Page:The American Cyclopædia (1879) Volume XIII.djvu/595

 PLANT 579 FIG. 8. Section of Melon Stem. Bundles of "Wood Fibre surrounded by Cel- lular Tissue. wood, and are termed bast cells or bast fibres. The bast tissue is so developed in the linden or basswood tree that it is used for coarse fabrics; it is this that constitutes the usual fibres of the flax and hemp plants, and of others which furnish fibre in their bark. Mix- ed with the woody fibres, and modifications of them, are ducts or vessels, as they are some- times called; they are much larger than the ordinary wood cells, and are often continuous tubes of considerable length, evidently formed from a row of cells placed end to end with the intervening partition obliterated ; these are dotted by the appearance of thin places when young, and of pores when old; other ducts are found with the lining disposed in rings (annular ducts), and others (spiral vessels) are lined by a continuous spiral thread, which may be seen on carefully breaking a rose leaf across and gently separa- ting the two parts. Besides these, there are milk tubes and oil tubes in plants, with special secretions, and intercellular spaces. The cell is the elementary organ of the plant, every portion of which is made up of an ag- gregation of cells of some kind. In the ger- minating seedling the stem or radicle is main- ly cellular tissue, but very early there appear bundles of woody fibre, which increase in num- ber as the stem grows upward and produces leaves. Growth of Dicotyledonous Stems. In dicotyledonous plants the fibro-vascular bun- dles are placed to form a ring between the centre and circumference of the stem ; in an- nual stems the woody fibre is not so strong or so abundant as in perennial stems; in the former there is often a considerable space be- tween the bundles of wood, while in the other they are crowded and show on a cross section as a nearly complete ring separating the cen- tral portion or pith from the outer portion or bark ; it will be noticed that plates of cellu- lar tissue pass from the central pith between the bundles or wedges of wood and keep up a communication with the outer portion, or bark. A cross section of a woody stem of a dicotyledon shows in the centre the pith, then a ring of wood, and outside of this the bark, with medullary rays or lines of pith running through the wood and connecting the two; the pith is simple cellular tissue; the wood, besides proper woody fibre, contains spiral vessels and ducts, often so large as to be visi- ble without a glass. The bark, which early in the life of the stem was simply cellular tis- sue covered by the epidermis, is found at the end of the season to consist of several parts ; that portion next to the wood of the stem has bast cells formed in it, and becomes the in- ner bark or liber ; outside of this is the green layer, which is much like the cellular part FIG. 9. Maple Stem one year old. a to c, bark; c to , wood;, corky layer; pe, green layer;/, liber; c, cam- bium layer ; /, woody fibre ; vp, dotted ducts ;, spiral vessels. FIG. 10. Maple Stem three years old. 1, 2, and 8, wood of first, second, and third years; c, cambium layer, be- yond which is the bark of three years; me, cellular tis- sue connecting by medullary rays with the pith m; I, liber. Other letters refer to the same parts as in fig. 9. of leaves; this soon becomes covered by the corky layer, and to this is due the color most