Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/23

13 VEGETABLE.] I S T O L O G Y 13 bll for- ! ation. rijuvcn- icencc. onjnga on. ree-cell orma- ion. distinguished in the cell. When those spheres arc of syrup they quickly disappear, no trace remaining in a few minutes, while, as Kraus (Bot. Zcitung, 1877, p. 329) has shown, if the substance be inulin the formation of sphserocrystals rapidly occurs. Hespericlin may be obtained from the unripe fruit of orange, &c., in the form of spheerocrystals when treated with alcohol. Tannin is present in the cells of many plants, and may be seen, when water is applied to the section, in the bark of the oak or birch in the form of fine granules which soon dissolve. A bluish black or greenish colour or precipitate is produced by the action of salt of iron, and a dark red-brown with bichromate of potash. Colouring matter (anthocyan) gives red and blue colours to ilowers and a red colour to stems and leaves, and is dissolved in the cell-sap. Lastly, calcium oxalate, which is formed in plants by the metastasis of nutrient matters during growth, is got rid of in many parts of plants, or rendered harmless in others, by crystallizing out, either as large crystals, prismatic or octohedral, or in masses of small crystals, or in the form of long needle-like crystals or raphides belonging to the trimctrie system. The two forms differ only in the quantity of water of crystallization present : the raphides have two equivalents, the prisms six of water. Cytoyenesis. The enlargement of organs of plants is not only accompanied by an increase in the size of the individual cells, but new cells are also formed in the part, these new cells, which .are at first small, soon enlarging to their full dimensions. Usually the formation of a new cell takes place by the division of the protoplasm of a pre-existing cell, the mother-cell, into two portions of equal or unequal size, the daughter-cells. These daughter-ceils in turn enlarge, and may each become the mother-cells of new daughter-cells. In this way by cell-division the vegetative cells of plants are increased in number. The process of reproduction in plants is invariably associated with the formation of a new cell or cells, and in general the process is very different from that of division, there being often a diminution in the number of cells, instead of an increase. Four types of Cytogenesis may be distinguished : (1) Rejuven escence ; (2) Conjugation; (3) Free-cell formation ; and (4) Division. In rejuvenescence, the whole protoplasm of the mother-cell under goes contraction and rounding ; water is eliminated, and an entire rearrangement of the molecules of the protoplasm may be noticed by changes in the contents. As a result of these changes one new daughter-cell is formed from the entire protoplasm of the mother- cell. Rejuvenescence is observed in the formation of the swarm- spores, non-sexual reproductive organs, of some algre, such as CEdogonium and Vauchcria, as also in the formation of single spermatozoids. The egg-cell of many algre and fungi, as well as of the vascular cryptogams, is formed by rejuvenescence, the only difference being that here the daughter-cell remains inside the wall of the mother-cell until fertilization, when it forms a wall and begins to divide. Conjugation consists in the union of two, rarely more, masses of protoplasm, nearly or. quite similar in size and appearance, to form a single new daughter-cell, which then becomes surrounded by a wall and forms a zygospore. The union of the two masses is always accompanied by rounding and contraction of the masses and a complete molecular rearrangement of the protoplasm. Con jugation is seen in the group of the Conjugates among the alga?, and also in the Zygowycctcs and Myxomycctcs among the fungi. In all cases conjugation is a reproductive process. The conjugation in the Myxomycctcs is very peculiar, the numerous small masses of protoplasm (the myxoamrebfe) fusing into a naked mass of proto plasm (the plasmodium). Free-cell formation consists of tho formation of several (rarely one) cells from and in the protoplasm of the mother-cell, the whole of the protoplasm not going to form daughter-cells. Free-cell for mation may be typically observed in the formation of the aseospores of the Ascomyc.ctcs. The nucleus of the large mother-cell or ascus disappears, and two new ones form, which again and again divide, thus forming eight, each nucleus forming the centre of a new mass of protoplasm, which at length becomes surrounded by a wall. In other cases many new masses of protoplasm form after the disappearance of the nucleus of the mother-cell ; and these new masses develop a wall and a nucleus, or very rarely no nucleus forms. The endosperm in the embryo-sac ofPhascolus and other phanerogams is formed by free-cell formation, the cells after attaining a certain size fusing together and forming a tissue, the individual cells of which divide. In some fungi, Feronospora, Cystopus, &c., only one daughter-cell is formed in the protoplasm of the mother-cell. The last variety is cell-division, the whole of tho protoplasm of the mother-cell going to