Lecture Notes for Friday, October 9; BTNY 1210, Fall 1998

Handout today - Pet Plant Seeds and Sowing Lab Exercise

Outline

Primary growth of plants (continued)

Ground tissue and vascular tissue

Monocots and dicots

Plant organs - roots

PRIMARY GROWTH IN PLANTS (continued)

Ground tissue (soft tissue) Ground tissue is the most abundant tissue in herbaceous (i.e. non-woody plants). The cells that make up ground tissue are not very specialized compared to other plant cells. They are enlarged, but not very elongated. They are alive at maturity and only have primary cell walls (all plant cells have primary cell walls). Air spaces which allow for diffusion of gasses are commonly found between cells in ground tissue. The cells in ground tissue function in food and water storage (especially in roots), in photosynthesis (in leaves) and they have the capacity to start dividing again when needed (as in wound healing and in the initiation of branch roots). Examine Fig. 24-5 and note the air spaces between these ground tissue cells. Examine Fig. 26-21 noting the large air (intercellular) spaces that help this water lily leaf float and notice the ground tissue that makes up most of the leaf tissue. Fig. 25-10 shows the extensive amount of ground tissue that makes up roots and the stained starch grains that are stored in these cells.

Vascular tissue is the plumbing in the plant; part of it also functions in support. Vascular tissue consists of xylem tissue and phloem tissue and is usually associated with cells called fibers.

Xylem tissue functions in water transport and also provides support. Xylem cells are dead at maturity and contain secondary cell walls in which the polymer lignin permanently binds cellulose fibers together into a very strong wall. Xylem cells function as pipes through which water is pulled up a plant following evaporation of water from the leaf (through stomata). The energy for the movement of water is heat energy in sunlight which is used to evaporate water from the leaves. Examine Fig. 24-13; (d) is a primitive type of xylem cell found in ferns and conifers, but is also present in flowering plants. It functions to transport water and is also strong enough to function in support. Evolution has produced more efficient water conducting cells (shown in c, b and a) which are better at conducting water but are not as strong as the more primitive cell shown in (d). A series of the type of cell shown in (a) is shown in Fig. 24-15. As evolution produced the type of cell shown in (a), another type of cell shown in Fig. 24-13 (f) and called a fiber was also evolving. Fibers are much stronger than the other types of cells shown in Fig. 24-13, but they are no good at conducting water because they have such a narrow opening in the center of the cell.

Phloem tissue functions in food (sucrose) transport. Phloem cells are alive at maturity and only have primary cell walls. The cells in phloem tissue are lined up end to end to produce pipes through which a sucrose solution is pumped through a plant. The energy required is metabolic energy (ATP) which is used for active transport of sucrose into and out of phloem tissue. The direction of pumping is often from leaves where sucrose is produced in photosynthesis to the roots where sucrose is used in respiration. Movement may also occur up a plant - e.g. in the spring when sucrose stored in tree roots (as starch) is moved up the plant to support the growth of young leaves, or when sucrose is moved to apical meristems or to developing fruits. Fig. 24-21 shows phloem tissue.

Examine the cross section of a stem in Fig. 24-12 and find xylem tissue (large, empty cells with thick cell walls) and phloem tissue (small cells, with thin cell walls and some remaining cytoplasm).

Fibers are cells that are found in several places in the plant, but usually near the phloem. Fibers are long, dead cells with a very thick secondary cell walls containing lignin. Fibers function in support.

A Brief Detour to Introduce MONOCOTS AND DICOTS.

There are two classes (evolutionary lineages) of flowering plants (Angiosperms). Monocots (e.g. corn) have one cotyledon in the seed and their flower parts occur in 3s. The gladiolus flower you dissected had 3 sepals, 3 petals, 3 stamens and an ovary with 3 compartments. Dicots (e.g. bean) have two cotyledons in the seed and their flower parts occur in 4s or 5s. The tobacco flower you dissected in lab had 5 sepals, 5 petals and 5 stamens. Your pet plants had 4 petals.

PLANT ORGANS (back to Primary Growth in Plants)

Roots. Examine Fig. 25-2 noticing that dicots tend to have a taproot type of root system and that monocots tend to have a fibrous type of root system. Examine Fig. 25-3. Notice the root cap which forms a sacrificial tissue of slime producing cells that protects the apical meristem as it is forced through the soil by elongating cells in the root. Notice the root hairs that form a short distance behind the root tip; if they were produced closer to the root tip, they would be torn off because that part of the root is still elongating. Notice that branch roots are very different structures and that they develop from a tissue deep within the root and emerge from the side of the root by forcing their way through. Examine the pictures of the rootcap in Fig. 25-24.

Fig. 25-5 shows a longitudinal section of a corn root. Notice the distinct rootcap. Find the position of the apical meristem as the point of convergence of the columns of cells extending back from the root tip. Elongating cells behind the root tip provide the force that drives the root tip through the soil. Examine the root cross sections in Fig. 25-10. Notice how much of the root is occupied by ground tissue (with darkly staining starch grains). Notice central cylinder of vascular tissue (xylem and phloem). Examine the close up of the vascular tissue (d) and identify the phloem and xylem tissues by their cell characteristics.

In Fig. 25-18 notice how lateral (branch) roots form from a tissue deep within the root. The branch root has its origin from ground tissue cells near the xylem tissue. These cells start to divide, forming an apical meristem which produces primary tissues of the new branch root. The elongating branch root with it developing rootcap soon forces its way out the side of the main root.

Fig. 25-19 shows what are called adventitious roots - roots that develop from tissue other than root tissue - such as stem tissue or sometimes leaf tissue. In this picture of a corn stem, adventitious (or prop) roots are developing from the node of the corn stem. Orchids also produce adventitious (or aerial) roots with a multiple epidermis (see Fig. 25-21).