HISTOLOGY. 1*14 HISTOLOGY. have nil their walls cutiiiizod. They constitute a tissue known as cork ( Fip. 4). (2) Collenchiiinn tlilTers from parenoliyma. of which it is ha nil v more than a variety, in the Fni. 4. COKK TIBSl'E. LODgitudinal section from bottle-cork. elonfration of the wlls. 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 bands, closely ass(K-iated with the vascular bundles. They are often called bast-libres, but do not always belong to the bast or phloem bundles. The individual cells taper at each end. the ends of adjacent i-ells above and Im-Iow overlap, and thus give to the strands great llcxibility and a tensile strength which in many cases exceeds that of the U'st steel. The cells arc relatively very FlO. «.. n. I ,.ni >. HVMA. 1, Irregular form (idiobliiHt) from leaf of tea; 2. surface vietv of a fibre hIiowIuk taiii*rlnK end and oblUiue pitA; .'i. transviTHe a^-ctlon of ft fibre showlug concentric struc- ture of wall with pore-pits. long, 1 to 4 millimeters in jute. 10 millimeters or more in hemi). 20 tn 40 millimeters in llax. and as much as '220 millimeters in 'ramie.' with diam- eters from .01 to 0.4 millimeter. The strands of FlO. G. COLLENCBYMA. 1, Longitudinal section; 2, transverse section. only in elongated organs (stems, petioles, etc.), where it forms a strengthening tissue beneath the epidermis. (3) Sclerenrhiima occurs in two forms, in one of which the cells have their three dimensions almost equal : in the other they are greatly elon- gated (Fig. (i). In both the wall is excessively thickened, sometimes so nuu-h 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 -celled sclerenchyma 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 scleren- chyma cells are most abundant in stems and leaves, in which they form continuous strands or FlO. 7. TRACREIDS. a. from st«m of pine; /.. foniiinie; the termination of a xylem bundle in a leaf of /mjiutlens purviffora. sclerenchyma 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) Trnchcw and trnclicidx. Tracheids are usually elongated cells, whose walls have become lignified (by which they are made very pervious
