orders. The uniformity of the results in particular genera of widely separated orders shows that the differences in question are the result of external conditions rather than of hereditary tendencies; the following are the more important points in which the tissues become modified by being buried in the soil. The epidermis, when present, is modified. Suberin attacks first of all its external wall, and may even form a very thick layer; it ascends only slowly into the lateral and internal walls. The cortex increases, either by increase of the size or number of its cells. The collenchyma either diminishes or disappears altogether, especially when this tissue is enveloped in the angles of the aërial stem. There is a tendency towards the early production of a suberous layer, which appears at different points of the epidermis, in the cortical parenchyma, in the endoderm, in the peripheral layer, and in the liber. This layer is sometimes a substitute for a ring of fibres which is often found outside the liber-bundles in the aërial stem. The underground stem sometimes contains a few fibres, but they are much less numerous. In the greater number of perennial plants examined the liberbundles of the aërial stem are closed, being shut up in this ring of fibres; while in the underground stem they are open. The activity of the formative layer is very variable; but lignification almost always takes place irregularly in the woody bundles. The pith is less developed in proportion to the cortex than in the aërial parts. Food-materials, especially starch, exist in it in great abundance. The angles of the aerial stem, when projecting, tend to disappear. The following phenomena in the underground stem may therefore be attributed to the influence of the environment:-The great development of protective tissues, such as a suberous layer and a suberized epidermis; the reduction or disappearance of the means of support, collenchyma, liber-fibres, &c.; the great development of cortex and relative reduction of pith; feeble lignification; and the production of reserve food-materials. The proportion of perennial plants increases with the altitude above the sea-level; and the same specics is sometimes annual at low altitudes, perennial at high altitudes. The duration of a plant, therefore, and the presence of a rhizome or other form of underground stem, are to a certain extent dependent on external circumstances. Junction of Root and Stem in Dicotyledons and Monocotyledons.-M. C. Potter draws the following comparison between the passage from root to stem in these two classes of plants:-In the procambium of the root the protoxylem or spiral vessels and the protophloem or bast-fibres are first differentiated, the differentiation in each bundle proceeding from without inwards, and thus the separate xylem and phloem bundles are produced. In the stem each bundle consists of xylem and phloem. The protoxylem is first differentiated at the most external part of each bundle, and the differentiation proceeds from within outwards, while the protophloem is first differentiated *Proc. Camb. Phil. Soc., iv. (1883) pp. 395–9 (1 pl.). at the most external part of each bundle, and the differentiation proceeds from without inwards. In Dicotyledons the transformation from the arrangement of the bundles in the stem to that of the root generally takes place in the tigellum; while in Monocotyledons the root arrangement of the bundles continues nearly as far as the point of insertion of the cotyledons in Phoenix dactylifera, or of the scutellum in Zea Mais. Suberin of the Cork-oak.-A. Meyer gives the general results of some investigations made by Kügler as to the nature of the suberin of Quercus suber. The micro-chemical reactions of suberin show that it is nearly allied to the fatty oils. Its molecules are so closely associated with those of cellulose, that boiling chloroform, while extracting the whole of the crystallizable cerin, removes only about 25 per cent. of the suberin. It is, however, completely extracted by treating first with chloroform and alcohol and then with an alcoholic potash-ley. Kügler regards it as a fatty oil, composed chiefly of stearin (CHO)3C3H, and the glycerin-base of a new acid, phellonic acid C20H4O3, with meltingpoint 96° C. Forty per cent. of the mixture of these acids, and 2.5 per cent. glycerin was obtained from cork. Suberin is therefore closely allied to the tallows, and especially to Japan tallow, which, besides palmitin, contains the base of an acid with high melting-point 95° C., obtained from the parenchyma-cells of Rhus succedanea, a substance apparently identical with that which causes the suberization of the cell-walls. Influence of Pressure on the Growth and Structure of Bark.t— A. Gehmacher finds that pressure exercises a considerable influence on the growth of bark, the separate elements being altered as definitely as those of the wood. As regards cork, the greater the pressure the fewer cork-cells are formed, and the less the pressure the more numerous are they. The radial diameter of the cells is also affected by the pressure. The cells of the primary cortical parenchyma undergo a similar change; but they appear to be compressed not only radially, but also laterally, becoming more or less angular towards those cells which were formed under less tension and have a more nearly globular form. The intercellular spaces disappear entirely with increased pressure, increasing perceptibly in size with its decrease. The sclerenchymatous elements are least affected by change of pressure. The bast-fibres increase considerably in number with diminution of pressure; when the pressure is very great very few bast-fibres or none at all are formed. Both the wood-fibres and bast-fibres increase in size with diminished pressure. Relation of Transpiration to Internal Processes of Growth.