Appendix 2: Roots, Meristems, and Mieosis/Mitosis Tree Roots

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1 Appendix 2: Roots, Meristems, and Mieosis/Mitosis Tree Roots The roots of trees anchor the tree to the substrate, absorb water and nutrients from the substrate, and form mutualistic relationships with organisms in the environment. Root tissues produce some vital plant growth regulators, and important food storage tissues are also located in the roots. A tree is often considered simply to be a tall plant with secondary growth of the stem. In order to secure such a stature, the underground portions, the roots, must be tough and widespread. Roots, like stems, have secondary growth. All roots originate from the primary root in the seedling. The vascular system of roots is connected directly to the vascular system in the tree trunk and branches, resulting in continuous conduit of vascular tissue between the roots, branches and leaves of the tree. However, the distribution of the vascular tissue in the roots is different from that of the stem. Roots are traditionally thought of as being positively geotropic, meaning they grow downwards It is important to realize the tree roots do grow down, but that they also grow horizontally, enabling the root system of the tree to cover areas much greater than the canopy of the tree. Root growth occurs at the root tip. As cells in the meristematic region of the root tip are formed by mitosis, those on the outer (soil) side become the root cap, a protective layer of cells that facilitates the root s movement into the surrounding soil by coating the root with mucilage. The root cap cells are worn off and destroyed as the root grows into new soil. Cells on the inner (root) edge that will form the root tissue first elongate, the differentiate into specialized absorptive, vascular, storage, protective and meristematic tissues characteristic of roots. The attached diagram (Figure 1) of a longitudinal section through a growing root tip shows the locations of the root cap, the meristematic tissue, the region of the cell growth and elongation, the region of differentiation. Please note that the root hairs are modified epidermal cells that have a tremendous surface area, which enhances contact with soil particles. These cells are large enough to observe without magnification. A cross-section through the root in a region with fully differentiated tissues shows that the root is protected by an epidermis (Figure 2). The cortex, composed of loosely packed parenchyma cells, lies with the protective epidermis. This tissue functions as storage tissue, support tissue and is involved in the formation of symbiotic associations with fungi (mycorrhizae) and bacteria (actinorrhizae). Centrally located and surrounded by the cortex is the central vascular cylinder, or stele, of 1

2 the root. The outer edge of this is the endodermis composed of cells that are cemented together by what is called the Casparian strip, a water impenetrable layer of suberins and lignins. This is very different from the loosely packed cortical cells through which and around which water and dissolved materials move. Water, dissolved minerals, and organic compounds must pass through endodermal cells in order to enter the central cylinder of the root. Figure 1. The characteristic star-shaped structure inside the endodermis that can be visualized in a magnified cross-section of a root is composed of thick walled primary xylem cells. Between the rays of this xylem star are patches of phloem cells. In addition to this, immediately inside the endodermal layer, there are patches of cells, pericycle cells that retain their meristematic potential. These pericycle cells give rise to secondary roots. Note that the secondary roots developing on the root in the diagram originate in the endodermis. Roots become woody, or develop the capacity to grow secondary xylem and phloem, because the patches of pericycle cells enlarge and fuse forming a layer of secondary vascular cambium that allows for the formation of secondary xylem and phloem in the root. As this occurs, the cortex is stretched and destroyed. A cork 2

3 cambium develops outside the secondary phloem that forms the tough cells of the protective periderm. To summarize then, the root tips are regions of cell formation, elongation and differentiation. The newly differentiated tissues are the regions of maximum absorption because of the presence of the root hairs. Secondary roots arise from pericycle cells inside the endodermis. Woody roots are formed by the growth and development of secondary xylem and phloem. Figure 2. Meristems Plants increase in size and mass by adding cells to their body and by subsequent growth of those cells. The process of cell addition takes place in distinct regions of the plant called meristems. The meristem is a region in which mitotic divisions regularly take place. The number and position of the cells derived from the meristems determines at least in part, the form of the plant, which will develop. Some plants show only primary growth or growth of apical meristems, these are the meristems at the tip of roots and stems. All of the new cells in these plants 3

