ribosomes and a vacuole. This reduction in cellular organelles allows nutrients to pass through the cells more easily. Companion cells, are found adjacent to sieve-tube elements, and connected to them by plasmodesmata . The nucleus and ribosomes of the companion cell serve not only the companion cell, but also the adjacent sieve-tube element. Companion cells may also aid in moving substances into and out of the sap. Gravity assists in the downward movement of sap. However pressure gradients , between the sites of sap production and the sites where sap is unloaded or used, also moves the fluids either upward or downward in the plant. As sugars are produced in the photosynthetic leaves ( sugar source ), companion cells pump them into the sieve tubes. 2 Figure 2. Coleus stemtip showing the apical meristem and cells that will give rise tothe three tissue systems including the protoderm, procambium and ground meristem.
Activity #2 Examination of phloem Observe the slides showing a cross section and longitudinal section of phloem in a stem. Locate a sieve-tube element, companion cell, sieve-plate and p-proteins . P- proteins in the sieve tube elements aggregate when the phloem is mechanically damaged; and act as a “plug” or clot” to prevent leakage of the phloem contents. Part III. Structure and function of xylem The upward movement of water in a plant occurs in opposition to gravity but does not require energy on the part of the plant. Loss of water into the atmosphere, when the stomata of the plant leaves are open drives the movement of water up the xylem. This water loss is called transpiration . Xylem is the main water conducting tissue of vascular 3 Figure 3. Cross section through phloem showing sieve-tube elements and companion cells Figure 4. Longitudinal section through phloem showing sieve-tube elements and sieve plates. Companion cell Sieve-tube element showing sieve plate Sieve-tube element p-protein forming a plug at sieve plate
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- Spring '14
- phloem, Xylem, Plant anatomy