Plants with a woody simple or branched axis, giving rise to a shrub or tree habit, are widely distributed. Among the orchids, the terrestrial genus Sobralia is almost shrubby. The Velloziaceae contains many shrubby forms including Vellozia which may be as tall as 6 m and branched.
The Bromeliaceae bears largely rosette plants, but massive columnar tall forms are produced in Puya. The Bambusoideae is mostly tall and woody, a branched shrubby habit being common among grasses of drier zones. A few shrubby species are present in Paepalanthus (Eriocaulaceae), Prionium (Juncaceae) and Microdracoides (Cyperaceae).
Though the woody habit (Ravenala) is rare in the Musaceae, the family comprises such tall plants as the herbaceous Musa. In the Palmae, there are many species with massive columnar trunks. In contrast, branching of the trunk is common in the Pandanaceae which offers a habit akin to that of Hyphaene (Palmae).
However, several species of Freycinetia are neither shrubs nor trees but embrace mostly epiphytic rhizomatous herbs. While the Araceae are predominantly herbaceous, Alocasia and Philodendron have “columnar stems up to 2 m tall surmounted by a crown of leaves”. There are grass trees (Xanthorrhoea), rosette trees (Agave, Furcraea, Yucca, etc.) and tree lilies (Dracaena). Scandent shrubs include species of Petermannia, Philesia, Ripogonum and Ruscus. Only the Iridaceae has any woody members showing actual secondary growth, e.g. Klattia, Nivenia and Witsenia.
The distribution of bulbs is almost exclusively restricted to the Liliaceae. Bulbs also exist in Triglochin bulbosum which is associated with the unusual habit (saline swamps subjected to seasonal drying out). Corms are known mostly from Colchicum and are of somewhat general occurrence in the Iridaceae. Hypocotyledonary tubers form a general feature in the Dioscoreaceae and appear to occur in many Araceae.
The genera of the Potamogetonaceae occur in brackish or fresh water, but Ruppia sometimes in salt ponds with distinctly high salinity. Species of Crinum, Pancratium and Triglochin grow in saline environments. Several species of Pandanus are seashore plants. Among the Palmae, Nypa thrives in banks of tidal waters and Phoenix is highly tolerant of saline soil.
In the stem, the vascular bundles are scattered throughout and are not arranged in a cylinder. Secondary thickening growth is usually lacking except in the woody Amaryllidaceae, Iridaceae and Liliaceae.
Various methods of seed germination have been described for the monocots. In the Alisma type, the radicle is little or not developed at the start of germination; the cotyledon comes out of the seed and becomes straight and elongated. In the Allium type, the cotyledon comes out and assumes a green cylindrical structure.
In the grass type, the cotyledon does not come out of the seed and forms a green structure; it remains within the seed and functions as a sucker. In the palm type, the upper half of the cotyledon remains permanently within the seed and the lower half forms a huge tubular sheath covering the axis. In the orchid type, certain cellular mass in the embryonic region grows out and forms a green primary tubercle and capillary roots from the lower part.
The Cyperaceae and Juncaceae possess chromosomes with diffuse centromeres. This chromosomal type has developed in common ancestors of these taxa, supporting their affinity.
Species accumulating aluminium (excess of 1.000 ppm dry weight of the element in over dried material) are rare amongst monocotyledons. One species of Eleochciris, several species of Aletris and most members of the Rapateaceae are considered as accumulators.
Cyanogenic compounds are known from various taxa. The cyanogenesis in nearly all monocotyledons is tyrosine derived, exceptions being Chlorophytum capense and C. comosum, with holocalin, i.e. the phenylalanine pathway. There is a certain tendency for concentration of cyanogenic compounds in the Araceae, Gramineae and Palmae, whereas the Liliaceae seems to be poor in these compounds.
Chelidonic acid occurs mainly in the Dioscoreaceae and Haemodoraceae. However, the Iridaceae is surprisingly poor and the Liliaceae apparently deficient in the compound. A number of records are known in the Gramineae and one in Canna.
