September 27, 1900] 



NA TURE 



541 



demonstrated that, whilst absorbing carbon dioxide and 

 evolving oxygen, green plants gain in dry weight ; and he 

 further contributed to the elucidation of the problem of nutrition 

 by showing that, whilst assimilating carbon dioxide, green 

 plants also assimilate the hydrogen and oxygen of water. 



Three questions naturally arose in connection with de 

 Saussure's statement of the case : What is the nature of the 

 organic substance formed? What is the function of the 

 chlorophyll ? What is the part played by light ? It was far on 

 in the century before answers were forthcoming. 



With regard to the first of these questions the researches of 

 Boussingault (1864) and others established the fact that the 

 volume of carbon dioxide absorbed and that of the oxygen 

 evolved in connection with the process are approximately equal. 

 Further, the frequent presence of starch in the chloroplastids, to 

 which Mohl first drew attention (1837), was subsequently found 

 by Sachs (1862) to be closely connected with the assimilation 

 of carbon dioxide. The conclusion drawn from these facts is 

 that the gain in dry weight accompanying the assimilation of 

 carbon dioxide is due to the formation, in the first instance, of 

 organic substance having the composition of a carbohydrate ; a 

 conclusion which may be expressed by the equation 



CO2 + H2O = CH2O + O2. 



The questions with regard to chlorophyll and to light are so 

 intimately connected that they must be considered together. 

 The first step towards their solution was the investigation of the 

 relative activity of light of different colours, originally under- 

 taken by Senebier (1782) and subsequently repeated by Daubeny 

 (1836), with the result that red and orange light was found to 

 promote assimilation in a higher degree than blue or violet 

 light. Shortly afterwards Draper (1843), experimenting with 

 an actual solar spectrum, concluded that the most active rays 

 are the orange and yellow ; a conclusion which was generally 

 accepted for many years. But in the meantime the properties 

 of the green colouring matter of plants (to which Pelletier and 

 Caventou gave the name " chlorophyll " in 1817) were being 

 investigated. Brewster discovered in 1834 that an alcoholic 

 extract of green leaves presents a characteristic absorption 

 spectrum ; but many years elapsed before any attempt was made 

 to connect this property with the physiological activity of 

 chlorophyll. It was not until 1871-72 that Lommel and 

 N. T- C. Miiller pointed out that the rays of the spectrum 

 which are most completely absorbed by chlorophyll are just 

 those which are most efficient in the assimilation of carbon 

 dioxide. Subsequent researches, particularly those of Timiriazeff 

 (1877), and those of Engelmann (1882-84) based on his inge- 

 nious Bacterium-method, have confirmed the views of Lommel 

 and of Miiller, and have placed it beyond doubt that the import- 

 ance of light in the assimilatory process is that it is the form 

 of kinetic energy necessary to effect the chemical changes, and 

 that the function of chlorophyll is to serve as the means of 

 absorbing this energy and of making it available for the plant. 



These are perhaps the most striking discoveries in relation 

 to the nutrition of plants, but there are others of not less import- 

 ance to which brief allusion must be made. We owe to de 

 Saussure (1804) the first clear demonstration of the fact that 

 plants derive an important part of their food from the soil ; 

 but the relative nutritive value of the inorganic salts absorbed in 

 solution was not ascertained until Sachs (1858) reintroduced the 

 method of water-culture which had originated centuries before 

 with Woodward (1699) and had been practised by Duhamel 

 (1768) and de Saussure. Special interest centres around the 

 question of the nitrogenous nutrition of plants. It was long held 

 chiefly on the authority of Priestley and of Ingen-Housz, and in 

 spite of the contrary opinion expressed by Senebier, Woodhouse 

 (1803) and de Saussure, that plants absorb the free nitrogen of 

 the atmosphere by their leaves. This view was not finally 

 abandoned until i860, when the researches of Boussingault and 

 of Lawes and Gilbert deprived it of all foundation. Since then 

 we have learned that the free nitrogen of the air can be made 

 available for nutrition — not indeed directly by green plants 

 themselves, but, as Berthelot and Winogradsky more especially 

 have shown, by Bacteria in the soil, or, as apparently in the 

 Leguminos;x;,by Bacteria actually enclosed in the roots of the 

 plants with which they live symbiotically. 



