Anatomy in Relation to Plant Taxonomy

In this article we will discuss about the Anatomy in Relation to Plant Taxonomy:- 1. Uses of Anatomy 2. Types of Anatomy 3. Ultra Structural Systematics.

Uses of Anatomy:

Anatomical characters of vegetative and floral parts of flowering plants have been successfully employed to solve taxonomic problems and for the elucidation of phylogenetic relationships. It was Bureau, who for the first time used anatomical characters in plant classification for the delimitation of taxa of various levels, within the family Bignoniaceae.

However, anatomical data have been used extensively as a taxonomic tool only after the nineteenth century. Anatomical data has not only been useful at the higher levels but in certain instances, have been successfully employed even at the specific level. Auguste Mathiew is one of the pioneer taxonomists, who used features of wood anatomy in the description of forest plants in Florae forestiere.

Later, another taxonomist Solereder, discussed the systematic value of anatomical structures in dicotyledons in his classic book Systematische Anatomic der Dicotyledonen, the English translation of which was later published in a modified form in the two-volume book Anatomy of the Dicotyledons by Metcalfe and Chalk.

Anatomical evidence can be useful in systematics in several ways:

(i) It can well be exploited taxonomically in the identification of fragmentary material, say a piece of wood.

(ii) When morphological characters prove to be of no help in the preliminary identification of herbarium material, anatomical study may prove helpful.

(iii) Anatomical data has proved to be very useful in discerning evolutionary trends and interrelationships of taxa at and above the species level and at higher taxonomic categories. They are most useful in determining relationship between different genera, families, orders and other taxonomic categories.

While studying anatomical data, it is advised to study the ranges of variability of these characters within the same individual and between different individuals of the same species and not rely on data from a single sample of an organ or tissue, as similarities in structural organization may not necessarily reflect close relationship but may be the result of parallel and convergent evolution.

Types of Anatomy:

1. Vegetative Anatomy:

(a) Leaf Anatomy:

Leaf anatomy provides various characters of taxonomic importance as has been rightly stated by Carlquist, that “the leaf is perhaps anatomically most varied organ of angiosperms and its anatomical variations often concur closely with generic and specific and occasionally familial lines”.

Leaf anatomy has been used widely in several taxonomically different groups such as Euphorbiaceae, Cyperaceae and Gramineae of Angiosperms and Coniferae of Gymnosperms.

It has been one of the most reliable characters in grass systematics. For example, the leaf anatomy of several species of Cyperaceae, was studied by Koyama and Govindrajalu and they formulated keys to identify various species of Cyperus, Fuirena, etc. Brown surveyed, 72 genera of grasses and on the basis of their tissue arrangement, six main types were recognized.

However, they could not, be segregated into the two traditional subfamilies, Pooideae and Panicoideae. Similarly, Vidakovie have used several characters of leaf anatomy in differentiating species in Pinus. Taxonomic implication of leaf anatomy of several genera of Musaceae, Zingiberaceae, Xanthorrhoeaceae and Ericaceae have also been established by several workers.

Some of the important characters of taxonomic significance in leaf anatomy include the following:

(i) Nature and thickness of epidermis:

The size and shape of epidermal cells is of great value in the taxonomy of several taxa (Fig. 8.2). Cuticular characters of the epidermis and stomata have also proved to be of great value.

For example, Conde studied 5 species of the genus Opuntia with respect to cuticular thickness, epidermal papillosity, stomatal size and frequency, hypodermal thickness, vessel number, etc. and found that each species was distinct in respect of the degree of papillosity of epidermal cells, hypodermal thickness and vessel width.

The information from trichome anatomy has also proved useful in certain taxa, e.g. trichomes have furnished as diagnostic characters in certain species of Veronica.