While in the Gramineae the saponins are chiefly of the triterpene type, they are of the steroid type in the Commelinaceae and zingiberaceae. In the palms, they may be both of triterpene and steriod types. Steriod saponins are generally poisonous and serve mainly as a device to protect plants. This may explain their probable absence in the alkaloid rich Amaryllidaceae where they would otherwise be expected from a phylogenetic viewpoint.
The flavonol kaempferol and quercetin are extremely widespread in plants. A third common flavonol, myricetin, are generally less frequent in the monocotyledons. It has a very restricted distribution- iridaceae, Marantaceae, Restionaceae, Sparganiaceae and Zingiberaceae.
While it is a useful marker at tribal level within the last family, its overall distribution is so sporadic that no general significance can be attached to its existence in these plants. Among the various flavones, tricin is almost univeral in the Gramineae. It is also very common in the Cyperaceae and Palmae. The two common flavones of angiosperms include apigenin and luteolin.
Their frequency varies widely, with as high as 95 percent in the Juncaceae to as low as six percent in the Araceae and 1 percent in the Orchidaceae. These flavones also occur in plants in C-glycosidic combination. Glycoflavones are the most common class of flavonoid encountered in the Araceae, Commelinaceae, Gramineae and Palmae.
There is a geographical element in their frequency of occurrence, being more common in tropical and subtropical species of the Orchidaceae and restricted to South African members of the Restionaceae.
Finally, o-methylation may take place at the 5-position of luteolin and produce luteolin 5-methyl ether; the latter compound is unique to the Cyperaceae and Juncaceae. forging a link between the two families. 6-Hydroxy flavonoids are regular components of several families- Cyperaceae and Commelinaceae (with 6-hydroxyluteolin), Eriocaulaceae (with quercetagetin and its 6-methyl either patuletin), Orchidaceae (with 6-hydroxyapigenin methyl ethers) and Bromeliaceae.
In contrast, 8-hydroxy flavonoids have been reported in two families- Restionaceae and Bromeliaceae. While the occurrence of sulphated flavonoids may sometimes be associated with a fresh water or saline habitat, they are of some interest as taxonomic markers. Flavone sulphates occur abundantly in the Gramineae, Juncaceae and Palmae; they are also reported in the Cyperaceae and Restionaceae.
Moreover, they are accompanied by flavonol sulphates in the grasses and palms. Interestingly, sulphates found in the Alismataceae and Hydrocharitaceae differ slightly from those of the grasses and palms; these families are distinctive in possessing sulphates of simpler phenolic acids, e.g. caffeic acid.
There is a great taxonomic regularity in the distribution of dehydroquinate dehydrolyases. Its occurrence in the Cyperaceae, Gramineae and Juncaceae is of interest on account of the existence of certain flavonoid compounds in these families.
Attempts to correlate the host-preferences of parasitizing fungi with monocotyledon taxonomy and phylogeny have been made by Savile (1979). Because of the host distribution of Uromyces spargani, the Typhales is thought to be closely allied to the Arales (Acorus). The Commeliniflorae is considered a relatively homogeneous group with the exclusion of the Typhales. The Juncales is treated as derived from ancestral Typhales as viewed from their Puccinia and Uromyces rusts. The Liliiflorae is assumed to be phylogenetically younger than the Commeliniflorean core groups (Cyperales, Juncales, Poales) and unrelated to them.
The distribution of angiosperms has been surveyed by Raven and Axelrod (1974). Many families are restricted in their distribution to the southern hemisphere and thus were possibly differentiated on the Gondwanaland. Others are partly or entirely concentrated on the northern hemisphere and appear to have their past history on the Laurasian block.
The fact that the Bromeliaceae and Cyclanthaceae are restricted to America in itself does not necessarily mean that they made their appearance after the period with an Antarctic connection. They might have prevailed in a limited portion of Gondwanaland and hence had a narrow range.
