The epidermal structure in angiosperms has been considered as a good taxonomic character specially when used with other characters. Epidermal studies have paid attention during the third and fourth quarter of twentieth century. During the beginning of the third quarter of the twentieth century, importance has been given to a greater extent. Interest in this subject increased considerably in second half of the twentieth century. Foliar structures in several angiosperms families has been discussed by Kondo (1962), Stace (1961, 1963, 1965, 1966, 1969, 1973), Pant (1965), Ahmad (1964a, 1974a), Singh et al. (1976), Trivedi and Upadhyay (1973, 1974, 1976, 1977).
Importance of cuticular studies in angiosperms is currently well recognized as evidenced by the numerous recent publications and review on this subject.
As early as 1810, Brown recognized the Asclepiadaceae as a well defined family and separated it from the Apocynaceae (Brown, 1810a). Recent morphological, palynological and molecular studies have shown that while the Asclepiads do indeed form a natural group they are inserted with in the Apocynaceae as a whole and are better considered a subfamily of the later.
The Asclepiadaceae, as traditionally defined has separately been shown to be a promorphic derivative of the family Apocynaceae; it has often been recommended that the Asclepiadaceae be submerged with the Apocynaceae in order to make the later monophyletic. Till date, however, no comprehensive unified classification has been established (Endress and Bruns, 2000; Endress and Doyle, 2009).
Ahmad (1974a) had worked on two genera Erathemum and Pseuderanthemum. Six species of each genus were studied for their epidermal characters viz., size of intercostals epidermal cells, stomatal frequency, stomatal size, glandular hair and non glandular hairs. He had shown that the two genera were quite distinct on the basis of epidermal features.
Different components of epidermis, the epidermal cell complex, however, seems to have received lesser attention than others. In retrospect, the available information in this one seems to be comparatively scantly and especially there is little information about the general characters of structures and distribution of the epidermal cell complex. The works of the epidermal structures have been discussed by various workers during third phase of the present century.
Studies have been undertaken on cuticles by various workers in the second half of the century (Prillieux, 1856; Weiss, 1865; Oudemans, 1866; Strasburger, 1866-1867, 1901; Rauter, 1870; Prantl, 1872; Vesque, 1881, 1883; Bachmann, 1886; Hildebrand, 1866; Grob, 1896; Tognini, 1897). During this period little attention was devoted to the study of epidermal characters.
Halliar (1912) derived Asclepiadaceae from Linaceae and included Apocynaceae with in the family.
Hutchinson (1959) had placed the family in a separate order Apocynales which includes two families Apocynaceae and Asclepiadaceae.
Stace (1964, 1965) has studied leaf cuticles extensively as well as intensively. Since then importance of the subject increased considerably and many attempts have been made to classify various species of a taxon on the basis of cuticular structure.
Floral characters have long been used by taxonomists for the identification of the taxa. In addition to floral morphology, taxonomist often looks for clues for the solution of their problems to disciplines like plant anatomy, embryology, cytology and palynology.
Linnaeus was the first to list a number of Asclepiads such as Asclepias, Cynanchum, Ceropegia, Pergularia etc. in his book “Species Plantarum” (1753) and “Genera Plantarum” (1754). Jussieu (1789) placed all known Asclepiads with Apocynads under the natural order Apocyneae (Family Apocynaceae). Brown reported his natural order Asclepiadaceae from Apocyneae of Jussieu on the basis of the highly specialized pollinia of the former. He divided the family in to the following three groups as “Suborder”:
a. Asclepiadeae verae
b. Secamone group
Endlicher followed Brown and proposed a system that now appears to reflect suprageneric relationships with in the Asclepiadaceae more accurately than does any other. However, he placed the “Periploceae”. First, the “Secamoneae” second and the “Asclepiadeae verae” that Rosatti (1989) pointed out that according to Article 18.3 (ICBN, 1886) these should be treated as names of subfamilies.
