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In this article we will discuss about the classification of plant tissues based on development.
Tissues are generally classified on the basis of development and the types of constituent cell — a brief account of which is as follows.
Classification of tissues based on development:
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(a) Meristematic Tissue:
Meristematic tissues can be defined as a group of cells which have the power of division, usually situated at the apices of roots, stems and leaves. These cells are always in a state of division and their derivatives either compose or supplement the dermal, vascular and ground tissue system of a plant.
(b) Permanent Tissue:
Permanent tissues can be defined as a group of cells, which do not divide and are the differentiated products of meristematic tissues. Sometimes these cells may regain the power of division—a phenomenon termed dedifferentiation. In some plants dedifferentiation occurs at definite regions at certain stages of development to supplement the conducting and protective tissue system.
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Haberlandt (1914) reviewed ‘the various anatomico-physiological systems’ in plants and recognized twelve types of tissue systems. The types are grouped into four tissue systems, namely — meristematic tissue system, protective tissue system, tissue system concerned with metabolic activity and tissue system comprising ‘the execution of movements for the perception or transmission of stimuli’.
A brief account of the various tissue systems is given below:
i. Meristematic system:
The tissues of this system are always in a state of division and compose or supplement the dermal, conducting and ground tissue system of a plant. Primordial, primary and secondary meristem belong to this category.
Meristematic tissue system is placed at the top of all tissue systems due to the fact that:
(a) It consists of embryonic condition of all permanent tissues, and
(b) It is an independent tissue system and provides cells to increase the respective permanent tissues.
Protective system:
The tissues, belonging to this category, protect the inner tissues from the adverse natural calamities and mechanical injury; they also add mechanical strength to the plant.
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These tissues are grouped into two systems, namely:
ii. The tegumentary:
The tegumentary or dermal system which includes epidermis and periderm.
iii. The mechanical system:
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The mechanical system which is composed of collenchyma, sclerenchyma, wood-fibres and bast-fibres.
System concerned in metabolic activity:
In this category the following six systems are recognized:
iv. The absorbing system:
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The tissues of this system have absorptive function.
Example:
Rhizoids, roots, haustoria, velamen etc.
v. The photosynthetic system:
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The tissues of this system can photosynthesize.
Example:
Mesophyll cells of leaves, peripheral chlorophyllous cells of stem etc.
vi. The conducting system:
Xylem and phloem compose this system and they are concerned with upward conduction of water and downward translocation of solutes. Xylem parenchyma, medullary rays and latex-tubes are also included in this system.
vii. The storage system:
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This system consists of water tissues and storage tissues of seeds, tubers and bulbs.
viii. The-ventilating system:
The tissues of this system are meant for gaseous exchange between internal tissues and external atmosphere.
Example:
Stomata, lenticels etc.
ix. Secretory organs and excretory reservoirs:
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The nectaries, digestive glands, hydathodes etc. compose this system and they are concerned with secretion and excretion. Mucilage glands, oil glands and resin glands etc., are also included in this tissue system.
Systems concerned with the execution of movements for the perception or transmission of stimuli:
The followings belong to this system:
x. Motor tissues:
The insectivorous plant Dionaea possesses these tissues on their leaves, which trap the insects. When the insects rest on leaves, the motor tissues direct the leaves to fold.
xi. Sense-organs:
The hairs, statolith and ocelli (Fig. 6.1) etc., which perceive the external stimuli, compose this system.
xii. Stimulus-transmitting organs and tissues:
The tissues of this system transmit the external stimulus.
Example:
Leaves of Mimosa pudica droop down when touched. Protoplasmic connecting threads are also included in this system.