The monocots have been variously treated by taxonomists: 12 orders and 42 families; seven series and 16 natural orders; four orders and 27 families; 10 orders and 42 families; nine orders and 46 families; seven orders and 66 families; seven orders and 36 families; nine orders and 39 families; eight orders and 45 families; nine orders and 33 families; four orders and 48 families; ten orders and 35 families; 12 orders and 53 families; 11 orders and 35 families; eight orders and 64 families; 28 orders and 64 families; 29 orders and 69 families; 18 orders and 58 families; 14 orders and 51 families; 18 orders and 61 families ; 20 orders and 67 families; 19 orders and 60 families; 26 orders and 84 families; 11 orders and 45 families; 18 orders and 33 families and 29 orders and 93 families. The present author takes into account 11 orders and 33 families.
The various monocot orders may be distinguished by the following characters:
Order 1. Helobiales:
The Helobiales is characterised by the aquatic habit, hemicyclic to cyclic flowers, free carpels, seeds without endosperm and seedlings with a large hypocotyl.
The order is regarded as the most primitive among the monocotyledons points favouring such a concept are six-fold:
(i) The presence of floral parts which are free and disposed in several whorls;
(ii) The abortion of perianth and reduction in stamen number in some genera of the Najadaceae;
(iii) The occurrence of numerous stamens and carpels, showing the transition from the spiral to whorled arrangement;
(iv) The prevalence of wide lamina-like filaments with basifixed anthers in the members of the Juncaginaceae and Potamogetonaceae;
(v) The incidence of laminal placentation in some members of the Alismataceae and Hydrocharitaceae and
(vi) The incomplete closure of carpels in some taxa of the Hydrocharitaceae.
In the monocots, the solitary bracteole is either median posterior or median lateral. This type of bracteole occurs in the Nymphaeaceae and Ranunculaceae, attaching significance to the idea that the Helobiales is the direct descendent of early dicots and direct ancestor of other monocots.
In fact, Mitra (1974) conjectured such an origin from the Ranales:
The Helobiales, also called Fluviales and Najadales, represents an unnatural assemblage, contrary to the dictum of Engler. Bessey included most of the families in the Liliales. Rendle placed the order after the Pandanales, but did not consider its derivation from the latter. Hutchinson split up the entire taxon into six orders, such as Butomales, Alismatales, Juncaginales, Aponogeionales, Potamogetonales and Najadales. He opined that the Aponogetonales was ancestral to the Najadales.
Moreover, he treated the Potamogetonales as an off-shoot somewhat more primitive than the Juncaginales. The studies of Cheadle (1942) and Uhl (1947) revealed the primitiveness of the Aponogetonaceae, Scheuchzeriaceae and Lilaeaceae as well as the probable highly advanced condition of the Najadaceae.
Cronquist (1968) created three orders and 12 families: Alismatales (Butomaceae, Limnocharitaceae. Alismataceae), Hydrocharitales (Hydrocharitaceae), Najadales (Aponogetonaceae, Scheuchzeriaceae, Juncaginaceae, Najadaceae, Potamogetonaceae, Ruppiaceae, Zannichelliaceae. Zosteraceae).
Stebbins (1974) followed likewise, but Takhtajan (1969) placed two extra families: Posidoniaceae and Cymodoceae—under the Najadales. But Dahlgren (1975) removed the Butomaceae from the Alismatales and placeu it in the Hydrocharitales along with the Hydrocharitaceae and Aponogetonaceae; here the Najadales was meant to accommodate the Najadaceae and seven families (Scheuchzeriaceae, Juncaginaceae, Potamogetonaceae, Zosteraceae, Posidoniaceae, Zannichelliaceae, Cymodoceaceae) remained in the Zosterales.
Thome (1976) accepted three orders and 10 families: Alismatales (Butomaceae, Alismataceae, Hydrocharitaceae), Zosterales (Aponogetonaceae, Scheuchzeriaceae incl. Juncaginaceae, Potamogetonaceae, Posidoniaceae, Zannichelliaceae, Zosteraceae) and Najadales (Najadaceae).
Ehrendorfer (1978) favoured the Alismatales for the Butomaceae and Alismataceae, the Hydrocharitales for the Hydrocharitaceae and the Najadales for the Scheuchzeriaceae, Juncaginaceae. Potamogetonaceae, Zosteraceae, Zannichelliaceae and Najadaceae. Dahlgren and Clifford (1982) proposed the Hydrocharitales for the Butomaceae, Aponogetonaceae, Hydrocharitaceae (incl. Thalassiaceae and Halophilaceae), the Alismatales for the Alismataceae (incl. Limnocharitaceae) and the Zosterales for the Scheuchzeriaceae, Juncaginaceae (incl. Lilaeaceae), Najadaceae, Potamogetonaceae (incl. Ruppiaceae), Zosteraceae, Posidoniaceae, Cymodoceaceae and Zannichelliaceae.