(ii) Structure and types of mesophyll, storage parenchyma, mid vein structure, bundle sheath, secretory apparatus, etc:

For example, Anderson & Crech suggested precise groupings of Solidago and other species of Asteraceae based on their study of leaf anatomy, including qualitative and quantitative differences in mesophyll, storage parenchyma, secretory apparatus, bundle-sheath extension and midvein structure.

(iii) Pattern of sclerenchyma:

Patterns of the distribution of sclerenchyma in Carex and Festuca have been used in distinguishing species.

(iv) Silica bodies:

Silica bodies in the epidermal cells of members of certain families like Zingiberaceae, Musaceae and Palmae among Monocotyledons and Rosaceae in the Dicotyledons have been used as diagnostic character in systematics at generic as well as specific levels.

(v) Chloroplast structure can also prove to be of taxonomic significance.

(b) Stem Anatomy:

Stem anatomy has also been long relied on as a taxonomic tool (Fig. 8.3). The two-volume work by Metcalfe & Chalk is an excellent example of an illustrated encyclopaedia of this and of other aspects of plant anatomy, which reveals the taxonomical significance of anatomical characters in plant classification and can be used at various levels from Dicotyledon-

Monocotyledon distinction, to the separation of various species of the same genus. Stem anatomy has particularly proved to be of diagnostic value in the herbaceous members. For example, anatomy of stems has been successfully employed in the delimitation of species of Dioscorea which otherwise are not easily separable on exomorphic grounds.

Carlquist has used anatomical features of the genus Fitchia (Asteraceae) in the classification of various species. Further, it is also possible to identify parents of several hybrids on anatomical grounds.

The anatomical features, which can be taken into account as diagnostically useful characters of the stem include:

I. Degree of elevation of stem ridges.

II. The distribution and abundance of collenchyma.

III. Pattern of collenchyma thickenings.

IV. Transformation of ground tissue cells of cortex into transfusion cells — e.g. Casuarina.

V. Distribution of fibres — e.g. Genista.

VI. Variation in the structure of the cells of stem endodermis — e.g. in families like Piperaceae, Asteraceae and Lamiaceae.

VII. Features of the stem pith — e.g. species of Dubantia and Fitchia have been distinguished on the basis of anatomical differences in pith.

VIII. Shape and size of sclerenchyma girders — e.g. used to distinguish species of the subgenus Genuini of Juncus.

IX. Arrangement and type of vascular bundles — e.g. two species of Dioscorea, viz. D. cayenensis and D. rotundata have been distinguished on the basis of arrangement of vascular bundles in the stem, which otherwise are difficult to distinguish on exomorphic grounds.

(c) Petiole Anatomy:

Metcalfe and Chalk and Howard have suggested that the petiole anatomy might also be of taxonomic significance (Fig. 8.4). According to Howard families, genera and even species in some cases may he identified by petiole characters.

Some of the important diagnostic characters of petiole anatomy include:

I. Position of petiole on stem.

II. Presence or absence of stipules.

III. Petiole outline.

IV. Number of layers of parenchyma in the cortex.

V. Vascularization of petioles.

VI. Distribution of perivascular fibres.

VII. Number of traces.

Example:

Petiole anatomy of 64 species of Baphia of Leguminosae have been studied by Soladoye, and some species of Phlomis and Eremostachys of Labiatae by Azizian and Cutler, which provide clear support of its use in the taxonomy of these genera.

(d) Nodal Anatomy:

Nodal anatomy has also gained much importance in taxonomy and phylogeny of angiosperms in recent years. Correlations of nodal anatomy with some other features might help significantly in tracing the phylogeny of angiosperms. A comparative study of nodal anatomy may show important relationships or distinctness of genera or even species (Fig. 8.4).

Based on his studies on megaphyllous plants, provided a classification of nodal types. In general there are three major types of nodes:

I. Unilacunar — occur in Laurales, Caryophyllales, Ericales, Ebenales, Primulales, Myrtales, some families of Tubiflorae and a majority of families of Asteridae.

