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| Complete Q&A Review
1. How are the plant tissues classified according to their functions? Plant tissues are divided into growth (embryonic) tissues, supporting, filling and photosynthetic tissues (ground tissues), conductive (vascular) tissues and covering (dermal) tissues. Embryonic tissues: primary meristems; secondary meristems. Supporting tissues: collenchyma; sclerenchyma. Filling and photosynthetic tissues: photosynthetic parenchyma; storage parenchyma. Conductive tissues: xylem; phloem. Covering tissues: epidermis; periderm. 2. Which are the growth tissues of plants? How do they classify and where can they be found? The growth tissues of the plants are the meristems. Meristems are the tissues that produce the plant growth giving birth to all other tissues; they are formed of undifferentiated cells having intense cell division rate. Meristems classify as primary meristems and as secondary meristems. Primary meristems are found in the apex of the stem, in the lateral buds of the stem, in the basis and tips of the shoots and within the root cap. Primary meristems are responsible for the primary growth (lengthening) of the plant. Secondary meristems are those that make the plant grow in thickness (secondary growth) and they are formed by tissues that thicken the stem: cambium and phellogen (cork cambium). Image Diversity: meristem 3. What is the difference between the lateral and the apical buds of the plants? Lateral buds are portions of meristematic tissue located in the base of the shoots. Apical buds are portions of meristematic tissue situated in the tip of the stem and shoots. 4. What are apical meristems? Which type of plant growth does this meristem promote? Apical meristems are those primary meristems found in the apex of the stem and in the tips of shoots and roots. The apical meristems are responsible for the primary growth of the plants. Image Diversity: apical meristems 5. What are lateral meristems? Where can they be found and which type of plant growth do they promote? Lateral, or secondary, meristems, are the cambium and the phellogen (also known as cork cambium), tissues from the stem, branches and roots that by mitosis generate other tissues. These tissues participate in the secondary growth of plant, i.e., in the thickening of the stem, branches and roots. Image Diversity: lateral meristems 6. What are the main features of the meristematic cells? Why do these cells need to have a high mitotic rate? Meristematic cells have very thin cell walls, small vacuoles, a well-centralized nucleus and they are constantly undergoing mitosis. Meristematic cells need a high mitotic rate because they are responsible for the plant growth. Image Diversity: meristematic cells 7. What is the best identification hypothesis for a plant tissue seen under the microscope having most cells undergoing cell division? The best hypothesis is that the tissue is a sample of meristematic tissue. Meristematic tissues seen under the microscope have many cells undergoing mitosis. 8. Which are the plant tissues responsible for the supporting of the plant? The plant supporting tissues are the collenchyma and the sclerenchyma. The collenchyma is made of living and elongated cells that accumulate cellulose and pectin in some regions of the cell wall making them unequally thick and thus providing flexibility. The sclerenchyma is made mostly of dead cells killed by lignin deposition (lignin is an impermeable biopolymer) forming elongated, rigid and impermeable fibers. The sclerenchyma is a plant tissue widely used in the textile industry. Image Diversity: collenchyma sclerenchyma 9. Which is the plant tissue responsible for the filling of the space between other tissues? The plant-filling tissue is generically called parenchyma. The plant parenchyma can be divided into photosynthetic parenchyma, a tissue that has cells with many chloroplasts and a high photosynthesis rate found mainly in leaves, and storage parenchyma, specialized in the storage of water (e.g., in cactus), starch or air (e.g., in aquatic plants). Image Diversity: plant parenchyma 10. Where in the leaves is the photosynthetic tissue often located? The main photosynthetic tissue is the photosynthetic parenchyma (also known as chlorenchyma, do not confuse with collenchyma) often located between the superior and the inferior epidermis of the leaves. 11. How are water, mineral salts and food (sugar) transported throughout the plant? Water, mineral salts and sugar are transported throughout the plant through conductive vessels formed by specialized tissues. 12. Which are the specialized conductive tissues of the plants? The vascular tissues of the plants are the xylem and the phloem. Xylem is the plant tissue that forms the vessels that conduct water and mineral salts absorbed from the soil to the plant cells. Phloem is the plant tissue that forms the vessels that conduct dissolved sugar from the leaves (where they are produced by photosynthesis) to other plant cells. Image Diversity: xylem phloem 13. What are the cell types that form the xylem? What are the main features of those cells? The main cells of the xylem are the tracheids and the vessel elements (these only in angiosperms). The tracheids and the vessel elements are dead cells that have lost their cytoplasm and only their cell wall impregnated with lignin (an impermeable biopolymer) remained. The tracheids form tubes that communicate with neighboring tubes through pores; the vessel elements do not present pores but instead they communicate with the successive vessel element through perforations in their extremities. 