The members of the Helobiales show trends in the floral evolution from the simple haplochlamydeous unisexual to the complex diplochlamydeous bisexual types. In the Najadaceae, the flowers are axial structures consisting of a solitary stamen or an uniovuled ovary which is either perianthless or provided with a sac-like perianth.
In the Potamogetonaceae. the flowers may be unisexual (Zannichellia) or bisexual (Zostera). Although the perianth is present in Althenia and absent, in other members of the Potamogetonaceae, the connective tends to become petaloid in Posidonia or Potamogeton.
In other families, the floral structure confirms to or is derived from the regular trimerous arrangement. There is a tendency to multiplication of the components of the androecium and gynoecium either by doubling or by increment of the whorl numbers.
The Juncaginaceae and Alismataceae contain diplochlamydeous. bisexual, regular and hypogynous flowers. The epigynous flowers of the Hydrocharitaceae are a step towards further advancement, reaching a climax in Vallisneria where the flowers are irregular.
The six families under the order, according to Core (1955), may be identified as follows:
Order 2. Triuridales:
The plants are yellowish or reddish saprophytic herbs, growing on rotting tree trunks or rich humus of the forest-floor. The leaves are small, scaly and non-green. The flowers are small and drawn out into tail-like appendages’, they are perfect or unisexual. The perianth segments are valvate in bud, numerous and free. The ovules are solitary with a single integument. The seed contains a spherical embryo and fatty endosperm.
The order is represented by a single family, the Triuridaceae, the status of which is doubtful. It is a fairly primitive taxon and often stands second and next to the Helobiales. “The apocarpy and sometimes gynobasic stylodia of the carpels have a superficial similarity to those of …Alisma”. It is allied to the genus Scheuchzeria of the Juncaginaceae and Petrosavia and Protolirion of the Liliaceae.
The Triuridaceae includes seven genera and 80 species. The members of this family extend from Guatemala to Brazil.
Andruris khasiana (Benth. & Hook, f.) Schlecht. is found in Khasi Hills, Meghalaya.
Economic uses for plants of the family are unknown.
Order 3. Pandanales:
The order is characterised by the plants dioecious or monoecious, leaves stiffly long and sword-shaped, perianth bristle-like or scale-like, pollen in diads, gynoecium monopistillate and seeds with a copious endosperm.
The Pandanales was treated by Engler and Rendle as the most primitive of the angiosperms due to the remarkably simple flowers and wind pollination. It also formed the first order of Lawrence’s and Core’s monocots. Wettstein included the order in his ninth series after the Spadiciflorae.
Hutchinson removed the Typhaceae and Sparganiaceae from the Pandanales and incorporated them into the Typhales near the Arales, but retained the Pandanaceae in the Pandanales next to the Palmales. Mitra placed it in his sixth monocot order between the Spathiflorae and Juncales.
Cronquist and Stebbins put the Typhales in his subclass Commelinidae and the Pandanales in his subclass Arecidae. Takhtajan accommodated the Pandanales and Typhales in the superorder Arecanae under the subclass Arecidae. Thome transferred the Sparganiaceae and Typhaceae to the Arales, keeping the Pandanaceae within the Pandanales.
Dahlgren shifted the Typhales to the Typhanae and the Pandanales to the Arecanae. Ehrendorfer considered the Typhales under the Juncanae of the subclass Liliidae and the Pandanales under the subclass Arecidae.
Hutchinson reckoned the Pandanales as a very advanced group, somewhat parallel with the palms. He also interpreted the Typhales as a reduced series which took up the aquatic habit and derived from the Liliaceae. But the Pandanales has a very indistinctive development of tap root in the seedling during germination and a primitive type of cotyledon without the differentiation into a lamina and base.
