The morphological characteristics are very useful to taxonomists for the diagnosis and classification of plants. With the progress of scientific research, it becomes clear that only the morphological characteristics are not sufficient enough as the data of studies of other branches of botany have considerable value to assess the proper systematic status and phylogeny of taxa. Findings through the recent scientific studies indicate that data from many different branches of botany are equally useful in systematic studies.
The importance of external and internal morphological characteristics of botany is discussed here:
External Morphology
The morphological characteristics give valuable information which helps taxonomists to a great extent because they are easy and can conventionally be used in the classification of plants. Besides the conventional morphological characteristics, others like habit, underground organs, leaves, seedling morphology, stipules, non-conventional characteristics of floral parts, seeds etc are now very popular to the students. In recent years, due to thorough and careful studies, some characteristics neglected in the past came into focus and helped in taxonomic practices.
Habit
The shape of a tree i.e. bushy, umbrella-shaped, flat-topped, cylindrical, oblong etc are used for the recognition of a tree. The characteristics of bark (colour, thickness, fissuring, texture) are used to distinguish different species of Betulla and Pinus. In many cases, the stem becomes much condensed which looks stemless but different in others.
Underground organs
During the preparation of a Herbarium, generally underground parts are not collected. But in some cases, they have been shown to be very useful in the identification of different taxa. In the classification of Scilleae of Liliaceae, Chouard (1936) has used the characteristics of bulbs and underground parts. Some are morphologically alike except the characteristics of underground parts, Ex- In Liliaceae two species of Chlorophytum viz. C. glaucous Blatt. and C. glaucum Dalz. In many other species the root fibres are thick and without terminal tubers but in others root fibres are slender. For the differentiation of a large number of species in Dioscorea of Dioscoreaceae (Burkill and Prain,1936), the structure and morphology of root tubers are used.
Leaves
Leaf characteristics are extensively used in the differentiation of species of Betula (Natho, 1959), Ulmus (Melville, 1955) and in many other genera. Different shapes, sizes, structures and arrangements of the leaflet on the rachis, are important characteristics that are useful in the case of the Papilionaceae family, different species of Dalbergia viz. D. sisso Roxb., D. latifolia Roxb and D. sympathetica Nimmo. Numerous Venetian patterns in the fossil genus Glossopteris were described by Pant(1958).
Stipules
The stipule characteristic is also very useful and very important in the differentiation of species. In the case of the Rosaceae family, the characteristics and arrangement of the stipule are variable but almost constant within a species. To differentiate the different species of Viola and Trifolium, external structure and morphology are very much effective.
Seedlings morphology
The various characteristics of seedlings have also been used in tracing phylogenetic relationships among different families and genus and they sometimes provide very important characteristics about the taxa.
Non-conventional characteristics of floral parts
The different types of characteristics of flowers and inflorescences are commonly used in the classification of plants. These characteristics have become very useful in the identification of different taxa and their differentiation with others. Some of them are –
- The configuration of the floral disc and also nectarines have great value in the diagnosis of Cruciferae (Brassicaceae) and Tamarix of Tamaricaceae.
- In the species of Alyssum of Cruciferae, staminal appendages have given very useful data.
Seeds
The characteristics of seed are also differentiation of taxa at both generic and specific levels. Among the species of Drymaria of Caryophyllaceae seed shape, the colour sculpture may contribute a critical indication. In Liliaceae, the two genera Anthurium and Chlorophytum can be differentiated only by the shape and number of seeds. To differentiate the genera and species of different families like Acanthaceae, Verbanaece, Malvaceae, Convolvulaceae, many of these characteristics are really useful like – hairy growth pattern on testa, their colour, length, texture etc contribute a great value.
Micromorphology
Epidermis
The characteristics of the epidermis, such as shape, thickness, nature of sculpturing and inclusions of epidermis cells provide important data useful in taxonomic consideration. Sometimes the divisions in the epidermal cell are also considered in the taxonomic analysis. Narrow epidermal cells are the characteristics of the family Stylidiaceae (Mildbread,1908) and sclerification of the epidermal cell wall is of taxonomic importance in the tribe Mutisiae of Composite.
In Cyperaceae, silica-body distribution on the surface of the epidermis provides important data of taxonomic importance (Metcalfe,1968):
- In Rhynchospora spherical and hemispherical echinulate bodies, both are found.
- Hemispherical type of smooth bodies found mainly in Lophoschoenus etc.
- Silica bodies (wedge-shaped) are restricted in Ptilianthelium, Scirpodendron and Neesenbeckia etc.
Based on epidermal characteristics like outline of epidermal cells in surface view, type of hairs, structure of stomata, scientists classified different species though they are placed in the same genus. Jain and Singh (1974) and Singh and Jain (1975) differentiated the species of Pyrus, Prunus and also distinguished Himalayan oaks and many others, based on epidermal characteristics.
Trichomes
It had been great variation in the structure and are used in taxonomy and phylogeny of angiosperms successfully used in the classification of genera and species. Some minor modifications change one species from another.
Some important observations:
- Cruciferae: Arabis the trichomes are commonly unicellular and glandular but sometimes they are simple and in some others, they are branched, either star-shaped or dendritic type (tree-like). They are also terete, flattened or with swollen bases.
- Compositae: Two species of Parthenium distinguished by trichomes, viz. P. argentatum and P. incanum were observed by many scientists. The hybrids between the species show intermediate types of trichomes.
- Ericaceae: In Rhododendron, trichome structures are useful in the taxonomic separation in infrageneric and also specific levels, observed by Cowan (1950).
- Oleaceae: Imander (1967) studied the structure and ontogeny of trichomes and confirmed the position of Nyctanthes in Oleaceae.
Trichomes of flowering plants and their phylogeny are well explained thoroughly by Ramayya ( in 1972). He said that unicellular Stigmatic papillae are the most primitive among the angiosperms. Thus the order Magnoliales is a heterogeneous group and the angiosperms are a polyphyletic group. The Heliantheae family is the most primitive tribe of Asteraceae; this view is supported by Ramayya (1962,1969). In the classification of some members of Capparidaceae and in Caesalpiniaceae, trichome characteristics are very much useful.
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