II. Trilacunar — occur in the majority of Dicotyledons.

III. Multilacunar — occur in the primitive orders such as Magnoliales, Piperales, Trochodendrales and a few advanced orders such as Umbellule’s and Asterales.

Sinnott considered the tri-lacunar node as primitive, and unilacunar and multi-lacunar nodes as advanced. A fourth type of node was discovered by Marsden & Bailey, viz.

iv. Unilacunar two trace — It is now considered as the basic type of node found in angiosperms.

Usually, the mode of nodal vasculature is uniform in the family, but exceptions have also been reported, where different types of node occur even in the same individual plant.

Example:

On the basis of nodal structure, the subfamily Icacinoideae of the family Icacinaceae has been divided into two distinct groups i.e., one section, which is characterized by tri-lacunar nodes, while the other section, which is characterized by unilacunar nodes.

(e) Wood Anatomy:

Wood anatomy has been used at almost all taxonomic levels. Because of their conservative nature, anatomical features of the secondary wood have been very useful in taxonomy and phylogeny.

Along with other lines of evidence, it has been successfully used in deciding the systematic position of primitive vessel less families such as Amborellaceae, Tetracentraceae, Trochodendraceae and Winteraceae, all included under the Magnoliales of angiosperms.

Similarly, due to the presence of specialized wood, it has been agreed by all phylogenists that the Englerian group of primitive angiosperms, namely, Amentiferae (including families like the Salicaceae, Betulaceae, Fagaceae, Juglandaceae, etc.) cannot be considered primitive.

Some of the important features of wood anatomy of taxonomical significance are as follows:

1. Vessel elements:

They are considered to have been derived from the tracheids and their evolution (advancement) is considered to have occurred along the following lines:

(i) Decrease in the length of vessel element.

(ii) Transition from vessels with angular outline to nearly circular outline.

(iii) Loss of borders and decrease of bars on perforation plates.

(iv) Alteration from oblique to nearly transverse angle of end wall.

(v) Pitting of lateral walls of vessels showing evolutionary series from scalariform to transitional to opposite to alternate.

Apart from the structure, the abundance of vessels, distribution of vessels, and sculpturing on vessel walls have also proved to be of taxonomic significance (Fig. 8.5). Thus, solitary vessels are considered to be primitive to aggregate groupings, such as pore clusters, pore multiples and pore chains. Similarly, diffuse-porous woods are considered primitive to ring-porous woods.

2. Axial parenchyma:

The distribution and characteristics of the cells of axial parenchyma and the length and thickness and lignification of their walls, can be useful in taxonomic considerations.

Like vessel elements, evolution (advancement) is considered to have occurred along the following lines:

(i) Absence of parenchyma is primitive at least in some cases e.g., Winteraceae.

(ii) Diffuse arrangement is primitive to diffuse-in-aggregates, such as apotracheal or paratracheal types.

Based on the distribution of parenchyma cells, they can be of two types:

(i) Apotracheal — Parenchyma distributed without any specific relation to vessels.

(ii) Paratracheal — Parenchyma distributed in close association with vessels.

3. Vascular rays:

The characteristics of vascular rays, which can prove useful as taxonomic criteria include ray abundance, cellular composition of rays, dimensions of rays in tangential section, degree of wall thickness and pitting of ray parenchyma cells. Rays with all the cells radially elongated i.e., homogeneous rays are considered advanced to the heterogeneous rays, i.e. rays with both vertically and radially elongated cells.

4. Storied wood:

Storied structure of wood refers to the planes of divisions of cambial initials. Woods with non-stratified cells are considered primitive to storied structures.

5. Presence or absence of latex vessels, resins, gums, crystals, etc. in the wood are also the characters of taxonomic importance.

(f) Sclereids:

Sclereids, i.e. cells with very thick lignified walls, which are widely distributed in the plant body, have been used as diagnostic tools in several taxa like Connaraceae, Nymphaeaceae, Oleaceae, Theaceae, Umoniaceae, and a few genera of Araceae, Acanthaceae, Ericaceae and Melastomaceae (Fig. 8.6).