The application of cytological principles in the Pandanales has produced interesting results. In Sparganiam and Typha, the chromosome number is x = 15, while in Pandanus, it is 30, i.e., a multiple of 15. The similarity in chromosome number of the three genera is striking. Moreover, the homogeneity of chromosomes is evident in very small chromosome size with identical types of constriction.
On the basis of their karyotypes, it is very difficult to distinguish the trio as they look very similar to each other. Hence, the evidence from chromosome studies justify their inclusion under the Pandanales as done by Engler. Hutchinson split up Engler’s Pandanales into two taxa: the Pandanales comprising Pandanus and the Typhales embracing Sparganium and Typha, the former being nearly terrestrial and the latter aquatic types.
Remarkable cytological resemblance and genetical relationship exhibited by these genera reveal the artificiality of Hutchinson’s circumscription and the futility of stressing unduly on habitat. But the primitive status of the Pandanales as assigned by Engler cannot be approved. The high chromosome number, coupled with small chromosome size, represents undoubtedly an advanced level of evolution.
Cytological data confirm the homogeneity of the Pandanales, but in no way indicate its primitive condition. Pandanus differs from Typha in the absence of catechol-tannins and leuco-anthocyanins, presence of alkaloids and positive activity of enzyme polyphenolase.
Gibbs (1974) reported the absence of such phenolics compounds as kaempferol, cyanidin, sinapic and ferulic acids in Pandanus and presence of the same in Typha. Sparganium shows more similarities in chemical characters with Typha than with Pandanus. Thus, there appears to be a chemical homogeneity among these three taxa.
The totality of evidence and numerical assessment of the characters drawn from diverse disciplines lend support to the close kinship among the three taxa and their retention under one Englerian taxon.
Due to their unisexual flowers and spathe formation, the Pandanaceae suggests an affinity with the Cyclanthaceae and Palmae. The coalescence of the fruits of Pandanus recalls similar infructescences in Artocarpus.
The Pandanales is a natural taxon. The Pandanaceae contains tropical shrubs or trees, whereas the Typhaceae and Sparganiaceae represent marshy herbs. In all the three families, the form and arrangement of flowers are strikingly alike. The flower is often reduced to a sporophyll, consisting of a single stamen or carpel at the axil of a bract.
In Pandanus and Sparganium, the flowers are crowded in heads to form a compound racemose inflorescence. The occasional union of carpels in Sparganium speaks for their characteristic cohesion in Pandanus. Furthermore, the method of stem branching in Sparganium can be compared to the apparent dichotomy in Pandanus.
The families under the Pandanales according to Core (1955), may be characterised as follows:
Order 4. Palmales:
The order contains a solitary family, the Palmae. The characters of the order are the same as those for the family.
Order 5. Synanthales:
This is a monotypic order with the characters of its one family, the Cyclanthaceae. The order is equivalent to the Cyclanthales of modern taxonomists.
The Cyclanthaceae (Panama hat-palm family) is distinguished by the herbs of palm-like habit, leaves deeply bilobed, flowers minute, unisexual, apetalous and densely crowded in a spadix, carpels united and many-ovuled and fruit a fleshy syncarp.
The family is divided into two subfamilies:
Subfamily I. Carludovicoideae:
Leaves bifid, flabelliform or entire. Male and female flowers in spirally arranged groups. Fruiting spadix not screw-like. Example- Carludovica.
Subfamily II. Cyclanthoideae:
Leaves deeply 2-partite, with forked costa. Male and female flowers in separate alternating whorls or sometimes part spirals. Fruiting spadix screw-like. Example- Cyclanthus.
The Cyclanthaceae indicates great advancement and specialisation in the floral structure; this is evident in the axillary spadix and much-reduced unisexual flowers. This family seems to stand as an intermediate between the palms and aroids in evolutionary development—a view advocated by Engler and Diels (1936) and supported by Harling (1946).
Hutchinson (1959) regarded the taxon as a very advanced climax group, equivalent to the Araceae in so far as the line of development is concerned. “It seems likely that the similarity of the Cyclanthales to the palms in leaf ontogeny and structure reflects an independent realization of similar potentialities rather than direct inheritance from a common ancestor. Neither group could have given rise to the other”.
