Plants: Roots, Stems and Leaves - Laboratory | BOT 1010, Lab Reports of Botany and Agronomy

Download Plants: Roots, Stems and Leaves - Laboratory | BOT 1010 and more Lab Reports Botany and Agronomy in PDF only on Docsity! Plants: Roots, Stems and Leaves 85 Plants: Roots, Stems and Leaves Unlike animals, plants only have 3 organs, the roots, the stems and the leaves. Stems and leaves together form the shoot of a plant. These 3 organs, elaborated in different ways, make up everything that you find on a plant, whether it’s the trunk of a tree, the pitcher of a pitcher plant, or the spines of a bouganvillea. Because there are only 3 organs, these are often highly modified or altered in appearance between different plants or even on the same plant. One aspect of understanding plants is to be able to figure out what is root, stem, and leaf. Roots, stems, and leaves are produced by the activity of apical meristems . Apical meristems are at the tips (or apices) of plant parts. The shoot apical meristem is at the tip of the shoot, while the root apical meristem is at the tip of the root. The cells of the meristem divide actively; some cells stay to maintain the meristem, while other cells differentiate. In the shoot apical meristem, the differentiating cells produce the stem and the leaves, which are lateral outgrowths on the stem. The presence of leaves along the stem divides the stem into nodes, where the leaves are attached, and internodes, the part of the stem between the nodes (Figure 1). Figure 1. Shoot structure. Plants: Roots, Stems and Leaves 86 Leaves differ from stems in not having an apical meristem, so leaves are determinate (i.e., limited in their growth), while stems are indeterminate (theoretically capable of grow- ing forever). In the root apical meristem, the differentiating cells produce the root cap, a structure that protects the root apical meristem as it pushes its way through the soil, and the root body, which is the part of the root that we see. Thus, the apical meristems of the root and shoot differ in their structure—the root apical meristem is internal, surrounded by cells on all sides, whereas the shoot apical meristem is external and not covered by cells. You usually need to look at sections of plants under the compound microscope to see these differences, but on some plants, such as the screw pine or Pandanus, next to the OE pond on campus, you can clearly see the root cap of the prop roots before they enter the ground. Examine plants on campus, identifying roots, stems, leaves, apical meristems and axillary buds. Both roots and shoots can branch. The branches form more roots, if they are root branches, and more shoots, if they are shoot branches. Root branches are produced inside the root itself, breaking out through the root, while shoot branches form from axillary buds. Axil- lary buds are produced in the upper angle between the leaf and the stem, which is called the axil of the leaf (Figure 1). Leaves are produced in a very organized manner at the shoot apex. This results in a predictable arrangement of the mature leaves on the stem. This arrangement is called the phyllotaxis of the leaf. Common patterns are for the plant to produce 1 leaf at a time at the apex, resulting in an alternate phyllotaxis. Sometimes twp leaves are produced at a time at the apex, with successive leaf pairs at 90o from each other. This is an opposite phyllotaxis. If more than twp leaves are produced at a time, the phyllotaxis is whorled, but this is a much more rare occurrence. See the examples in Figure 4. Figure 2. Plants: Roots, Stems and Leaves 89 Phyllotaxis and Fibonacci Numbers Interestingly, alternate phyllotaxis of plant parts is often associated with spiral patterns. These patterns can be easily observed in the arrangements of sunflower seeds in a sunflower head, florets of daisys, scales of pinecones, and segments of pineapples. In these patterns, two types of spirals are apparent to the eye: clockwise spirals and counterclockwise spirals. Surpris- ingly, the number of spirals in each direction are usually adjacent numbers in the Fibonacci sequence of mathematics. The Fibonacci sequence has the following form: 1, 1, 2, 3, 5, 8, 13,... where each succeeding number is the sum of the two before it. This remarkable connec- tion between botany and mathematics has been studied by botanists, mathematicians, and reproduced from: http://ccins.camosun.bc.ca/~jbritton/fibslide/fib31.gif Plants: Roots, Stems and Leaves 90 Wood Wood is secondary xylem. Xylem is the plant tissue that conducts water and mineral nutrients. The conducting cells of xylem (tracheids and vessels), which are dead at maturity, have secondary cell walls with lignin in addition to cellulose (the main component of primary cell walls). Lignin is a complex polymer that is strong and resists decay. Some wood also has lots of fibers , which don’t conduct but have VERY thick, lignified, secondary cell walls. These cells provide additional strength to wood. Secondary xylem is produced by a meristem that is on the sides of the stem or root, so it is called a lateral meristem. These completely surround the stem or root. Not all plants have lateral meristems. For example, most monocots, such as palms, grasses, and orchids, do not. The primary meristems are the apical meristems. They cause the plant to lengthen, while the lateral meristems cause the plant to increase in width (Figure 5). There are 2 lateral meris- tems. The lateral meristem producing wood is the vascular cambium; the other lateral meris- tem makes the outer part of the bark, called cork. Figure 5. Plants: Roots, Stems and Leaves 91 The vascular cambium produces xylem cells to its interior and phloem, or carbohy- drate-conducting cells, to its exterior (Figure 6). The phloem does not have many lignified cells, and because it is to the outside of the growing xylem and vascular cambium, it is con- tinually crushed by their growth. Thus, the secondary phloem is not persistent, becoming part of the bark. The secondary xylem, however, can last for many years, with the rest of the plant growing like a skin outside of it. In addition to conducting cells and fibers, the wood can have parenchyma cells, which are living cells. It can also have other specialized cells, such as ones that make resin in pine trees. Most of the cells of wood are elongated parallel to the direction of growth. Some cells, however, run radially, making up the rays of the wood. When you cut a piece of wood, it looks very different, depending on the orientation of the cut. If you cut a cross-section or transection, you look down on the ends of the elongated cells. If you make a longitudinal cut along a radius, you parallel the rays and can see the length of the elongated cells. If you make a longitudinal cut tangential to the stem, you see the elongated cells, but you also look end-on at the rays (Figure 5). The arrangement and types of cells in wood define its grain. Different species have different grains. Two major differences are softwoods , which come from coni- fers, and hardwoods , which come from angiosperms. The wood of softwoods has tracheids but no fibers or vessels and little parenchyma, except in the rays. The wood of hardwoods has tracheids, vessels , parenchyma and fibers. Thus, softwoods have a very homogeneous Figure 6.