Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/96

86 diameter. The spiral thickened portion of the wall of the vessel may become loosened from the membrane of the wall and form a spiral fibre in the interior. These fibres are elastic, usually rounded and simple ; but sometimes two or move are combined so as to form a flat band. These flat ribands, consisting of fibres which vary _in number from two to twenty-five, or more, are met with abundantly in the stems of Bananas and Plantains, and in the shoots of Asparagus. The spiral in such cases is called compound, and the vessels pleiotrackece. The spiral fibres have such tenacity that when the vessels are ruptured they can be pulled out. This capability of being unrolled characterizes true spiral vessels (fig. 13). When the spiral is not loosened from the cell-wall and cannot therefore be unrolled, it is said to be closed. On breaking the young shoots or leaf-stalks of the Geranium, Straw berry, and Rose, or the leaves of the Hyacinth, Amaryllis, and Banana, and pulling the parts gently asunder, the fibres can be easily seen in the form of a fine cobweb. When the aerial stems of the Banana and Plantain are cut across, the spiral fibres may be pulled out in large quantity so as to be used for tinder. Generally, the coils or volu tions of the fibre are said to be left-handed, that is, turning to the left of a person supposed to be in the axis. In the garden Lettuce vessels are met with, some having the fibre turning to the left, others to the right. In the Scarlet Bean the coils of the fibres are left-handed, while the plant itself turns to the right in twining. Spiral vessels are abundant in young plants and shoots, while in the hard stems of trees and shrubs they chiefly surround the pith. Spiral vessels occasionally exhibit a branched appearance. This may arise from the union of separate vessels, or it may depend on a regular division of the fibres, as is seen in the Mistleto, House-leek, and Gourd (fig. 14). Annular vessels are those in which the thickening (or, if it be loosened from the wall of the vessel, the fibre) is in the form of rings (fig. 15). These rings in Mamrmllaria quadrispina, and in some other plants of the Cactus tribe, are very thick, and leave only a small canal in the centre of the vessel. Annular vessels are from -y^-yth to -j^tlx of an inch in diameter. In reticulated vessels (fig. 1C) the thickenings take the form of a network. All vessels of this type lose very early their protoplasmic contents, and serve to convey air. Fii Fig. 16. Fig. 13. FIG. 13. Spiral vessels of tlie Melon, showing the elastic fibres uncoiled, and the vessels overlapping at their pointed extremities a. FIG. 14. Branching fibre, from spiral vessels of Gourd (Cucitrbita Pepo). FIG. 15. An annular vessel taken from the Melon plant. FIG. 16. Reticulated vessel taken from the Melon plant.

In the process of thickening of the cell-wall, if large spaces of the cell-wall remain thin, and the thickening mass growing in a circular manner projects into the interior of the cell and gradually arches over the thin portion of cell-wall, a dome-shaped cavity is enclosed betwixt the thin cell-wall and the thickening mass. The growing thickening mass gradually contracts the opening into this cavity, but never completely closes it. On front view this presents the appearance of two concentric circles, an outer marking the edge of the original thin portion of the cell-wall, and an inner indicating the under edge of the gradually contracting ring of thickening matter (iig. 17). When this process takes place on opposite sides of the parti tion wall between two cells, there are then two similar cavi ties separated by the thin partition wall of the cells, each communicating freely by a small circular aperture with the cell in which it has been formed (figs. 1 7 and 18). In process of growth the partition wall is absorbed, and then a lenticular cavity is formed, connected by a circular aperture on each side with adjacent cells (fig. 19). W r hen viewed by trans mitted light these present the appearance seen in fig. 17 t ; such structures are termed bordered pits, and a collec tion of such cells constitutes the disk-bearing or punctated tissue of authors. It is well seen in Firs and other cone- bearing plants. It has been called glandular tissue. In the case of some fossil woods, pieces of silica, like double convex lenses, have been found in the cavities. When a vertical radial section is made of the stem of Fir, bordered pits, arranged in two rows, with individual pits on the same level, are seen. In Araucaria double and triple alter nating rows are seen; whilst in the Yewa prominent striation line winding spirally amongst the pits is noticeable. When the thickening begins by the formation of transverse ridges extending right across the wall of the cell, and the inwardly projected ridges gradually arch over the thin membranous portion of the wall, a narrow fissure only is left leading into the cavity enclosed by the thickening masses and the thin por tion of the partition wall, on the opposite side of which a similar process has pro ceeded. By the absorption of the parti tion wall a single cavity between two cells is thus produced, communicating with both, just as in the last case. Viewed by transmitted light these present an appear ance like rungs of a ladder, and hence the name scalariform applied to the cells in which they occur (fig. 20). They are specially seen in Ferns, where they give rise to long prismatic vessels. FIG. 17. Kadial longitudinal section of wood of Finns sylvestris. a, cr.mbiurn ; &, c, d, e,f, wood cells; t, bordered pits. in an early stage of formation, before the thickening ring has arched over the thin portion of cell-wall; t, bordered pit after the thickening mass has arched over the thin cell-wall ; st, very largo pits where in contact with medullary rays. Fio. 18. Diagram to show thickening dome on both sides of the partition wall. FIG. 19. 1 artition wall has given way and a single cavity is formed, com municating on both sides with adjacent cells. (These figures after Sachs.) Fig. 20. [ missing text ]

When the thickening takes place over seaianform or ladder- nearly the whole of the cell- wall, thin J ) i f k ^;.^^ vessds portions may be left here and there, which appear as pits when viewed by transmitted light, 