14. What are the cell types that form the phloem? What are the main features of those cells? The main cells that form the phloem are the sieve elements and the companion cells. The sieve elements form the vessel walls; they are living enucleated cells positioned in series forming the sieve tubes. Between successive vessel elements there are communicating pores. The companion cells are located outside and alongside the sieve tubes and they help in the absorption of the material to be transported. 15. What is the vascular cambium? What is its function? Vascular cambium is the secondary meristematic tissue that in roots and in the stem forms the vascular tissues (xylem and phloem) of the plant. Usually the outer side of the vascular cambium produces a layer of phloem and the inner (more central) side of the tissue produces a layer of xylem. Image Diversity: vascular cambium 16. What are vascular bundles? How does the configuration of the vascular bundles within the stem differentiate monocots from dicots? Vascular bundles are segments of xylem and associated phloem that run longitudinally within the stem. In dicots the vascular bundles are organized side-by-side forming concentric rings. In monocots the vascular bundles are scattered and do not form rings. 17. What are the main plant tissues that form the rings observed on stem sections of some trees? The rings observed on a stem cross section of dicot trees are made of conductive tissues: xylem and phloem. Image Diversity: tree rings 18. How can the age of a tree be estimated from the analysis of the rings present on a cross section of its stem? For the growth of the tree it is necessary to have formation of new vessels within the stem, a task performed by the vascular cambium. The vascular cambium is more active in hot seasons (summer and spring) generating a lighter band made of large calibered vessels. During winter and fall the vascular cambium produces the opposite, so small calibered vessels and a darker band appears outside the previous lighter band. Therefore two ring bands are made yearly, one lighter and the other darker. By a direct count of these band pairs one can estimate the age of the tree. 19. What are the plant tissues that constitute the functional structures of the leaf veins? Leaf veins are made of vascular tissues. They are constituted by xylem and phloem that respectively conduct water and mineral nutrients (xylem) and sugar (phloem). 20. Which are the plant tissues specialized in covering? The covering tissues, or dermal tissues, of the plants are the epidermis (that covers the leaves and the young stems and shoots) and the periderm (a tissue that substitutes the epidermis in stems, shoots and roots). The periderm is made of phelloderm, phellogen and suber (cork). Image Diversity: plant epidermis plant periderm 21. Which are the plant tissues that cover the stem and the leaves? The stem may be covered by epidermis (having stomata, cuticle and photosynthetic cells) as in monocots or, alternatively, the epidermis is substituted by the periderm (phelloderm, phellogen and cork) as in dicots and gymnosperms. The leaves are covered by epidermis. 22. What is phellogen? What is its function? Phellogen, also known as cork cambium, is the meristematic plant tissue responsible for the formation of the periderm (the covering of the stem, shoots and roots). The inner side of the phellogen layer forms the phelloderm and its outer side forms the cork (suber). The suber secretes suberin, an impermeable substance that impregnates the tissue. 23. Which type of plant tissue is cork? Cork, the material, for example, used to cap wine bottles, is extracted from the suber of a special oak called cork oak. Image Diversity: cork 24. What are the plant root hairs? Where can they be found and what is their function? The root hairs are external elongated projections of the root epidermis. Their role is to increase the absorption of water by the root. 25. Why does bark often die and break naturally? The bark is the mature periderm of the stem, branches and roots. It dies and breaks when these structures grow and thus the peridermal suber formed of already dead cells ruptures. 26. What is the leaf cuticle? The leaf cuticle is a thin waxy layer made of cutin and waxes on the outer surface of the leaf epidermis. Its function is to control the cellular transpiration. Image Diversity: leaf cuticle 27. Which are the plant tissues that form the plant roots? The roots have a central portion called medulla made of vascular tissue (inner xylem and outer phloem). The medulla is surrounded by the medullary parenchyma and delimited by pericycle, a meristem that originates the secondary roots (ramifications). Externally to the medulla lies the cortical portion formed of endodermis (that surrounds the pericycle) and cortical parenchyma. The covering of the roots is epidermis (with root hairs) later substituted by suberized (corky) periderm. Image Diversity: root structure root hairs 28. What is the root cap? The root cap is a protective structure located in the tip of the growing root. It protects the meristematic tissue of the root forming a cap that surrounds the tip. The cover is necessary since during the growth of the root the meristem otherwise would be injured by the friction with the soil. Image Diversity: root cap 29. What are secondary roots? In origin how secondary roots differentiate from shoots? Secondary roots are ramifications of the primary (main) root. The secondary roots emerge from the pericycle, inner tissue of the root. The shoots originate from the lateral buds of the stem. Therefore the origin of the secondary roots is endogenous and the origin of the shoots is exogenous. 30. Why do roots of many swamp plants have a special morphology? Swamp and marsh plants generally present supporting roots that ramify from portions of the stem above the ground helping the plant to fixate down the muddy and sandy soil. They may also have respiratory roots (pneumatophores), structures that emerge from buried roots to catch oxygen. Image Diversity: respiratory roots |