There are, however, in certain families of Angiosperms peculiar tissues which are not
with
the Pteridophytes. Such, for example, is the
found in Compositae and Euphorbiaceae, which takes the form of long, usually branched tubes which penetrate the other tissues of the plant in a general longitudinal direction. The tubes possess a delicate layer of protoplasm with numerous nuclei lining the walls, while the interior of the tube (corresponding with the cell-vacuole or cavity) contains a fluid called latex, consisting of an emulsion of fine granules and drops of very various sub stances, suspended in a watery medium, in which other substances (salts, sugars, rubber-producers, tannins, alkaloids and various enzymes) are dissolved. Of the suspended substances, grains of caoutchouc, drops of resin and oil, proteid crystals and starch grains may be mentioned. Of this varied mixture of substances some are undoubtedly plastic (i.e., of use in constructing new plant tissue), while others are apparently end-products of meta bolism, secreted within the plant-body. The relation of the laticiferous tissue to the assimilating cells, under which they often run, and the fact that where this tissue is richly developed the conducting parenchyma of the bundles, and sometimes also the sieve-tubes, are poorly developed, as well as various other facts, point to the conclusion that the laticiferous tissue has an im portant function in conducting plastic materials, in addition to acting as a reservoir for excreta. As a secondary function we may
recognize, in certain cases, the power of closing wounds, which results from the rapid coagulation of latex in contact with the air. The use of certain plants as rubber-producers (notably Hevea brasiliensis, the Para rubber tree), depends on this property of coagulation. The trees are regularly tapped and the coagulated latex which exudes is collected and worked up into rubber. Opium is obtained from the latex of the opium poppy (Papaver somni ferum) which contains the alkaloid morphine.
Laticiferous tissue is of two kinds : (1 ) laticiferous cells, which branch but do not anastomose, and the apices of which keep pace in their growth with the other tissues of the plant (Apo cynaceae and most Euphorbiaceae), (2) laticiferous vessels, which are formed from rows of actively dividing (meristematic) cells. The end walls of these cells break down so that a network of laticiferous tubes arises (Papaveraceae, Hevea). In some cases, as in Alllium and in the Convolvulaceae, rows of cells with latex-like contents occur, but the walls separating the individual cells do not break down.
Such facts serve to illustrate the advanced specialization of the higher plants compared with the land bryophytes, but by no means exhaust the anatomical peculiarities of the former. In particular, mention may be made of tissues which arise by modi fication of primary living tissues, and are accordingly known as secondary tissues.