PHYSIOLOGY [VEGETABLE. low temperature the storing -up of intramolecular oxygen is relatively more active than the decomposition of the protoplasm-molecules, whereas at a high temperature the converse is the case. At medium temperatures these pro cesses are about equally active, for it has been ascertained in various cases that the volumes of oxygen absorbed and of carbon dioxide exhaled are under these circumstances approximately equal. It must not, however, be concluded that the exhalation of carbon dioxide is entirely independ ent of the absorption of oxygen, for the observations of Broughton, Wilson, and Wortmann all show that when plants are deprived of a supply of free oxygen the activity of the exhalation of carbon dioxide rapidly diminishes. Among the other waste-products the following are those which are of most common occurrence, organic acids, aromatic substances, colouring matters, bitter principles, certain fatty bodies, alkaloids. Organic (1.) Organic Acids. The organic acids are very generally pre- acids. sent in plants, either free or in combination with organic or in organic bases, and it is to the presence of these acids or of their acid -salts that the acid reaction of plant* tissues is due. Those most commonly occurring are the malic, tartaric, citric, oxalic, and fatty acids, the last-named being generally in combination with glycerin, forming fats (glycerides). There can be little doubt that they are to be regarded as products of destructive metabolic processes, though Liebig regarded some of the more highly-oxidized acids as the first products of constructive metabolism, and as being formed from carbon dioxide and water in the cells which contain chlorophyll. It is not so clear that they are all to be regarded as waste -products ; it appears possible that some of the less highly- oxidized may undergo reduction with the formation in carbo hydrates, for it has been observed, especially by Beyer, that in ripening fruits the acids diminish and the sugar increases in quantity. Again, there can be no doubt that fats enter into con structive metabolism, and hence the fatty acids must be regarded as plastic products. The more highly -oxidized acids are almost certainly waste-products. Oxalic acid, for instance, is commonly found as crystals of calcium oxalate which, in most cases at any rate, undergo no alteration. It appears that the oxalic acid is withdrawn in this way from the sphere of metabolism, and, inas much as these crystals are deposited especially in the deciduous parts of the plant, it is also ultimately got rid of. It is probable that the organic acids are largely produced as the result of oxidative decompositions (see sujjra). There can be no doubt that the self- decomposition of protoplasm is attended by a formation of acids, especially of nitrogenous acids, such as the aspartic and gluta- minic, and of fatty acids. In addition to their significance in the constructive metabolism of plants the organic acids are of use in other ways. Their presence in the living cells contributes to the maintenance of the turgid con dition ; the presence of acid-sap in the root-hairs renders possible the solution and absorption of mineral substances which are insoluble in water ; oxalic acid, at least, decomposes the salts absorbed by the roots ; and finally it appears that the organic acids are capable of inducing the conversion of one carbohydrate into another cane- sugar into glucose, for instance and they may in this way play an important, though hitherto undetermined, part in the general metabolism of plants. Aroma- (2.) Aromatic Substances. These occur generally in the form of tic sub- glucosides, the most common of which is tannin. The glucosides stances, are bodies, for the most part non-nitrogenous, which yield sugar on decomposition amongst other substances. In so far as they yield sugar they may be regarded as plastic products ; but the aromatic substances to which they give rise on decomposition are waste-products, for it appears from the observations which have been made on this point that the higher plants, at least, cannot avail themselves of carbon when combined in an aromatic molecule for the purposes of their constructive metabolism. Probably the resins which are so commonly present in plants are derived from tannin. The first step is the formation of a terpene (C 10 H ]6 ) in the secreting cells ; this is then excreted into the ducts and undergoes partial oxidation with the formation of resin. In connexion with the terpenes two hydrocarbons, caoutchouc and gutta-percha (C 5 H 8 )a,-, may be mentioned, which occur in the latex of certain plants. It is not possible to make any definitive statement as to the mode of origin of the aromatic substances in the plant, but the fact that tannin is constantly present in