11 Bio Chapter Summary

Plant Growth

Chapter Summary

  • A permanent and irreversible increase in size of an organ or its part or even of an individual cell is called growth.
  • The growth can be divided into three phases, viz. meristematic, elongation and maturation.
  • The constantly dividing cells of the root apex and the shoot apex represent the meristematic phase of growth.
  • The proximal cells which are just next to the meristematic zone represent the phase of elongation. In this phase, there is increased vacuolation, cell enlargement and new cell wall deposition.
  • Next to the phase of elongation lies the maturation zone. The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications.
  • The increased growth per unit time is called growth rate. The growth rate can be arithmetic or geometrical.

  • Arithmetic Growth Rate: In this type of growth, only one daughter cell continues to divide after the mitosis. Another daughter cell differentiates and matures.
  • Geometrical Growth Rate: In most of the cases, the initial growth is slow and is called the lag phase. After this, the growth is quite rapid and at an exponential rate. This phase is called the log or exponential phase.
  • Plant growth is closely linked to water status of the plant. Water is required for cell enlargement and also for turgidity.
  • Oxygen helps in releasing energy which is utilised in growth activities.
  • Various nutrients are also required by plants for synthesis of protoplasm.
  • A range of optimum temperature is also necessary for growth in a plant. Any deviation from the optimum range can be detrimental for the survival of plant.
  • The process which leads to maturation of cells is called differentiation. During differentiation, a few or major changes happen in protoplasm and cell walls of the cells.
  • A differentiated cell can regain its capacity for cell division under certain conditions. This phenomenon is called dedifferentiation.
  • A dedifferentiated plant cell once again loses its capacity to divide and becomes mature. This phenomenon is called redifferentiation.
  • Some plants show different growth pathways in response to environment or to phases of life to form different types of structures. This ability of plants is called plasticity.
  • Auxins help to initiate rooting in stem cuttings. Auxins promote flowering. Auxins help to prevent fruit and leaf drop at early stages but promote abscission of older and mature leaves and fruits.
  • Gibberellins cause an increase in length of axis. They cause fruit elongation and also delay senescence.
  • Cytokinin help to produce new leaves, chloroplast in leaves, lateral shoot growth and adventitious shoot formation. Cytokinins help in overcoming the apical dominance. Cytokinins promote nutrient mobilization which helps in the delay of leaf senescence.
  • Horizontal growth of seedlings, swelling of axis and apical hook formation (in dicot seedlings) are some of the examples of activities of ethylene.
  • ABA is a plant growth inhibitor. It plays a major role in seed development, maturation and dormancy. ABA stimulates closure of stomata and increases the tolerance of plants to various types of stresses. Hence, it is also called the stress hormone.
  • Flowering in certain plants depends on a combination of light and dark exposures and also on the relative duration of light and dark periods. This response of plants to variable duration of sunlight is called photoperiodism.
  • In some plants, flowering is quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is called vernalisation. Flowering is promoted during the period of low temperature because of vernalisation.