Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/63

Rh VEGETABLE.] PHYSIOLOGY 53 junction of the yellow and the green, and a fourth more distinct band in the green near F. When an alcoholic extract of leaves is used, as is ordinarily the case, the whole of the blue end of the spectrum beyond F is absorbed, in consequence of the coalescence of three broad bands, which can be seen separately when a very dilute solution is used, two of the bands being in the blue between F and G, and one at the end of the violet. The spectrum of the alcoholic extract presents then seven bands in all. According to Hansen, the spectrum of solutions of his crystallized chlorophyll possesses only the first four of the above-mentioned bands, and it is only when very thick layers are used that the blue end of the spectrum is absorbed ; this is true also of the spectrum of the green colouring matter obtained by Tschirch. Little is as yet known as to the chemical composition of chlorophyll. Gautier and Hoppe-Seyler have both obtained a crystalline green substance from the alcoholic extracts of leaves, termed by the latter &quot; chlorophyllan,&quot; which is not to be regarded as pure chlorophyll. The following are their analyses of this substance, to which is added for comparison Hansen s analysis of the green crystalline substance which he obtained Gautier. C 73-97 H 9-80 N 4-15 . ...10-33 Ash 1-75 Hoppe-Seyler. 73-34 972 5-68 9-54 P. 1-38 Mg. 0-34 Hansen. 60-33 9-37 4-77 14-77 10-76 Hansen states that the ash found by him is due to the previous processes of preparation, and that the only normal ash-constituent is iron, which neither Gautier nor Hoppe- Seyler had discovered. Tschirch on reducing chlorophyllan by means of zinc-dust has obtained a green substance which does not crystallize, and is soluble in alcohol, ether, and oils, but not in water. This he believes to be pure chloro phyll. From the percentage composition of the crystals of chlorophyllan Gautier deduces the formula C 19 H 22 N 2 O 3 , and draws attention to the similarity between this and the formula of bilirubin (C 16 H n N 2 3 ). Hoppe-Seyler concludes that chlorophyllan contains phosphorus in its molecule, and is either a lecithin or a lecithin compound. Schunck has found that the residue of an ethereal solution of chlorophyll when treated with sulphuric or hydrochloric acid yields glucose amongst other products ; he therefore regards chlorophyll as a glucoside. tter (4.) Bitter Principles. It has been ascertained that some of in- these are glucosides, and soine alkaloids, but the chemical nature pies. of many of them is still undetermined. Such are santonin (C ]8 H 18 3 ), aloin (C ]5 H 1(i 7 ), quasiin (C 10 H 1:i 3 ). It is impossible at present to say anything as to the possible mode of their origin or as to their physiological significance in the plant. itty (5.) Certain Fatty Bodies. The ordinary fats (glycerides) are to )dies. be regarded as plastic products, and they will be subsequently treated of under that head. But there are certain fatty bodies of which this statement cannot be made ; these are cholesterin, lecithin, and wax. It is not known how these substances are formed, but probably they, like the ordinary fats, are derived from protoplasm. This view is especially probable with regard to lecithin, which is a nitrogenous and phosphorized fat. Wax occurs especi ally in the external cell-walls or on the surface of those parts of plants which have a cuticularized epidermis; the &quot;bloom&quot; on fruits, for example, is a layer of wax. Jka- (6.) Alkaloids. Tlw alkaloids are regarded as waste-products, rids. because, as the observations of Knop and Wolff show, the demand for combined nitrogen cannot be met by supplying the plant with it in the form of alkaloids, though the plant can avail itself of such organic nitrogenous substances as urea, uric acid, leucin, tyrosin, or glycocoll. The alkaloids are compound ammonias which are not volatile at ordinary temperatures. With regard to their mode of origin in the plant, there can be little doubt that they are derived more or less directly from protoplasm, or at least from proteid, as are urea and uric acid in the animal body. But, although these nitrogenous waste -products are formed in the destructive metabolism of plants, their formation is not accom panied by a loss of nitrogen, for they are not excreted, as is the case in animals, but are deposited in the cells. 2. Excretion. Of the waste -products, some, such as oxygen, water, and carbon dioxide, are excreted in the gaseous form the oxygen and the carbon dioxide through the superficial cell-walls of the plant, the watery vapour through the stomata. Some of the carbon dioxide may combine with earthy bases to form carbonates, which are either retained in the plant or excreted in solution. The resins and ethereal oils, as well as wax, are frequently excreted. The mechanism of excretion is widely different Gland- in different cases. The resins and ethereal oils are usually ular ex- excreted by means of special glandular organs. The gland cr IOI! may be a hair on the surface, and it is then commonly the terminal cell at the free end which is secretory ; or it may be a group of epidermal cells between which large inter cellular spaces are formed, which serve as receptacles for the excreted substance ; or it may be formed by the absorp tion of the adjoining walls of a group of cells belonging partly to the epidermis and partly to the underlying ground-tissue, a cavity being thus constructed, which con tains the excreted substance ; or again, longitudinal strands of cells may become separated so as to enclose an elongated intercellular space into which they excrete (resin-ducts). In many cases the substance to be excreted may be detected in the glandular cells ; not unfrequently, however, and always in the case of wax, no trace of it can be discovered in the cells themselves ; it is first to be found in the cell- walls between the cuticular and the deeper layers. The actual excretion is usually effected, in the case of super ficial glands, by the rupture of the cuticle which is con tinuous over the gland, and by the consequent escape of the contents ; in some cases the gland remains closed, and any volatile substances (ethereal oil) which may be present escape by evaporation. The excretion of the earthy carbonates in solution is most commonly effected by means of a well -developed gland. Such a gland consists of a group of modified parenchymatous cells in connexion with the termination of a fibro- vascular bundle; and one or two openings, termed &quot; water-pores,&quot; and somewhat resembling stomata, are present in the epidermis immediately over it. Under the action of the root-pressure the gland excretes water which holds the carbonates in solution. Glands of this kind are present in the leaves of various Saxifragaceous and Crassulaceous plants. In other cases these salts appear to be excreted by ordinary epidermal cells. In certain Ferns (various species of Polypodium and Asjridium), for instance, scales of calcium carbonate are found on depres sions in the surface of the leaves which are situated imme diately over the terminations of the fibro-vascular bundles. It not unfrequently happens that plants excrete sub- Nectary stances other than waste-products, but this has the effect excre - of securing indirect advantages. In the great majority of flowers there are glandular organs which excrete a watery fluid holding principally sugar in solution ; these organs are termed &quot;nectaries, &quot;and the excretion &quot;nectar.&quot; The nectary has essentially the same structure as the water-gland de scribed above, the only important difference being that, whereas the gland is sunk in the tissue and is covered by the epidermis, the nectary has a large free surface, so that the nectar is at once poured out on to the exterior. But there is an important functional difference between them, namely, that, whereas excretion by the gland can only take place under the influence of the root -pressure, excretion by the nectary is independent of the root -pressure, for it will continue when the flower has been removed from the plant. Another instance of an excretion of this kind is afforded by the carnivorous plants. The glands of their leaves excrete a watery liquid which holds in solution a peptic