1.What are Tracheids?
Tracheids are elongated with blunt ends. Its lumen is broader than that of fibres. Their secondary wall is lignified. In cross section, the tracheids appear polygonal and thick walled. The pits are simple or bordered. There are different types of cell wall thickening due to deposition of secondary wall substances. They are annular (ring like), spiral (spring like), scalariform (ladder like), reticulate (net like), and pitted (uniformly thick except at pits).Tracheids are imperforate cells with bordered pits on their end walls. They are arranged one above the other. Tracheids are chief water conducting elements in gymnosperms and pteridophytes. Here, the conduction of water and mineral salts takes place through the bordered pits. They also offer mechanical support to the plants.
2. What are the different cells of phloem?
Phloem is composed of four kinds of cells: sieve elements, companion cells, phloem parenchyma and phloem fibres.
3. Explain Phloem.
Like xylem, phloem is also a complex tissue. It conducts food materials to various parts of the plant. The phloem elements which are formed from the procambium of apical meristem are called primary phloem.
4. Explain Xylem.
Xylem (Greek word ‘xylos’ = wood) is a complex tissue that is mainly responsible for the conduction of water and mineral salts from roots to other parts of the plant. The xylem, which is derived from procambium, is called primary xylem and the xylem, which is derived from vascular cambium, is called secondary xylem. Earlier formed xylem elements are called protoxylem, whereas the later formed xylem elements are called metaxylem. Xylem is made up of four kind of cells – tracheids, vessels or tracheae, xylem fibres and xylem parenchyma.
5. What are Sclereids?
Sclereids are dead cells. They vary greatly in shape and thickness. The cell wall is very thick due to lignification. Lumen is very much reduced. The pits may be simple or branched. Usually sclereids are isodiametric, but in some plants they are elongated. They are responsible for the rigidity of the seed-coat. The isodiametric sclereids are called brachy-sclereids (stone cells). They are found in bark, pith, cortex, hard endocarp and fleshy portions of some fruits. Example: pulp of Pyrus.
6. What is a simple tissue?
A tissue with the cells of similar structure and function is called simple tissue.
7. Explain Parenchyma cells?
It is generally present in all organs of the plant. It constitutes the ground tissue in a plant. Parenchyma is the precursor of all the other tissues. Parenchyma is a living tissue and made up of thin walled cells. The cell wall is made up of cellulose.
Parenchyma cells may be oval, spherical, rectangular, cylindrical or stellate. The cells are usually polyhedral with 10 to 12 facets.
8. What are the characteristics of meristematic cells?
The meristematic cells may be round, oval, polygonal or rectangular in shape. They are closely arranged without intercellular spaces. They have dense cytoplasm with large nucleus. They have smaller vacuoles, which are scattered throughout the cytoplasm. Their cells walls are thin, elastic and made up of cellulose.
9. Explain Sclerenchyma.
Sclerenchyma is a dead tissue. The cells have lignified secondary walls. They lack protoplasts. On the basis of origin, structure and function, sclerenchyma is divided into two types – sclereids and fibres. The sclereids are different from fibres in the following respects. Sclereids are shorter whereas fibres are longer. Sclereids possess numerous pits as compared to the fibres.
10. What are Sclereids?
Sclereids are dead cells. They vary greatly in shape and thickness. The cell wall is very thick due to lignification. Lumen is very much reduced. The pits may be simple or branched. Usually sclereids are isodiametric, but in some plants they are elongated. They are responsible for the rigidity of the seed-coat. The isodiametric sclereids are called brachy-sclereids (stone cells). They are found in bark, pith, cortex, hard endocarp and fleshy portions of some fruits. Example: Pulp of Pyrus.
11. What is intercalary meristem?
It is present in the nodal region and is prominently found in monocotyledons, example: grasses. As the name indicates, it is present in between the permanent tissues. It is derived from the apical meristem and is responsible for the elongation of internodes.
12. What are the sub-divisions of parenchyma cells?
The parenchyma found in the cortex region possesses well-developed large intercellular spaces called air spaces. This air filled parenchyma tissue is called aerenchyma. It helps the plant to float in water. Example: Nymphaea and Hydrilla.The parenchyma cells that are stored with starch grains are called storage parenchyma, example., stem and root tubers. In the petioles of Banana and Canna, star shaped parenchyma cells are found. These cells have stellate parenchyma. In green parts of the plants, the parenchymatous cells have chloroplasts. These cells are called chlorenchyma. Its important function is photosynthesis.
13. What are vessels or Tracheae?
Vessels are perforated at the end walls. Its lumen is wider than that of tracheids. The perforated plates at the end wall separate the vessels. They occur parallel to the long axis of the plant body. Due to dissolution of entire cell wall, a single pore is formed at the perforation plate. It is called simple perforation plate. Example: Mangifera. If the perforation plate has many pores, then it is called multiple perforation plate. Example: Liriodendron.
The secondary wall thickenings of vessels are annular, spiral, scalariform, reticulate, or pitted as in tracheids conducting elements in angiosperms and they are absent.
In pteridophytes and gymnosperms. The main function of vessel is conduction of water and minerals. It also offers mechanical strength to the plant.
14. What is lacunate collenchyma?
In the hypodermis of Ipomoea, the cell wall thickening materials are deposited on the walls bordering the intercellular spaces. This type is called lacunate collenchyma.
15. Define companion cells?
They are connected to the sieve tubes through pits found in the lateral walls.
Practice in Related Chapters
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