NEET Biology Notes Anatomy of Flowering Plants Plant Anatomy
Plant anatomy is the branch of biological science, which deals with the study of gross internal structure of plants. N Grew (1682) is known as Father of Plant Anatomy.
Plants have cell as the basic unit. Cells are organised into tissues and in turn the tissues are organised into organs. Different organs in a plant show differences in their internal structure. Internal structures also show adaptations to diverse environment.
- Tissue is a group of similar or dissimilar cells of common origin that have same method of development and similar structure and function. The study of tissues is called Histology. Tissues can be conveniently grouped under two categories,
- i. e. meristematic and permanent tissues.
- i. Meristematic Tissues
- These represents a group of cells that are in a continuous state of division to produce new cells or retain their power of division. Meristematic cells have abundant, dense cytoplasm and large conspicuous nucleus. These cells remain in active state of metabolism. There is no intercellular spaces, crystals and vacuoles in meristematic cells. On the basis of origin and development of initiating cells meristem can be Promeristem (e.g. tip of plumule and radicle), primary meristem (e.g. apical meristems, intrafascicular cambium, intercalary meristem) and secondary meristem (e.g. vascular cambium in dicot root, interfascicular cambium, cork cambium, wound cambium and accessory cambia of monocots).
On the basis of position in the plant body meristems can be divided into three parts :
- Apical meristem is present at the apices of primary and secondary shoots and roots of the plants. Apical meristem is without vascular tissue, free from virus and is used as explant in tissue culture to get virus free plants. Apical meristems are responsible for increase in length.
- Intercalary meristem help in elongation of the organs and also allow the fallen stems of cereals to become erect. It lies between the regions of permanent tissues, grasses grow by intercalary meristem and lack apical meristem that is why cutting causes no damage to their growth.
- Lateral meristems are present along the sides of organs. Lateral meristems are responsible for increase in girth of stem and roots.
- Different Theories of Apical Meristems
Many theories have been proposed to explain the mode of growth of the shoot apical meristem such as
- Apical cell theory was proposed by Nageli (1858).
- Histogen theory was proposed by Hanstein (1870) advocated three distinct zones called dermatogen (epidermis),
- periblem (cortex) and plerome (central cylinder, pith). Haberlandt (1914), proposed new terms, i.e. protoderm for dermatogen, ground meristem for periblem and procambium for plerome.
- In case of root apex Hanstein proposed one more histogen, i.e. calyptrogen which is responsible for , the formation of root cap.
- Tunica-corpus theory was proposed by Schmidt (1924) advocated the presence of two distinct zones, i.e. the peripheral zone called tunica and the inner core of cells called corpus, which is surrounded by tunica.
- Mantle-core hypothesis was proposed by Popham and Chan (1950).
Korper-Kappe theory was given by Schuepp (1917) for root apices. According to this two zones are differentiated in root system, the korper (body) and kappe (cap). In many cases, a quiescent centre (Clowes; 1961) is found in the centre of root apex. Cell divisions are very few in quiescent centre as there is very little synthesis of new proteins, RNAs and DNA. It may function as reserve meristem. In stems, maximum growth occurs in shoot apices, whereas in roots, maximum growth occurs behind the root apex region.
- Permanent or mature tissues are composed of mature cells that after undergoing complete growth have acquired a definite shape, size and function. Permanent tissues can be of two types simple permanent tissues, e.g. parenchyma, collenchyma, sclerenchyma and complex permanent tissues
- e.g. phloem and xylem.
Some additional features related with xylem elements are following :
- Tracheids and vessels are tracheary elements. Vessels are absent in pteridophyte, gymnosperms and Winteraceae, Tetracentraceae and Trochodendraceae families of angiosperms.
- First formed xylem is protoxylem and later formed xylem is metaxylem.
On the basis of position of protoxylem in relation to metaxylem, xylem can be of four types, i.e. exarch, endarch, mesarch and centrarch.
- In exarch condition, protoxylem lies towards the outside of metaxylem.
- Protoxylem is on inner side in the endarch middle of metaxylem in the mesarch condition and at centre of metaxylem in centrarch xylem.