In dicots, they are more common in woody forms than in herbaceous ones, but they are extremely rare in monocots, except in certain genera of Araceae, Agavaceae, Arecaceae and a few other families. As they exhibit various shapes, sizes and characteristics of their walls, they have been of some taxonomic significance.

Two main types of sclereids have been recognized, viz. isomorphic and polymorphic types. The sclereid forms may be characteristic of a particular species and thus of taxonomic value.

(g) Cellular Contents:

Many types of microscopic characters of cell contents i.e., chemical deposits, can serve as important diagnostic tools, and at times prove extremely helpful in delineating species, genera and families.

Crystals and crystalliferous cells have been found to be of systematic importance in several families of angiosperms such as Euphorbiaceae, Leguminosae, Verbenaceae, etc.

Some of the important chemical deposits of systematic significance are as follows:

I. Albuminoids — e.g. Laportea.

II. Starch grains — The immense diversity in the types of starch grains (Fig. 8.7) may turn out to be a good taxonomic character for the angiosperms in general, e.g. Solarium tuberosum.

III. Protein bodies — Deposition of solid protein depositions have systematic use, e.g. some Cactaceae.

IV. Large silica bodies — Silica bodies in the epidermal cells of various families like Arecaceae, Musaceae and Zingiberaceae and Rosaceae can be used at generic as well as specific levels.

V. Calcium oxalate crystals — They are widely distributed in plants and are of different types, like prismatic, styloid and idioblasts (Fig. 8.8). Their distribution is very specific for a particular taxon and hence of taxonomic importance, e.g. Eichhornia, – Allium.

VI. Cystoliths (calcium carbonate crystals) (Fig. 8.9) — e.g. Cannabinaceae, Moraceae and Urticaceae.

VII. Tanniniferous cells — e.g. The presence and absence of tanniniferous cells in the root cortex of related families of Rapateaceae and Xyridaceae can be used as systematic criterion.

Apart from these, presence or absence of Laticifers, which are cells or a series of fused cells containing latex, and their structure, has also been of some taxonomic value. They are common features of many succulent plants and other plants of arid regions, and vary widely in their structure and the latex in their composition.

For example, certain species in Aroideae lack laticifers or any related structures, while others have longitudinal rows of elongated, cylindrical, sac-like cells.

2. Floral Anatomy:

As the reproductive organs show a high degree of conservation, they have been widely used in the classifications (Fig. 8.11A). At the same time, it is also quite likely that the vascular supply to these floral organs is also conservative and thus more reliable in taxonomic and phylogenetic interpretations (Fig. 8.11B).

The distribution and course of vascular bundles within the receptacle and floral parts have proved to be of systematic significance, particularly in ranking taxa of higher order such as genera and families. Even specific characters may be quite clear in some cases.

The significant role played by the floral anatomy in the solution of morphological problems has been greatly emphasized by Puri. Unlike other branches of anatomy, the application of floral anatomy to taxonomy is limited due to technical and interpretative difficulties.

However with the development of rapid clearing techniques, this branch of investigation received a great impetus. The floral anatomical characters of families and genera are generally well marked and have been useful in solving some fundamental questions, like the nature of flower, carpel, inferior ovary and also several problems related with homologies, phylogeny and taxonomy.

Following are some of the examples of the contribution of floral anatomy in resolving the taxonomic position of some disputed taxa:

I. Confirmation of the origin of the families of Annonaceae, Calycanthaceae and Menispermaceae from Ranunculaceae.

II. Separation of Paeonia from Ranunculaceae and its inclusion under a separate family Paeoniaceae.

III. Derivation of Polemoniaceae and Caryophyllaceae from a caryophyllaceous stock.

IV. Cyperaceae and Poaceae were formerly treated together in one single order. Later Hutchinson separated them and placed them in Cyperales and Poales respectively, which has been confirmed by floral anatomical studies of both the families.