‡— According to P. Sorauer, transpiration results from two sources, viz. the water derived from processes of oxidation within the plant, and * Ber. Deutsch. Bot. Gesell., i. (1883); Generalvers. in Freiburg, xxix.-xxx. + SB. K. Akad. Wiss. Wien, lxxxviii. (1883) (1 pl.). Forsch. aus d. Geb. der Agriculturphysik, vi. (1883) p. 79. See Naturforscher, xvi. (1883) p. 470. that which serves as a mechanical transport of material and passes unchanged through the plant. It may therefore be compared to the perspiration of animals, and is intimately connected with the process of oxidation within the plant. It results from this hypothesis that the transpiration from the leaf per unit of surface must be less, the less active the internal activity of growth, or, in other words, the larger the amount of surface which goes to the production of a given weight of dried substance. The correctness of this view was proved by the following experiments:-Young seedling cucumbers, 10 cm. long and of an average weight of 1.5 g., were each placed on June 14 in a vessel of two litres capacity, containing 1700 g. of leaf-mould, and 400 g. water. On July 17, the plants had an average leaf-surface of 1700 g., and had transpired 454 g. water. Five fully developed leaves were now removed from one plant, having a superficies of 525 2 sq. cm., and a weight of 9.42 g. These plants, from which one-half of the leaf-surface had now been removed, maintained the same amount of transpiration as the uninjured ones, showing that the surface which remained must have performed a portion of the work of the leaves that had been removed. On August 3 a still further quantity of leaves with a superficies of 88.8 sq. cm. and a weight of 8.2 g. was removed. Since the first denudation the plant had grown very quickly, having formed 10 leaves with a superficies of 1121.79 sq. cm. At the same time 16.2 g. were removed from a second plant, having a superficies of 264.1 sq. cm. After fourteen days the amount of transpiration was again nearly the same from all the plants. Those which had been denuded showed no decrease of transpiration, the substance removed being replaced by a rapid fresh production of leaf-surface. A second series of experiments gave similar results. Transpiration was also shown to be dependent on the concentration of the nutrient solutions. Experiments were made on four different species of cereals, with five different concentrated solutions, and the transpiration was found to be less in proportion to the concentration of the fluid. With those solutions in which the plant grew most rapidly, the absolute amount of transpiration was large, as was the general metastasis, but the relative proportion to the weight of newly formed substance was very small. The following is Sorauer's explanation of these phenomena. A maximum transpiration accompanies the rapid production of substance in an optimum nutrient solution. But for this fresh production a certain quantity of mineral constituents is indispensable, and these are absorbed by the roots out of the fluid. When this solution is very dilute, a larger quantity of water must be carried up; and thus, with the increase of the mechanical water of transpiration, the total quantity of water transpired increases above the optimum with the decreasing concentration of the fluid. Easily Oxidizable Constituents of Plants.*-It is a well-known fact that the juices of many plants become discoloured on exposure to * Zeitschr. Physiol. Chem., vi. (1883) pp. 263–79. See Journ. Chem. Soc.— Abstr., xliv. (1883) pp. 880-1. the air; so, too, sections of stems and roots, of leaves, and fleshy fruits which acquire a brown colour on exposure. Little has been ascertained in regard to the physiology of these changes. They obviously depend upon the oxidation of certain constituents; this is seen, for instance, on exposing grated potatoes to the air, when the uppermost layer assumes a brown colour, which by frequent turning over of the mass may be communicated throughout. The same is seen in the case of the expressed juice of the potato. Putrefaction or fermentation, and reducing agents, such as sulphurous or hydrosulphuric acid, decolorize these fluids. The juice of the white sugarbeet is even more sensitive, becoming on exposure to the air immediately of a dirty wine-red colour, then violet, brown, and finally almost black. These facts indicate the presence in plants of easily oxidizable bodies, and inasmuch as the products of