But in other woods, the ends of some cells which meet endwise are absorbed, thus forming a continuous series of elements which constitute an open tube. Such tubes are known as pores, or vessels, or "tracheæ," and sometimes extend thru the whole stem. Besides this marked difference between the porous and non-porous woods, the porous woods are also distinguished by the fact that instead of being made up, like the conifers of cells of practically only one kind, namely tracheids, they are composed of several varieties of cells. Besides the tracheae and tracheids already noted are such cells as "wood fiber," "fibrous cells," and "parenchyma." Fig. 19. Wood fiber proper has much thickened lignified walls and no pits, and its main function is mechanical support. Fibrous cells are like the wood fibers except that they retain their protoplasm. Parenchyma is composed of vertical groups of short cells, the end ones of each group tapering to a point, and each group originates from the transverse division of one cambium cell. They are commonly grouped around the vessels (tracheæ). Parenchyma constitutes the pith rays and other similar fibers, retains its protoplasm, and becomes filled with starch in autumn. [Illustration: Fig. 19. Isolated Fibers and Cells. _a_, four cells of wood parenchyma; _b_, two cells from a pith ray; _c_, a single cell or joint of a vessel, the openings, x, x, leading into its upper and lower neighbors; _d_, tracheid; _e_, wood fiber proper. _After Roth._] The most common type of structure among the broad-leaved trees contains tracheæ, trachæids, woody fiber, fibrous cells and parenchyma. Examples are poplars, birch, walnut, linden and locust. In some, as ash, the tracheids are wanting; apple and maple have no woody fiber, and oak and plum no fibrous cells. This recital is enough to show that the wood of the broad-leaved trees is much more complex in structure than that of the conifers. It is by means of the number and distribution of these elements that particular woods are identified microscopically. See p. 289. [Illustration: Fig. 20.] _Ring-porous woods._ Looking thru the microscope at a cross-section of ash, a ring-porous wood, Fig. 20: (1) The large round or oval pores or vessels grouped mostly in the spring wood first attract attention. Smaller ones, but still quite distinct, are to be seen scattered all thru the wood. It is by the number and distribution of these pores that the different oak woods are distinguished, those in white oak being smaller and more numerous, while in red oak they are fewer and larger. It is evident that the greater their share in the volume, the lighter in weight and the weaker will be the wood. In a magnified cross-section of some woods, as black locust, white elm and chestnut, see Chap. III, beautiful patterns are to be seen composed of these pores. It is because of the size of these pores and their great number that chestnut is so weak.