the cells of parts in which destruc tive metabolism is active growing points, mobile organs of leaves, galls, for example tends to prove that this glucoside at least may be deiived from protoplasm. It must not be overlooked, too, that substances like tyrosin, which contain an aromatic radical, occur in plants, and that they are derived more or less directly from protoplasm. (3.) Colouring Matters. The principal colouring matters of Colour- plants are (a) those which occur in the walls of the bark-cells of ing trees and shrubs (phlobaphenes) ; (6) those of woods, such as matters, logwood ; (c) those which occur in solution in the cell-sap, as in most flowers ; (d) those which occur in connexion with protoplasmic corpuscles, as in the Algir and in the leaves and other green parts of the higher plants. With regard to the three first-named groups it appears probable that they are derived in various ways from tannin. Of the colouring matters which occur in connexion with proto plasmic corpuscles by far the most important is chlorophyll, the substance to which plants owe their green colour. The corpuscle has a spongy structure, the interstices of which are occupied by the chlorophyll in solution in some fatty substance. The other colour ing matters which may be present in corpuscles are ctiolin, yellow, which is apparently present in all chlorophyll-corpuscles, conspicuously so in those in parts of normally green plants which have been growing in darkness, and is apparently an antecedent of chlorophyll ; xanthoplnjll, also yellow, and commonly present in chlorophyll - corpuscles, especially in those of fading leaves, prob ably a derivative of chlorophyll ; anthoxunthin, also yellow, the colouring matter of yellow flowers, and a derivative of chlorophyll ; phycoxanthin, brownish, present in the chlorophyll-corpuscles of the brown Algae, (Phseophyceas or Mclancyhycciv) ; phycocrythin, red, present in the chlorophyll-corpuscles of the red Algae, (Shodqphyceas, Floridcae). Chlorophyll is a substance of such great physiological Forma- importance that the conditions of its formation and its tion of properties must be treated of in some detail. The general C l lo , r 1 " conditions upon which its formation depends are (a) ex- * posure to light, (A) a sufficiently high temperature, (<) a supply of iron. Plants which are normally green are not green if they have been grown in the dark, or if the tem perature has been too low, or if they have not been supplied with iron ; they are usually yellow, and in the last case especially they may be quite colourless. Normally green plants which have been kept in the dark or at too low a temperature are said to be " etiolated," since they form etiolin ; plants which have grown in absence of a supply of iron are said to be " chlorotic." There are good grounds for regarding etiolin as an antecedent to chlorophyll. It is formed in the corpuscle in darkness and at a tempera ture lower than that which is necessary for the formation of chlorophyll. It appears from the researches of Gris, Mikosch, and others that when the corpuscle is about to form etiolin it contains a starch-granule, and that as it assumes a yellow colour the included starch-granule dimi nishes in size and may disappear. It must not be inferred from this observation that the etiolin is directly formed from the starch. It is more probable that it is derived from the protoplasm, and that, as the protoplasm is con sumed in the formation of the etiolin, the starch is used in the construction of fresh protoplasm. Under the in fluence of light and of a sufficiently high temperature the yellow etiolin is converted into the green chlorophyll, but nothing is known as to the nature of the process by which the conversion is effected. With regard to the physical properties of chlorophyll Pro- it has long been known that it is soluble in alcohol, ether, perties benzol, chloroform, carbon disulphide, and various oils. cl ln 1 r ] " Hansen has obtained, by a process of saponification, from * the alcoholic extract of leaves a green crystalline substance, probably the purest form of chlorophyll yet obtained, which is readily soluble in water. All solutions of chloro phyll in the above-mentioned media are fluorescent, that is, when they are viewed by reflected light they appear opaque and of a deep lake -red colour, but when thin layers are viewed by transmitted light they appear green. If the light which has passed through a layer of a moder ately strong solution be examined with the spectroscope a characteristic absorption -spectrum will be observed. Beginning at the red end of the spectrum, a well-marked dark band will be seen between Fraunhofer s lines B and C, extending rather beyond C, a second dark band in the
orange between C and D, a third very faint band at the