Plant Development Introduction


Plant Development Introduction

  • A plant is a nature’s highly efficient device, which very preciously uses all its resources to develop into a complete entity.
  • A plant starts its life in the form of a small seed, which contains a very small embryo along with a little amount of stored food.
  • When this seed gets proper external conditions, it starts imbibing water, which dilutes the otherwise concentrated cytoplasm of seed cells.
  • These cells become active and start production of active enzymes, which further take part in metabolic activities.
  • Increased rate of metabolism, leads to increase in cell mass and there by cell and nuclear inclusions.
  • Many other external and internal factors together activate the cells to be meristematic.
  • These factors include nucleic acids, available food stuff, hormones etc
  • The growth in plants is localized to the meristems.
  • These meristems together form the second level of structural units the metamers.
  • The repetitive occurrence of these metamers forms the modules.
  • These modules form the organs like shoot, root & leaf.
  • The small developing embryo breakes it’s covering, the seed coat, and comes out i.e. germinates.
  • When it receives proper light, temperature and air, it starts differentiating into various cells. 
  • Growth hormones influence all the development processes.
  • They influence the plant at cellular level, organellar level and even at the level of whole plant.
  • They are -actually chemical messengers, organic in nature, synthesized in response to some environmental stimulus in one part of the plant and translocated to another part of the plant where it has a controlling or regulatory effect.
  • These hormones also regulate the movement of plants in response to external factors.
  • In this unit, we shall try to understand the development of a new plant, from a tiny seed. 
  • How does this seed transform into a new plant, and what are the factors which affect the germination and differentiation?

Features of plant development

  • All the living beings have a common feature of growth. It can be defined as an irreversible and permanent increase in mass, weight, volume of cell, organ or organism.
  • Growth is a common word used to designate any change in an organism. But, it should not be confused with the development.
  • Development is an ordered change or progress towards a more complex state. It is also an irreversible process that means a differentiated organ can not become an undifferentiated one.
  • In unicellular and some other lower forms of plants, all the cells of the body divide, hence the growth is diffused.
  • But in higher plants, growth is restricted to definite regions of the body called meristems.
  • The plant body shows two types of growth-   
  1. Determinate growth – When a structure grows to a certain size and then stops, ultimately falls off after senescence eg. Leaves, flowers & fruits.
  2. Indeterminate growth- when a structure continues to grow for an indefinite time. eg root & shoot.
  • The growth and development of the plant is ultimately the growth and development of the plant cell.
  • So, we can express the development of the plant at following levels-

1. Cellular level-

  • The cells enlarge, elongate & then undergo karyokinensis & then cytokinensis.
  • These cells differentiate to form various tissues & tissue systems.

2. Organ level –

  • The single celled zygote undergoes predetermined divisions to form a well-organized embryo.
  • This embryo develops to form a seed, which in turn, develops into a seedling which after full maturation forms root, shoot, leaf, flower & fruits.
  • The site of the formation of these organs & the time of the formation is preset in the plant.
  • It also forms special organs like tubers, rhizomes, bulbs, corns, bulbils etc.
  • This level also decides and forms abscission layers in ripen leaves and fruits.
  • At this level, movements of organs, either environmentally controlled or Nastic movement are also included.

3. Whole Plant level –

  •  At this level, plant’s overall form, in terms of polarity, symmetry is controlled.
  • During the plant’s life cycle, certain morphological and genetical events take place.
  • They are in correlation with time.
  • Germination always takes place at a season favorable for the growth.
  • The onset of flowering is again a time, photoperiod and temperature influenced phenomenon.
  • Similarly, dormancy of germinating buds, seed and other vegetative parts is again a way to avoid particular time during the season.
  • So, we can say that all primary tissues of a plant are developed by the activity of embryonic cells.
  • Some of the embryonic cells at embryonic root and embryonic shoot become meristematic.
  • They are apical in position and by their activity, root and shoot apices are formed.
  • Here, other types of meristems as marginal, intercalary, plate, rib are formed; all of them contribute to the longitudinal and latitudinal development of the plant.
  • An important aspect of apical meristem is their repetitive activity.
  • They divide at regular intervals to form group of cells as leaf primordium in stem apex & root primordia in root apex.
  • The time interval between the two successive leaf primordia is 3-4 days.
  • At the higher level of the whole plant, there are periods of meristems activity and inactivity.
  • Thus, the higher the level of organization; the longer the period of the rhythm.
  • These rhythmic activities result in the construction of 3 types of structural units that are fundamental to plant form.
  • The first – the cell is the source of other cells and of other structural units. Their auto reproduction results in the formation of the meristems.
  • The meristems form the second – the metamers eg.
  • The apical meristem of shoot produces leaves, internodes, nodes and buds, these four are known as metamers.
  • The metamers form a more complex unit – the module.
  • The modules are present as combined units of morphologically similar metamers.
  • The growth of module is either terminated in an inflorescence, tendril or spine or in parenchymatous axis.
  • Module development continues only as long as the apical meristem is active.
  • When it becomes inactive it gives a stimulus for axillary bud to become active and thus bring about branching of the module.
  • The position of a module is a relative and not a fixed property.
  • The addition and growth of new modules causes the relative positions and state of the already existing modules to change.
  • Combinations of modules construct an even higher level of organization the system of shoot and roots.
  • The spatial arrangement of modules determines the particular morphology of systems.
  • As many as 23 types of modules have been recognized in shoot system of trees.
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