Vascular Tissue -- Xylem and Phloem

Ground Tissue -forms the bulk of the plant. Parenchyma cells have thin walls and intercellular air spaces. The walls of collenchyma cells are much thicker than those of parenchyma cells. Sclerenchyma cells have very thick walls and are nonliving – their only function is to give strong support. -are hollow, nonliving support cells with secondary walls. -thin-walled and capable of photosynthesis when they contain chloroplasts. -have thicker walls for flexible support (celery strands).

Plant Ground Tissues Thick cell wall Strength and support Thin cell wall Storage & photosynthesis Uneven cell wall Flexible support SCLERENCHYMA PARENCHYMA COLLENCHYMA

Two Kinds of Plant Vascular Tissue - Vessel Elements- pitted cell wall. Water pipeline - Tracheids - pitted Xylem Phloem - Carries H2O, dissolved nutrients. - Upward movement. - Dead at maturity. - Carries products of photosynthsis. Sieve tube - perforated end walls - transport sugar - Companion Cells- swirl cytoplasm to push sugar up or down - Up and downward movement. - Alive at maturity Sieve-tube Companion cells Vessels Tracheids

Vascular Tissue Two types of vascular (transport) tissue: Xylem transports water and minerals from roots to leaves and contains two types of conducting cells: tracheids and vessel elements. Phloem transports organic nutrients from leaves to roots and has sieve-tube elements with companion cells, sieve plates. Both tracheids and vessel elements are hollow and nonliving, but vessel elements are larger, lack transverse end walls, and are arranged to form a continuous pipeline for water and minerals. The elongated tracheids, with tapered ends, form a less obvious means of transport, but water can move across the end walls and side walls through pits where the secondary wall does not form. Xylem also contains parenchyma cells that store various substances, and fibers and sclerenchyma cells that lend support. The conducting cells of phloem are sieve-tube elements each of which has a companion cell. Sieve-tube elements contain cytoplasm but no nuclei. These elements have channels in their end walls that in cross section make them resemble a sieve. Plasmodesmata (sing., plasmodesma), which are strands of cytoplasm, extend from one cell to another through this sieve plate. The smaller companion cells have a nucleus in addition to cytoplasm; the nucleus of the companion cell controls both the companion cell and the sieve-tube element. Vascular tissue is located in the vascular cylinder in dicot roots, in vascular bundles within stems, and in leaf veins in leaves.

Xylem structure Leaves Xylem transports water and minerals from roots to leaves Contains two types of conducting cells: tracheids and vessel elements. Water This drawing shows the general organization of xylem tissue. Roots

Tracheids lie along side other tracheids, over-lapping extensively, so that water can flow out of the pits of one cell into an adjacent cell. This allows long range transfer of water and solutes, although (since the cells are dead) the flow has to be passive, pulled by an external force. Water Flow (passive flow) The driving force for this flow is hydrostatic pressure, coming partly from root pressure (pushing up wards) but mainly from the suction pressure created by water being evaporated from leaves. Passive water flow in plants is upwards.

Softwoods (conifers) – tracheids only Hardwoods – note the larger bore of the vessel elements

In Angiosperms - Vessel elements, idealised Vessel element, here with a open end (simple perforation plate). A perforated (scalariform) perforation plate Tracheids

Phloem structure Transports organic nutrients from leaves to roots Has sieve-tube elements with companion cells cells at sieve plates. This drawing shows the general organization of phloem tissue.

Figure: 23.6 Title: The structure of xylem Caption: Xylem is a mixture of cell types, including parenchyma and sclerenchyma fibers (ground tissue) and two types of conducting cells, tracheids and vessel elements. Tracheids are thin with tapered, overlapping ends joined by pits. The pits are not holes; they consist of a water-permeable primary cell wall that separates the interiors of adjoining cells. Vessels consist of vessel elements stacked atop one another. The end walls of some vessel elements are absent, and others have narrow openings that connect adjacent vessel elements. Both tracheids and vessel elements have pits in their side walls, allowing water and dissolved minerals to move sideways between adjacent conducting tubes.

Figure: 23.7 Title: The structure of phloem Caption: Phloem is a mixture of cell types, including sclerenchyma, sieve-tube elements, and companion cells. Sieve-tube elements have primary cell walls and a thin layer of cytoplasm around a fluid-filled core. Sieve-tube elements, stacked end to end, form the conducting system of phloem. Where they join, sieve-tube elements form sieve plates, where membrane-lined pores allow fluid to pass from cell to cell. Each sieve-tube element has a companion cell that nourishes it and regulates its function.

Figure: 23.25 Title: The pressure-flow theory Caption: This theory relies on differences in hydrostatic pressure to move fluid through phloem sieve tubes. 1) A photosynthesizing leaf manufactures sucrose (red dots), which 2) is actively transported (red arrow) into a nearby companion cell in phloem. The sucrose diffuses into the adjacent sieve-tube element through plasmodesmata, raising the concentration of sucrose in the sieve-tube element. 3) Water (blue dots) leaves nearby xylem and moves into the “leaf end” of the sieve tube by osmosis (blue arrow), raising the hydrostatic pressure as increasing numbers of water molecules enter the fixed volume of the tube. 4) The same sieve tube connects to a developing fruit. At the “fruit end” of the tube, sucrose enters the companion cells by diffusion through plasmodesmata. It is then actively transported out of the companion cells and into the fruit cells. 5) Water moves out of the tube by osmosis, lowering the hydrostatic pressure within the tube. 6) High pressure in the leaf end of the phloem and low pressure in the fruit end cause water, together with any dissolved solutes, to flow in bulk from leaf to fruit.