(2) Collettchyrna differs from parenchyma. of which it is hardly inure than a variety, in the bands, (losely associated with the vascular bundles. hey are often called bastdibms, but do not always belong to the bast or ',liken, bundles. The individual eells taper at each end. the ends of adjacent cells above and below overlap, and thus give to the strands great flexibility and a tensile strength which in many eases pxecetb: that of the best steel. The cells are relatively very elongation of the cells. the absence of intercel lular spaces, and the thickening of the angles of the cells where three or more walls join (Fig. 5). These thickened parts are more highly refractive than other parts of the wall, and have a very pe culiar bluish-white lustre. Collenchyma occurs long, 1 to 4 millimeters in jute, 10 millimeters or more in hemp. 20 to It) millimeters in flax, and as much as 220 millimeters in 'ramie,' with diam eters from .01 to 0.4 millimeter. The strands of only in elongated organs (stems, petioles. etc.), where it forms a strengthening tissue beneath the epidermis.
(3) Sclerenrh yam occurs in two forms, in one of which the cells Imve their three dimensions almost equal: in the other they are greatly elon gated (Fig. 6). In both the wall is excessively thickened, sometimes so lunch so that the lumen is nearly obliterated. In all cases the protoplasm disappears at maturity, and the tissue is of use to the plant only by its mechanical strength. The short•eelled selerenchyma is common in the stone fruits (peach and cherry), in the shell of various nuts, in the gritty parts of the flesh of pears, quinces. etc.. and in the hard portions of bark, many dry fruits, and seeds. Elongated seleren ehyma cells are most abundant in stems and leaves, in which they form continuous strands or selerenehyma fibres constitute the so.called fibres of commerce, the finer ones of which are used for textile fabrics and the coarser for cordage, etc.
(4) Trachea and I racheids. Tracheills are usually elongated cells. whose walls have become Uplifted (by which they are made very pervious to water) and thickened in spiral or annular lines or in reticulate patterns (Fig. 1 ) . At maturity the protoplasm disappears, leaving only the cell wall of service to the plants. Trache:e are simi
lar to tracheids in the sculpturing of their walls, but instead of being single cells they are formed by the fusion of a row of cells lying originally end to end, the end walls being resorbed as the lateral walls thicken and the protoplasm disap pears. At maturity the long empty tubes thus formed show little trace of the cells from which they originated. In angiosperms they constitute the greater part of the xylem bundles, changing to tracheids as the bundles grow smaller and come to an end (Fig. 7b). But in gymnosperms (pines and their allies)traehea. arc formed only in the primary xylem, almost all the secondary xylem being tracheids with characteristic circu lar-bordered pits (Fig. 7a). Tracheie and tra cheids are the most efficient tissues for the trans port of water in the larger plants.
(5) Sieve tubes are cell fusions formed by the partial resorption of the end walls of a row of young cells. The end partitions and sometimes the lateral walls which adjoin other sieve tubes become perforated, forming a so-called 'sieve plate,' through which the contents of the sieve tubes (a slimy mixture of soluble proteids, car bohydrates, and other foods) pass freely. Sieve tubes are found in the phloem bundles, in which they constitute the most efficient tissue for the transport of foods (Fig. S).
(6) Latex tubes are long, much-branched tubes, with free or anastomosing branches, which con tain the milky or colored sap in certain plants. There are two sorts, articulated and non-articu lated. A non-articulated tube arises by early differentiation of certain cells in the embryo, which push their way among the other developing tissues by independent growth. The articulated tubes arise by the fusion of rather indefinite rows of cells, either longitudinal or transverse, which form a network of irregular tubes. Latex tubes are found in the phloem bundles, or just outside these bundles in the cortex of stems, and accom pany them into the leaves. where they ramify widely. Their terminals cone into close rela tion with the nutritive cells. (See illustration under LATEX.) They seem to constitute a trans portation system for foods. See GROWTH (in Plants) ; ANATOMY OF PLANTS; CELL ( in Plants); ROOT; STEM; CONDUCTION; LATEX.