Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/95

these will bo noticed hereafter. The protoplasm in old cells may disappear, and then all growth ceases, and the cells consist of a mere framework; or it may remain, and then growth of the cell continues. And it is by a re-arrangement of the molecules of this protoplasm that the formation of

new cells begins, the nucleus entering also into the process.

By the term cell-sap is meant the fluid contained in the vacuoli. It consists in great part of water, in which are dissolved various salts, derived from without, and compounds formed by assimilation in the plant itself. Amongst the latter we may mention inulin, a substance closely allied to starch and sugar, found in Composite plants.

The term parenchyma (areolar, utricular, or vesicular tissue) is a general name for any form of cellular tissue, in which thin-walled cells of a diameter nearly equal in every direction are united to one another by broad surfaces (fig. 2). If the cells are pointed at both ends and have a length greatly exceeding their breadth, there is formed prosen- ckyma (fig. 8). Both tissues may be complete (figs. 2 and 8) or incomplete (figs. 9 and 10), i.e., the component cells may touch each other on every side and leave no inter cellular spaces, or intercellular spaces may exist between the cells. According to the amount of surface-growth and thickening of the cell-wall various forms of parenchy- tnatous and prosenchymatous tissue result. Thus, in the Hush and Bean we have a stellate parenchyma, with large intercellular spaces (fig. 10), in the Elder pith a complete angular parenchyma (fig 2), and in the succulent stem of the Cactus a spherical incomplete parenchyma (fig. 9). Those forms of tissue in which the individual cells have bean altered by thickening of the cell-wall are the most important ; and the alterations in the cell-wall consequent on growth in thickness may be such as to produce oblitera tion of the septum between superposed cells, and their cavities, freely communicating, then give rise to a tube or vessel, a combination of which constitutes the vascular tissue of authors. Whilst this is the nature of a true vessel, considerable confusion has arisen from the term being applied to any cell-like structure in which the longitudinal diameter exceeds the transverse ; and thus the difference between a cell and a vessel became one of length only. The term vessel ought to be restricted to such as are formed by coalescence of cells.

Fig. 10.

Fig. 3. Fio. 8. ProscnchymatousJccIls FIG. 9. Incomplete parenchj ma. FIG. 10. Stellate or star-/ike cellular tissue of the Bean, with intercellular spaces or lacuna;

Under the term pleitrenchyma (fig. 8) is included tissue composed of such elongated prosenchymatous, flexible, thickened cells, as are found in the bast or phloem layers of ordinary trees. They also occur in the wood portion. Their walls are thickened regularly, and they con stitute when united what is commonly known as the woody or bast fibre. The diameter of the woody fibres varies from TTyVyth to 7r y th of sm inc}i. The materials used for ropes and cordage, linen, certain Indian muslins, mummy-cloths, and mats consist of the woody fibre of plants from which the more delicate tissues have been removed by maceration in water. Flax or lint is thus procured from the bark of Linum usitatissimum, hemp from Canuabis sativa, New Zealand flax from Phormium tenax, Pita flax from Agave americana, Sun-hemp from Hibiscus cannabinus, and bass or bast from the common Lime or Linden-tree. Fibres are also procured for manufacture from the Pine-apple plant (Ananassa sativa), from Yucca gloriosa, from Buh- meria nivca, which yields the Indian llheea fibre, from most of the plants belonging to the Mallow and Nettle tribes, and from some Leguminous plants, such as Crota- laria juncea, which supplies a kind of Bengal hemp. If the maceration of the fibre is carried to a great extent, a pulp is formed from which paper is manufactured. Pleu- renchyma does not occur in cellular plants, such as Lichens, Sea-weeds, and Mushrooms. The tissues of these plants speedily disappear under the action of water, and hence, perhaps, the reason of their rarity in a fossil state. In the very young state woody cells arc delicate, and it is only in proportion as they attain maturity that their walls acquire a thick consistence. In the sap-wood of ordinary trees the woody cells are thickened in their walls, but are pervious ; while in the heart-wood they are rendered solid by the thickening matter, which is often variously coloured.

Fl Sfc AimuUur ccl1 from fhe FIG. 12. Spiral ceil from an ...

If the thickening of the cell- wall takes place so that a network, ring, or spiral of thickening matter is formed, then the cells are reticulated, annular (fig. 11), or spiral (fig. 12), as in the leaves of Sphagnum, hairs of Cactacete, and seed-coat of Casuarina. In these cells the spiral thickening fre quently becomes loosened from the cell- wall as a spiral fibre, and can be unrolled. Such forms oc cur in the outer covering of the seed of Collomia linearis and of the fruit of Salvia Verbenaca. In these, when placed in water, the spirals rupture the softened membrane of the cells, and spread out wards. The spongy elastic character of the outer cellular covering of the roots of tropical Orchids and Aracese, of the sepals of Illecebrum verticillatum, of the pericarp of Cachrys Morisoni and C. odontalgica, and of the ribs of the fruit of vEthusa Cynapium, is due to the presence of spiral cells. In the reproductive cells of Hepaticaa spiral fibres called elaters are found in connection with the spores. Reticulated or netted cells, produced by fibres forming a sort of mesh or network, occur in the wing of the seed of Swietenia, in the pericarp of Picridium tingitanum and P. vulgare, in the seed-coat of Cucurbita Pepo, in the paren chyma of the leaf of Sanseviera guineensis, and in isolated cells of the pith of Ilubus odoratus and of Erythrina Corallodendron.

If spiral, annular, or reticulated cells are arranged

in a longitudinal series, and the septum between adjoin ing cells gives way, then we have a spiral, annular, or reti culated vessel (figs. 13, 15, 16) formed, and to this tissue authors have given the name trachenchyma, on account of its resemblance to the tracheae or air-tubes of animals.

These vessels vary from g^Vfftk to s^th of an inch in