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-

Determinate growth – When a structure grows to a certain size and then stops, ultimately falls off after senescence eg. Leaves, flowers & fruits.
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-

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.

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.