Schumman (1895) recognized 2 groups in the family as subfamilies the “Periplocoideae” and the “Cynanchoideae” which were further divided into 5 tribes. The tribe “Tylophoreae” was further distinguished into 2 subtribes as “Ceropeginae” and “Marsdeninae”.
Schlechter reported the subfamily “Periploceae” from the “Asclepiadaceae” as an independent family the “Periplocaceae”.
Bullock (1957) reported the Schlechter separation of the “Periplocaceae” as an independent family. He divided the family “Asclepiadoideae”, in later 5 tribes were recognized and Stapelieae was including in the Ceropegieae.
Takhtajan (1866), Cronquist (1968), have not recognized these two groups as separate families and consider them as two subfamilies of the family Asclepiadaceae.
Squire and Mansfield (1972) observed the importance of subsidiary cells by isolating stomata on detached epidermis by the low pH treatment. They worked on Commelina communis. In the study correlation between stomatal aperture and changes in solute potential, starch disappearance and potassium levels in the guard cell was observed.
Ahmad (1976) studied stomatal features of Acanthaceae and observed that within limitations, stomatal frequency appears to be quite constant as well as reliable characters and can used for distinguishing one species of the genus from another. Several type of abnormalities were also found by him such as:
a. Stomata with single guard cell
b. Stomata with aborted guard cell
c. Contiguous stomata (twin stomata)
d. Stomata with arrested development
Much emphasis on epidermal characters of leaves has been given by Dilcher (1974). These workers have given detailed description of architectural patterns in angiosperm leaves especially in dicotyledons. They have proposed a detailed classification of epidermal pattern occurring in dicotyledonous plants.
Interest in this subject increased considerably in the second half of the present century, after the publications of research papers by Stace (1961, 1965). Stace described stomatal types and their structures in detail. He emphasized that epidermal characters are most helpful in taxonomic considerations. Recently some workers have advocated that a study of ontogeny of stomata could provide a valid base for taxonomic conclusions (Kondo, 1962; Pant, 1965; Stace, 1965). Ontogenetical studies of stomata in India and elsewhere have been taken in large numbers often the publication of Pant (1965).
One of the most significant families of flowering plants from the evolutionary point of view is the Asclepiadaceae. Benthum and Hooker (1876) in their system of classification placed the Asclepiadaceae together with the Oleaceae, Selvadoraceae, and Apocynaceae.
Kondo (1962) has given epidermal patterns in dicotyledons leaves in relation to the plant taxonomy. Some of the angiospermous families have studied in quite detail as far as their cuticular structures are concerned. Family Apocynaceae has been studied by Kapoor et al., 1969; Sharma et al., 1970).
Rao (1939) was the first in this country to elucidate the epidermal structures of living taxa. He described in detail the epidermal structures in fourteen genera and fifty six species of the Magnoliales.
In broad level cuticular studies of the families Solanaceae and Acanthaceae was undertaken by Ahmad (1974). He has tried to classify the genera and species of these stomata in some Angiospermous families.
Besides Angiospermous, stomata of ferns and Gymnosperms and their structures has also received attention from Maheshwari and Vasil (1961), Nautiyal Singh and Pant (1976) etc. Important contributions on stomatal development have been undertaken by Ramayya et al. (1976), Trivedi and Upadhyay (1976). They studied trichomes morphology in detail and believe that trichomes are as important taxonomically as any other epidermal characters.
Bandulska (1931) have studied the cuticles of living as well as fossil Myrtaceae. Stace (1961, 1963, 1965, 1966, 1969, and 1973) gave a comprehensive account of epidermal structures and their appendages in some detail. He also described different stomatal types and slightly modified the concepts of Metacalfe and Chalk (1950). Stace (1965, 1966) emphasized the importance of cuticular characters in identifying taxa.
