a nucleus of four 

 pyrrole rings. Magnesium is also an essential 

 constituent, not electrolytically dissociable, but 

 believed to be directly united with the nitrogen. 

 The difference between the two green pigments 

 NO. 2520, VOL. 100] 



is simply this, that "chlorophyll h " contains one 

 atom more of oxygen (and two less of hydrogen) 

 than "chlorophyll a." In complete contrast to 

 this complexity is the simplicity of the yellow pig- 

 ments ; "carotin" is an unsaturated hydrocarbon, 

 and "xanthophyll " an additive oxidation deriva- 

 tive of it. Both the yellows, when isolated from 

 the cell, spontaneously absorb oxygen in abun- 

 I dance. It is easy to assume that these differences 

 ' of oxygen-potential occurring within both the 

 ! green and the yellow pairs are significant for the 

 reduction of carbon dioxide ; but there is no 

 evidence on this point at present. 



A second line of attack into which much work 

 has been put is the determination of the nature 

 i and amount of the carbon-containing substances 

 which arise in the leaf as CO^ disappears. Is the 

 I CO2 quantitatively reduced to its theoretical yield 

 of carbohydrates, or do other substances arise in 

 "multiple photosynthesis"? The measurement 

 of the CO, intake by the green leaf is not diflicult, 

 but difficulties attend the correction of this ap- 

 parent photosynthesis for the amount of CO2 

 simultaneously produced in the body of the leaf 

 by respiration — an amount which is large at high 

 temperatures, but must be known and added in 

 for exact statements of photosynthesis. At the 

 other end of the reaction the determination of the 

 carbohydrates produced continues to present con- 

 siderable difficulties, so that no one has yet man- 

 aged to measure in one experiment both the 

 initial CO2 used up and the final carbohydrates 

 produced whereby we might judge of their equi- 

 valence. Much discussion has taken place on the 

 question of what is the first sugar to appear in 

 photosynthesis, though this is largely a strife of 

 ideas rather than of facts. 



The identification and accurate determination of 

 individual sugars and polysaccharides in a mixture 

 of such bodies form a special field of analytical 

 work the difficulties of which have been much 

 lightened by recent English researches, set out in 

 chap. V. ; but these have not been, fully overcome 

 yet. Further, these carbohydrates have all to be 

 extracted from the leaf unaltered by the enzymes 

 j that lie in wait for them in the cell, and finally 

 not one determination, but two differential deter- 

 minations are required to establish changes due 

 to photosynthesis; one, at the beginning of the 

 i experiment, being on some other area of leaf 

 i that can be held to furnish a strictly comparable 

 j control. Progress in this important line of work 

 j waits upon absolutely trustworthy methods of ex- 

 j traction and analysis of carbohydrates. 



The third significant advance that has been 



I made is that towards an understanding of the in- 



j fluence and mode of interaction of the many ex- 



I ternal and internal factors that can influence the 



j rate of photosynthesis. The control or measure- 



! ment of the external factors of illumination (sun- 



I shine or artificial light), temperature, and COj 



I supply require elaborate apparatus and consider- 



\ able physical experience in the fields of radio- 



metry, photometry, scientific illumination, 



thermo-electric measurement of leaf temperature, 



etc. Of internal factors the amount of chloro- 



