_Latex cells._—Large, often much elongated cells, _m_, lying in the parenchyma, _p_, fig. 27, which are packed with contents. Compare fig. 107, _s_.] [Illustration: Fig. 28 _Sclerenchyma._—Thick-walled cells among parenchyma for strengthening, fig. 28. Compare fig. 34, _s_.] [Illustration: Fig. 29 _Cork._—Layers of cells replacing the epidermis in old stems. Outer cells, _o_, crushed; _k_, closely packed cork cells; stone cells, _s_, fig. 29. Compare fig. 95, _k_. _Cork cambium._—Narrow, actively dividing cells, _c_ in fig. 29, giving rise to new cork cells in consecutive rows.] [Illustration: Fig. 30 _Tracheides._—Specially thickened cells in the parenchyma, usually for water storage, _t_, fig. 30. Compare fig. 95, _t_.] Vascular Tissue [Illustration: Fig. 31 _Wood._—_Protoxylem_, tracheids and vessels, long, narrow elements, with spiral or ring-like thickenings, _s_^1 and _s_^2, fig. 31. Compare fig. 81, A, _px_, for fossil. _Metaxylem_, long elements, tracheids and vessels. Some with narrow pits, as _t_ in fig. 31; others with various kinds of pits. In transverse section seen in fig. 33, w, fossil in fig. 114, _w_. _Wood parenchyma._—Soft cells associated with the wood, _p_ in fig. 31. Fossil in fig. 81, B, _p_. _Wood sclerenchyma._—Hard thickened cells in the wood.] [Illustration: Fig. 32 _Bast._—_Sieve tubes_, long cells which carry foodstuffs, cross walls pitted like sieves, _s_, fig. 32. In transverse section in fig. 33. _Companion cells_, narrow cells with rich proteid contents, _c_, fig. 32. In transverse section at _c_, fig. 33. _Bast parenchyma._—Soft unspecialized cells mixed with the sieve tubes, _p_, fig. 32. _Bast fibres._—Thick-walled sclerenchymatous cells mixed with, or outside, the soft bast.] [Illustration: Fig. 33 _Cambium._—Narrow cells, like those of the cork cambium, which lie between the wood and bast, and give rise to new tissues of each kind, _cb_, fig. 33. Compare fig. 114, fossil.] There are, of course, many minor varieties of cells, but these illustrate all the main types. Among the early fossils, however, one type of wood cell and one type of bast cell, so far as we know, are not present. These cells are the true _vessels_ of the wood of flowering plants, and the long bast cells with their companion proteid cells. The figure of a metaxylem wood cell, shown in fig. 31, _t_, shows the more primitive type of wood cell, which has an oblique cross wall. This type of wood cell is found in all the fossil trees, and all the living plants except the flowering plants. The vessel type, which is that in the big wood vessels of the flowering plants, and has no cross wall, is seen in fig. 20, _x_.