V. Inclusion of Solanaceae and Scrophulariaceae under one single order, Scrophulariales due to uniformity in floral vasculature.

VI. Confirmation of the close relationship between Cyrtandromoea and members of Scrophulariaceae based on the presence of several lateral traces in carpels, a bilocular ovary, and absence of a disc in both.

VII. Support for the removal of Lilaea from Scheuchzeriaceae and be placed under an independent family Lilaeaceae, because both differ in their vascular supply of flower and number of ovules.

VIII. Confirmation of the transfer of Hydrocotyle asiatica L. to the genus Centella in the form of Centella asiatica L.

Ultra Structural Systematics of Anatomy:

Electron microscopy has brought revolution in all biological fields, and so also in the field of taxonomy. Heywood and Dakhshini and Meywood have demonstrated and reviewed the benefits of scanning electron microscopy to plant systematics and have suggested that this ultra structural device represents one of the most powerful taxonomic tools now available for systematic research.

Like the role of SEM in plant micromorphology, Transmission electron microscopy (TEM), aided by ultra microtome techniques, have proved a powerful tool in studying various anatomical aspects of taxonomic significance. However, till date only a few ultra structural characters have been exploited and applied in plant classification.

Some of these characters are as follows:

(a) Dilated Cisternae (DC):

These structures are of common occurrence in the Cruciferae and of the order Capparales. Dilated Cistemae (DC) in the endoplasmic reticulum was first reported by Bonnet & Newcomb in the root cells of Raphanus sativus (radish).

Later they were also reported in the phloem parenchyma of foliar veins in Brassica chinensis  and in Capparis cynophallophor«. The significance of the endoplasmic reticulum in sieve elements has been focused by Spanner & Moattari, in the light of its evolutionary origin.

Depending upon their location in the cells, the internal structure of Dilated Cistemae is of following types:

They usually contain filamentous structures when present in the root cells. They contain protein tubules when present in other parts.

(b) Sieve-Tube Plastids:

The potential uses of ultra structural features of sieve-element plastids, which are of systematic value, were noted almost a decade ago. Since then more than 1500 species from 380 families have been investigated by Behnke with regard to this character.

The plastid elements are of following types, depending upon their accumulation of starch and protein:

i. S-type—:

They are the plastid elements, which accumulate starch. About 65% of the flowering plants have such plastids in the sieve tube elements. They are present in the subclasses Dilleniidae, Hamamelidae, Ranunculidae, a great majority of the orders in the Rosidae, half the members of the Magnoliidae, a few orders in Caryophyllidae, Rhamnaceae.

ii. P-type—:

They are the plastid elements, which accumulate protein. They are further differentiated on the basis of their number and shape of the crystalloids as well as the nature of the filaments surrounding them.

They are present only in Pinaceae among Gymnosperms and among the Angiosperms, in all the 21 families of Monocotyledons and a few groups among Dicotyledons, such as Vitaceae and Leeaceae  and half the members of the Magnoliidae.

iii. S₀-type—:

They are the plastid elements, which have neither starch, nor protein accumulations, e.g. Crassulaceae, Rafflesiaceae and some species of Moraceae, Ulmaceae and Urticaceae. According to Behnke, classification and delimitation of higher taxa in flowering plants can be aided by utilizing the different types of sieve-element plastids.

P-type is considered to be ancestral and the S-type is considered to have been derived from the P-type by loss of protein.

The heterogeneity of the Magnoliidae as to its plastid types is attributed to its basal position in the evolution of flowering plants. The presence of P-type plastids in all groups of Monocotyledons, and the preponderance of S-type in the basal groups of Dicotyledons, have been interpreted as an additional evidence for the independent origin of Monocotyledons and Dicotyledons.

A very good example of the use of plastids has been made in elucidating the inter­relationships and circumscribing the order Caryophyllales (Centrospermae).