We now turn from the nutritive or anabolic processes to those 

 which are catabolic. The discovery of the latter, just as of the 

 former, was arrived at by the investigation of the gaseous inter- 

 change between the plant and the atmosphere. In the 



NO. I 61 3. VOL. 62] 



eighteenth century Scheele and Priestley had found that, under 

 certain circumstances, plants deteriorate the quality of air ; but 

 it is to Ingen-Housz that we owe the discovery that plants, like 

 animals, respire, taking in oxygen and giving off carbon 

 dioxide. And when Senebier (1800) had ascertained for the 

 inflorescence of Arum maciilatum, and later de Saussure (1822) 

 for other flowers, that active respiration is associated with an 

 evolution of heat, the connection between respiration and cata- 

 bolism was established for plants as it had been long before by 

 Lavoisier (1777) in the case of animals. 



Among the catabolic processes which have been investigated 

 none are of greater importance than those which are designated 

 by the general term fermentations. The first of these to be 

 discovered was the alcoholic fermentation of sugar. Towards 

 the end of the seventeenth century Leeuwenhoek had detected 

 minute globules in fermenting wort ; and a century later La- 

 voisier had ascertained that the chemical process consists in the 

 decomposition of sugar into alcohol and carbon dioxide ; but it 

 was not until 1837-38 that, almost simultaneously, Cagniard de 

 Latour, Schwann and KUtzing discovered that Leeuwenhoek's 

 globules were living organisms, and were the cause of the fer- 

 mentation. Shortly before, in 1833, Payen and Persoz ex- 

 tracted from malt a substance named diastase, which they found 

 could convert the starch of the grain into sugar. These two^ 

 classes of bodies, causing fermentative changes, were distin- 

 guished respectively as organised and unorganised ferments. 

 The number of the former was rapidly added to by the investi- 

 gation more especially of the Bacteria, in which Pasteur led the 

 way. The extension of our knowledge of the unorganised fer- 

 ments, or enzymes, has been even more remarkable ; we now 

 know that very many of the metabolic processes are effected by- 

 various enzymes, such as those which convert the more complex 

 carbohydrates into others of simpler constitution (diastase,, 

 cytase, glucase, inulase, invertase) ; those which decompose 

 glucosides (emulsin, myrosin, &c.) ; those which act on proteids 

 (trypsins) and on fats (lipases) ; the oxidases, which cause the 

 oxidation of various organic substances ; and the zymase^ 

 recently extracted from yeast, which causes alcoholic fermenta- 

 tion. 



The old distinction of the micro organisms as "organised fer- 

 ments" is no longer tenable; for, on the one hand, certain of 

 the chemical changes which they effect can be traced to extract-- 

 able enzymes which they produce ; and, on the other, as Pasteur 

 has asserted, every living cell may become an "organised fer- 

 ment " under appropriate conditions. The distinction now to 

 be drawn is between those processes which are due to enzymes 

 and those directly effected by living protoplasm. Many now 

 definitely included in the former class were, until lately, regarded 

 as belonging to the latter ; and no doubt future investigation 

 will still further increase the number of the former at the expense 

 of the latter. 



The consideration of the metabolic processes leads naturally 

 to that of the function of transpiration and of the means by 

 which water and substances in solution are distributed in the 

 plant. This is perhaps the department of physiology in whiclv 

 progress during the nineteenth century has been least marked. 

 We have got rid, it is true, of the old idea of an ascending 

 crude sap, and of a descending elaborated sap, but there have 

 been no fundamental discoveries. With regard to transpiration 

 itself, we know more of the detail of the process, but that is 

 all that can be said. As for root-pressure, Hofmeister (1858-82) 

 discovered that " bleeding " — as the phenomena of root-pressure 

 were termed by the earlier writers— is not confined, as had- 

 hitherto been thought, to trees and shrubs ; but the current 

 theory of the process, allowing for the discovery of protoplasm 

 and of osmosis, has advanced but little upon that given by Grew 

 in the third book of his "Anatomy of Plants" (1675). Again^ 

 the mechanism of the transpiration-current in lofty trees re- 

 mains an unsolved problem. To begin with, there is still some 

 doubt as to the exact channel in which the current travels. 

 Knight (1801-8) first proved that the current travels in the 

 alburnum of the trunk, but not, he thought, in the vessels, for 

 he found them to be dry in the summer, when transpiration is 

 most active; a view in which Dutrochet (1837) subsequently 

 concurred. Meyen (1838) then suggested that the water musi 

 travel, not in the lumina, but in the substance of the cells of 

 the vessels, and was supported by such eminent physiologistst 

 as Hofmeister (1858), Unger (1864, 1868) and Sachs (1878); 

 but it has since been strongly asserted by Boehm, Elfving, 

 Vesque, Hartig and Strasburger that the young vessels always 