Taxonomic significance of stomatal distribution and their important morphological characters have been discussed by Watson in families like Ericaceae (Watson, 1962, 1965). Besides dicotyledons, many genera belonging to monocotyledonous families have also been studied in detail. Stomatal types, ontogeny, epidermal cells and trichomes have been studied in monocotyledonous families by Metcalfe (1960-61), Tomlinson (1961, 1969, 1974), Stebbins and Jain (1960), Stebbins and Kush (1961), Stebbins and Shah (1960), Kidwai (1965, 1974), Pant and Kidwai (1966), Paliwal (1969), Culter (1969), Ghose and Davis (1973-1974), Trivedi and Upadhyay (1976) etc.
Carpenter (2005, 2006) has discussed the stomatal pattern and their evolution and specialized structure in the leaf of Angiosperms. He has discussed the morphology, distribution and homology of the specialized leaf structures and discussed the habitat related adaptive properties of plant cuticular lipids.
Various aspects of cuticles such as function, structure, development, chemistry, biosynthesis and commercial use have been dealt by Martin and Juniper (1970) in their work “The cuticles of plants”. The effect of environment on epidermal characters has been studied by Walker and Dunn (1967), Sharma and Dunn (1968). In 1810 Robert Brown published two seminal papers, “Prodromus Flora Novae Hollandiae” (Brown, 1910a) and “On the Asclepiadeae” (Brown, 1910b). In their publications he reported the Asclepiadeae (Asclepiadaceae) from the Apocineae (Apocynaceae) of Jussieu for the first time. In his prodromus, this came out about a week before the second paper (Forster, 1990). He listed the taxa in two separate orders (families), which he called the Asclepiadeae and the Apocineae. Although Brown’s classification has been universally accepted and implemented, controversy over the elimination of the two families has never been put to rest. They are clearly more similar to each other than to the rest of Gentianales, and in a number of characters there is a gradation from the Apocynaceae to the Asclepiadaceae Safwat (1962), Takhtajan (1966). As an order separate from the Gentianales and as a suborder within Gentianales (Rosatti; 1989), Endress and Bruvns (2000), Endress and Bruvns(2009).
Epidermal characters in relation to Pharmacognosy have been studied in family Asclepiadaceae in details (Krishnamurthy and Sundaram, 1967; Krishnamurthy and Kannabiran, 1970, Mitra et al., 1974).
Grace (2009) has discussed the taxonomic significance of cuticular morphology. Effect of various growth substrances on leaf cuticles have recently been investigated by Dehnel (1960), Gangadhara and Inamdar (1975), Rao et al. (1977), Bohra and David (1974-1975).
The fundamental knowledge on plant cuticles was discussed by Metcalfe and Chalk (1950), Martin and Juniper (1970) and Barthlott et al. (1998) gave a detailed classification and terminology on plant epicuticular waxes. The effect of pollution on leaf surface in Calotropis procera and Psidium guajava L. have been discussed by Yunus and Ahmad (1980, 1981). Yunus et al. (1979) worked on Ricinis communis. Yang et al. (2000) have studied leaf epidermis using light microscope and scanning microscope in Hyoscyameae (Solanaceae). They have also discussed the taxonomic significance of the genera studied by them. Epidermal cell structure, trichomes, stomatal frequency, types and their distribution and cuticular features obtained by scanning electron microscope (SEM) have taken into consideration by these workers. The importance of cuticular study and its relation to the taxonomy in some tropical Scrophulariaceae has been discussed by Nyauame et al. (1993). Bayon et al. (2005) discussed the types of stomata on the foliar surface of family Asclepiadaceae agreed with Metcalfe and Chalk (1950, 1961) and also with Bayon and Arambarri (1999) who mentioned that paracytic was the most frequent type of stomata. Also illustrated the epidermal and trichome ornamentations especially for Oxypetalum solanoides which have hairs, exclusively waxes ornamented on the apical cell.
Cotthem (1970), Fryns and Clacssens (1973), Pant and Kidwai (1969) explained the ontogenic type of stomata and its homologous structures.
Jefree (1996, 2006) discussed the structure and ontogeny of plant cuticles.