form two, rarely more, daughter-cells. The process may be observed in the cells of Sj)irof/i/ru, in the cells of the hairs of Tradcscantia,, or in the cells near the growing points of the roots or stems of plants. Spiroyyra and Tiadcscantia may be observed in a hanging drop of fluid, water in the case of Spiroyyra, a dilute sugar solution (1 per cent.) in the other. In Sjrirogyra a ring-like groove forms round the protoplasm in the centre of the cell, gradually deepening until the nucleus divides, and the two portions of protoplasm become separate. As the separation of the protoplasm goes on, the wall forms a ring-like projection of cellulose, which gradually extends inwards until only a small central hole is left ; this soon fills up, and the mother-cell is separated into the two daughter-cells. Cell-division can be seen in the hairs of the young stamen of Tradcscantia virginica. A small unopened bud about one-fifth of an inch long is recommended by Strasburger. 1 The entire stamens are removed, and one, with the small hairs attached, is to be placed in the 1 per cent, sugar solution in a hanging drop. The cover glass on the under side of which the hairs are arranged must be very thin, to permit of observation with an immersion object-glass magnifying about 600 diameters. The cell-division in the last three cells of tho hair can be readily observed, as well as the peculiar be haviour of the nucleus, its solution, and the formation of the barrel- like body &quot; Kerntonne.&quot; These and other changes, which had been fully described by Strasburger ( Ucbcr Zellbildung und Zelllhcilung) in Spirogyra and other cells from specimens hardened and fixed in absolute alcohol, can be seen in the living cell of Tradcscantia. In the pollen of monocotyledons and the tissue cells of many dicotyledons, as in the pith and epidermis, the division of the cell differs slightly from that seen in Spirogyra. The nucleus of the mother-cell divides into two sister nuclei, and the protoplasm separates into two portions, the wall forming at once as a plate stretching right across the mother-cell and cutting it into two daughter-cells. The process of division can rarely be observed in living cells; hence it is necessary to make use of specimens killed during the process of division by immersion in absolute alcohol, or in a 1 per cent, solution of osmic acid. Special modifications of the process of cell-division may be observed in yeast (Saccharomyccs), in the formation of styloconidia as in Penidllium, and of the basidiospores of the Basidiomycdcs, as also in CEdogonium, in the sporangia of Saprolcgnia, and in the spores of the higher cryptogams. In yeast a portion of the cell-wall enlarges in a sac-like manner, and into it a portion of the protoplasm of the mother-cell passes, thus forming two daughter-cells of very different sizes ; when the smaller cell is full-grown a wall separates the two, and they become detached. In Fenicillium, and in the formation of basidiospores, a very similar process is seen. In CEdogonium division of the cells is preceded by the formation of the curious cap-like structures at the apex of the cell due to local intercalar growth of the wall. 2 In Saprolcgnia the protoplasm of the mother-cell divides into a large number of daughter-cells, which are liberated as ciliated swarm-spores, and afterwards form a cell-wall. Lastly, in the spores of the higher cryptogams the division of the mother-cell into four daughter-cells is observed. Union of Cells to form Tissues. Cells are usually united Tissues together to form an aggregate governed by some common true and law of growth. Such an aggregate of cells is called a tissue. a Tissues are formed in different ways, and in accordance with their mode of formation are distinguished as true and false. A -true tissue is formed by cell-division. In the young growing part of the plant the young active cells are all capable of dividing, a transverse wall cutting the mother- cell into two daughter-cells, the process being repeated for some time. In this way the tissues of the higher plant are formed, either originally from a single cell (apical cell) at the apex of the part, or from several cells (initial cells) situated at the growing point. In some of the lower plants false tissues are formed, rarely in some of the higher ones. The first mode of formation of a false tissue is noticed in some of the algae, as in Pediaslrvm and in Jfydrodictyon, as well as in the formation of the endosperm in the embryo- sac of many plants, as in Phaseolus, Gnetum, &c. Here the cells are at first separate and distinct, but these loose cells become aggregated together, often, as in Pediastrum and Hydrodidyon, to form a beautiful and regular figure. In such instances the wall separating two cavities is a double structure formed by the union of two distinct walls. In the endosperm of the higher plants, when the false 1 &quot;Tidier ciii 7U Demonstration geeignetes Zelltheilungs-Objeot,&quot; Sitzunrjsberichte der Jcnaischcn Gcsdlschaft fur Medicin und Natur- wisscnschaft, July 18, 1879. 2 Fully described by Strasburger, Uclcr Zellbildung und Jdltheilung (2d eel.), p. 73.