their oxidation do not occur within the uninjured cells, it follows that there is either no free oxygen in the latter, or that these oxidizable substances are accompanied by other reducing substances, which hinder their oxidation, or again, that in the protoplasm oxidation affords other uncoloured products. Upon which of these three factors the colourless state of the protoplasm and cell-sap of living plants depends is not yet decided. In the study of oxidation processes in the living plant-cell, an important question presents itself, as to whether substances occur in the cell, which at ordinary temperatures unite with atmospheric oxygen without the essential co-operation in this process of the living protoplasm. Difficult as the problem is, the isolation and determination of the constitution of these easily oxidizable substances forms an indispensable preliminary step. It may be conjectured that they belong to the aromatic series. In this connection the numerous hydroxybenzene derivatives claim attention, of which many are known to be easily oxidizable. Pyrogallol in alkaline solutions greedily absorbs oxygen and becomes decomposed into carbonic anhydride, acetic acid, and a brown body of unknown nature. The dihydroxybenzenes (catechol, resorcinol, and quinol) are easily oxidizable bodies, and their methyl derivative, orcinol, is coloured red by the air. As regards derivatives of the anthraquinone series, there is the change of indigo white into indigo blue, and the behaviour of Boletus luridus, the colourless section of which becomes at once blue on exposure to the air. Lastly, there is a series of complex plant-constituents, undoubtedly benzene derivatives, although their constitution has not yet been ascertained, which exhibit many analogies to the discoloration of plant-juices. Of these brazilin may be named, the colourless aqueous solution of which becomes first yellow, then reddish yellow in the air. J. Reinke, in his endeavours to isolate the easily oxidizable constituents of the sugar-beet and potato to which the discoloration of their respective fluids is attributable, succeeded in the first instance in isolating from the beet-root a chromogen which on exposure to the air acquired a red colour. This substance he has accordingly designated Rhodogen. The product of its oxidation he terms beet-red, and he notes certain remarkable analogies between the absorption-bands of this substance, and of the colouring matter of Anchusa tinctoria, alkanet red, the spectrum of each showing three bands occupying identical positions. These investigations have therefore so far afforded proof of the existence in the colourless cells of the sugar-beet of an easily oxidizable colourless body, capable of isolation, which by itself, without the aid of the living protoplasm of the plant, can split up the oxygen molecule, forming a coloured substance. The isolation of the chromogen of the potato has not succeeded so satisfactorily. The presence of vanillin in the juice appeared to be shown by the strong odour of vanilla. Vanillin has been detected by Scheibler in raw beet-sugar. A substance resembling catechol, but not identical with it, was also separated. It would seem to be the same body discovered by Gorup-Desanez in the leaves of the Virginian creeper. It is undoubtedly an acid, and, amongst the known aromatic acids, most closely corresponds in its reactions with hydrocaffeic acid. In conclusion, the author suggests the hypothesis that these easily oxidizable bodies belong, in their physiological relations, to the retrogressive series, perhaps originating from the breaking up of albumin, or formed by the synthesis of the products of such decomposition, and that in these features the process is allied to that of respiration. Action of Light on the Elimination of Oxygen."-The following are the main results of a series of experiments by J. Reinke on Elodea : The evolution of oxygen which is dependent on light begins with a mean illumination and increases pari passu to a maximum with increasing intensity of light, this optimum corresponding nearly to direct sunlight; any further increase in the intensity of light does not increase the development of gas. Indicating the intensity of ordinary direct sunlight by 1, one-fourth that amount by 1/4, and four times that amount by 4/1, the two lower rows in the following table indicate the number of bubbles given off in 1/4 minute in two different experiments: In light of 800/1, the plant gave off in two minutes the same number of bubbles as in ordinary sunlight; the stream then ceased, the chlorophyll being bleached. In light of from 64/1 to 300/1 intensity, the gases exhaled do not contain more carbonic acid than that produced by the green plant in ordinary sunlight. From all these facts he draws a conclusion unfavourable to Pringsheim's hypothesis that chlorophyll acts as a protecting screen against the light. Red Pigment of Flowering Plants.+-H. Pick points out that those organs of flowering plants in which carbo-hydrates are present *Bot. Ztg., xli. (1883) pp. 697-707, 713-23, 732-8. + Bot. Centralbl., xvi. (1883) pp. 281-4, 314-8, 343-7, 375-83 (1 pl.). |