4 originate from these meristems and the regions immediately surrounding them. Each of these meristems is organized around a cell or group of cells that is undifferentiated and that undergoes mitotic divisions. As cells are added to these areas the plants increase in length. The mitotic activity is coordinated by hormones and tends to be seasonal. Once cells are formed they differentiate, and ultimately rigid cell walls are produced. All cells, regardless of their function and final morphology, develop from meristems. Each shoot or root and branch is capped by a meristem. Each leaf, flower or other structure develops from the meristem. Meristems on shoots are enclosed in and protected by buds. The increase in girth or diameter in trees is accounted for by the activity of another meristem. This meristem surrounds the woody parts of trees and is located under the bark (Figure 3). Because of its position it is referred to as a lateral meristem or technically, cambium. The cambium is made up of a continuous layer of cells, which are able to divide laterally. Cells produced by cambium accumulate on both sides of the cambium. Those on the inside develop into xylem while those on the outside develop into phloem. The cells composing the cambium remain undifferentiated; the cells produced by it become differentiated. The activity of the cambium accounts for the formation and accumulation of wood. Figure 3. 4

5 There may also be a cambium associated with the formation of bark. This, again, is located laterally, outside of the phloem and is called the phellogen (located in the layers of the periderm). As the tree grows and increases in diameter, the outer layers become cracked and broken. The phellogen or cork cambium maintains the continuity of the bark. This cambium adds cells; thus the bark increases to accommodate the increased girth by adding plates of cells laterally. The cork that we use for so many purposes is produced by the activity of the cork cambium. In summary, a tree has several types of meristems, each of which is noted by its location. Each branch terminated in an apical meristem. Each root and branch root terminated in an apical meristem, and the entire woody part of the tree is clothed in the vascular cambium. Phellogen contributes cells to the bark. Each meristem is a hot spot of mitotic division and each hot spot is accompanied by a zone of active and rapid differentiation. Mitosis and Meiosis Proteins are structural and biochemical components of cells. One function of proteins is to mediate chemical processes or to act as enzymes. Enzymes facilitate chemical reactions, the products of which give cells and organisms their biochemical and developmental capabilities and hence their traits. Genes are segments of deoxyribose nucleic acid (DNA) that are responsible for coding for the production of protein structure or protein synthesizing molecules. Genes are, in essence, the recipes for protein production. Genetic material is duplicated and separated accurately when new cells are formed because errors in duplication and separation of genetic material could result in biochemical changes or recipe deletions and additions that could be deleterious to the cell. In eukaryotic cells, a long strand of DNA with many genes encoded on it is combined with histone and non-histone proteins to form a unit called a chromosome. The length of the DNA molecule in a chromosome varies. An organism will have a mixture of long and short chromosomes. Different organisms have different numbers of chromosomes. In diploid (2n) organisms, the chromosomes are in pairs called homologous pairs (one chromosome having been contributed by the egg and the other by the sperm). Homologous chromosomes have genes called alleles that encode for the same traits. Such genes can, but need not be, identical. Meiosis is a process of cell division which separates the homologous chromosomes so that the gametes or spores formed have one half the number of chromosomes, one chromosome from each homologous pair in the parent cell. Meiosis is therefore, a reduction 5

6 division. A cell is said to be diploid (2n) if the homologous pairs are intact. If the homologous pairs are split, the chromosome number is reduced by a half and the cell is said to be haploid (n). For example, a human somatic cell has 46 chromosomes, 23 homologous pairs (n+23, 2n=46). A human sperm cell has 23 chromosomes, one each of 23 homologous pairs; so too does the egg cell. These gametes fuse during syngamy reestablishing the diploid number. Growth of multicellular plants and animals is the result of an increase in cell number and cell size. The cell process that reproduces, or duplicates, cells is called mitosis. In trees the cells of the apical meristem, vascular cambium, cork cambium, and pericycle retain the ability to undergo mitosis and they are therefore the sites of growth. In mitosis the genetic material is duplicated and the duplicated material is then separated into two cells, called daughter cells. This might be considered equivalent to holding two copies of a book. When chromosomes duplicate, the duplicates are called chromatids. They are held together by a centromere until they are pulled to opposite ends of the duplicating cell by structures called spindle fibers. The sequence of changes in mitosis and meiosis are usually broken down into phases. It is not important for this class to remember the stages of each process, but it is important to see how the products of each phase differ. Note that because the chromosomes are also duplicated in meiosis, there are two phases in meiosis whereas there is only one phase in mitosis. The first phase of meiotic division separated the homologous pairs; the second phase the chromatids. More application of the concepts of mitosis and meiosis will be built upon in the discussions of life cycles. For now keep in mind that mitosis results in the building of mass in the organism and that meiosis is involved in a fundamental rearrangement of the chromosomes. 6