Woody Plants BI237 Week 10 ROOTS FIGURE 1. ZONES OF THE ROOT ROOT CAP: thick layer of cells at forefront of growing tip. Cells secrete mucigel to help them glide through the soil but the soil is just plain scratchy and these cells are constantly being destroyed. ZONE OF CELL DIVISION: Includes the apical meristem, which is a group of cells specialized for the production of new cells, located at the farthest point of the tissue or the organ produced (found in both shoots and roots). Very different from animal growth! Animals have diffuse growth; plants have localized growth. It would be impossible for the whole root to expand at once; the entire root would have to slide through the soil. With apical growth, the tip is pushed through the soil. This zone also contains a region called the "quiescent center", where cells are not mitotically active. These cells are resistant to harmful factors such as radiation and toxic chemicals, and they serve as a reserve of healthy cells. They can form a new apical meristem if one is damaged by sharp objects, burrowing animals, nematodes, or pathogenic fungi. ZONE OF CELL ELONGATION: Cells enlarge here, pushing the apical zone forward. ZONE OF MATURATION (= ZONE OF CELL DIFFERENTIATION): The vascular tissues of the root become differentiated. Instead of forming bundles containing xylem and phloem, as in leaves and young shoots, the xylem forms a solid mass in the center, surrounded by strands of phloem. (See Figure 2.) Root hairs are unicellular and extremely transitory. They only live for 4-5 days (but new ones are always forming). Root hairs greatly increase the root's surface area (approximately double). They can also penetrate tiny pores between soil particles. Woody Plants BI237 Week 10 FIGURE 2. CROSS SECTION OF ROOT IN MATURATION ZONE. A mature root in a woody plant will also have secondary xylem and secondary phloem, produced by a vascular cambium. The vascular cambium arises between the primary xylem and primary phloem. Lateral roots will also grow. Cortex cells transfer minerals from epidermis to vascular tissue. The innermost layer of cortical cells differentiates into a cylinder called the endodermis. The endodermis contains a "Casparian strip" that controls the passage of minerals. FIGURE 3. ENDODERMIS SHOWING CASPARIAN STRIP The cell walls to the inside and outside of the endodermis are normal, thin primary walls, but the walls of cells touching other endodermis cells are waterproofed with lignin. If minerals are travelling through the intercellular spaces or by diffusion through cell walls, they are stopped by the Casparian strip. Minerals can flow freely as long as they are going through cell cytoplasm (this means that they at one point, had to gain entry to a cell by crossing the cell membrane). Woody Plants BI237 Week 10 FIGURE 4. DEVELOPMENT OF A LATERAL ROOT Some cells just became mitotically active at the inside of the cortex. They make a new apical meristem that comes crashing through the root, destroying cortex and epidermal cells. On the back of this page, you'll see a diagram of two types of mycorrhizae. Woody Plants BI237 Week 10 Before we can learn about these in detail, we need to take a detour to learn about fungus in general.... .