Morphogenetic phenomena
These are innate behavioural tendencies namely Regeneration, Correlation, Polarity, Differentiation and Symmetry- all vested in any embryonic tissue which is liable to the influence of physical factors. Without the influential role of these, manifestation of morphogenetic pattern of the plant organism is not possible, either internally or externally. These varied phenomena are interdependent for their combined expression of biological behavior.
Regeneration: It causes repetitive production of similar cells, tissues and organs in the embryonic tissue(meristem), consequent upon the stimulatory influence of environmental factors. Impact of light, temperature and water or humidity on the shoot apical embryonic tissue causes the behavioral expression. Regeneration in the latter resulting in the successive differentiation of leaf primordia, and the subtending procambial and fundamental tissues at the shoot polar end, even in the lateral buds. In response to water, temperature and gravity, root apical embryonic tissue also regenerates similarly causing differentiation of procambial and fundamental tissues and organs in developmental succession. The incision experiments on these apical meristems have proved beyond doubt the prevalence of regeneration phenomena.
Correlation: It establishes integration of plant areas through chemical correlation. Growth and growth regulating substances are conveyed to the embryonic regions for tissues and organs differentiation through correlation either directly by means of plasmodesmata and by or selective permeability of membrane systems and cell walls. Organizational stimulatory and inhibitory activities causing regeneration and differentiation of tissues and organs at the remote embryonic regions, are caused exclusively due to correlation through the growth regulating substances which are the chemical agents for such behavioral expressions.
Polarity: It causes bipolar asymmetrical orientation of biochemical constituents of cells in the embryonic area for the regeneration and differentiation of tissues and organs. An ultimate expression of symmetrical organic pattern at the unlike shoot and root polar apices is caused by polarity. It has been experimentally shown in the zygote of marine alga Fucus by Whitaker and his colleagues and Bloch that, induction of polarity in a dividing embryonic cell preparatory to resulting differentiation into unlike cells, is induced by the stimulatory physical factors namely light and its accompanying temperature. Direction of growth of the resulting upper shoot is towards the source of light. On the contrary, the root growth in the opposite direction is induced by the other physical factors namely gravity and water. Such a bipolar and bidirectional growth is caused by polarity and reorientation of cell constituents of the germinating zygote which is in response to these external stimuli from the physical environment.
The invisible hand of polarity is prevalent in every cell area of plant organisms causing differentiation. The underlying principle in its behavioral expression is same as shown experimentally by Whittakar in the germination of Fucus zygote. It is an outcome of stimulus and response reaction between the external physical and internal biochemical factors, the former having either direct or remote influence over the latter in the embryonic, multi issued and multi-organ plant organisms.
Differentiation: The resulting visible expression of unlikeness caused by polarity in the dividing cell or any embryonic area is differentiation. Creation of unlikeness in plant organisation is a biological need. Even the first division of a cell, as in the division of zygote, result in it. Cells, tissues and organs produced in the embryonic growth centers, specific to the position, function and the growth phase- are the outcome of this behavioral tendency, namely differentiation. Regeneration of these areas at the shoot and root apices is coupled with the differentiation and polarity. On went of the reproductive phase from the vegetative phase due to Topophysis- a cumulative increase in the body material and energy sources which is caused due to differentiation.
Symmetry: This phenomenon gives expression to the genetic pattern of the plant organism, both internally and externally, in fulfilling its mission, ultimately. Internal/anatomical pattern and external morphological pattern are through symmetry which is apparently an ultimate expression of behavioral tendencies of plant organism and which goes parallelly with polarity, regeneration, correlation and differentiation. The seats of symmetry are the embryonic growth centers namely shoot and root apices.
Thus, in Plant Morphogenesis plant development can be considered as an outcome of stimulus-response reaction between the external physical factors and internal biochemical factors. The innate behavioral tendencies namely phenomena which have been defined above cause the expansion and differentiation of the embryonic areas during development beginning with the zygote itself. These phenomena give vent to the biological urge of the plant organization resulting in the organic pattern for reproduction.