A family of 11 genera and 180 species, the Cyclanthaceae abounds in tropical America and West Indies.
The leaves of Carludovica angustifolia (Peru) are used for thatching native huts and those of C. sarmentosa (Guyana) for making brooms. The leaves of C. insignis (Eucador) are made into Panama hats.
Order 6. Arales:
The order is characterised by the presence of a thickened spadix subtended by a single large herbaceous spathe.
The Arales includes two families which, according to Core (1955), can be identified as follows:
Order 7. Graminales:
The order is distinguished by the prominent nodes, narrow leaves with a sheathing base and small flowers arranged in spikelets and borne in the axils of dry chaffy bracts.
The Gramineae is a very advanced taxon in which the apparently simple floral structure represents a drastic reduction from an unknown ancestor, possibly from a primitive liliaceous stock.
The usual two families of the Graminales may be characterised as follows:
(1) Culm mostly solid, triangular; leaf-sheath closed; fruit an achene – Cyperaceae
(1) Culm mostly hollow, cylindrical; leaf-sheath open; fruit a caryopsis – Gramineae
Order 8. Farinales:
The order is known by the flowers regular or nearly so, ovary superior and endosperm mealy.
The Farinales is not a homogeneous taxon. Engler considered thirteen families, belonging to six suborders. His basis in creating the order solely on the endosperm character was rejected by Bessey, Wettstein, Hutchinson and others. The families under the order have been distributed by them elsewhere among the monocots and there is no uniformity in their treatments. Within the superorder Commeliniflorae,
Dahlgren and Clifford (1982) recognised eight orders:
Commelinales,
Eriocaulales,
Typhales,
Juncales,
Cyperales,
Hydatellales,
Restionales and
Poales.
The present author follows the plan of Core (1955) and considers five families in the Farinales:
Order 9. Liliales:
The Liliales is characterised by the bisexual pentacyclic trimerous flowers and fleshy endosperm.
There are a number of views concerning the origin of the Liliales. Some have regarded it as originating from the Ranales. Others have derived it from the Commelinales (Farinales). Another idea accepted its derivation from the Triuridales. Still a few have supported its helobial origin.
The Liliales is of phyletic significance, as it is the general basal stock from which prolific evolution of other monocots like the Arales, Palmales, Amaryllidales, Iridaies, Haemodorales, Agavales, Orchidales and much-reduced Glumiflorae (Juncales, Cyperales, Graminales) have taken place.
The order is considered as a primitive group among the monocots, since it has not advanced far in the floral make-up except for the entomophilous habit and the tendency towards zygomorphy as well as epigyny. The primitiveness of the order is also supported by anatomical, morphological and palynological features.
In Engler’s arrangement, the order comprised three suborders and nine families- Juncineae (Juncaceae), Liliineae (Liliaceae, Stemonaceae, Haemodoraceae, Amaryllidaceae, Velloziaceae, Taccaceae, Dioscoreaceae), Iridineae (Iridaceae). Under this order, Rendle considered six of these families by ignoring the Stemonaceae, Haemodoraceae and Velloziaceae.
Core treated seven families in the order, but omitted the Stemonaceae and Taccaceae. Hutchinson constructed several separate orders (Liliales, Amaryllidales, Iridaies, Dioscoreales, etc.) and placed them apart from each other. Ehrendorfer considered six families in the order, Huber seven, Thorne as well as Dahlgren and Clifford eight each, Hamann ten, Cronquist 13, Stebbins 14 and Takhtajan 20.
The present author takes into account six families with their principal characters given below:
Order 10. Scitaminales:
From the stand-point of floral structure and adaptation for cross pollination, the Scitaminales seems to be quite advanced in the scale of monocot evolution. It has been considered as ancestral to or parallel with the Orchidales. It has probably sprung from the liliiflorean stock, since the floral construction of Ravenala and some wild species of Musa resemble that of the Liliiflorae (Liliales).
Bentham-Hooker considered the Scitamineae as a single natural order and put it in their second series Epigynae. Engler, Rendle. Lawrence, Core and Mitra treated the order just before the Microspermae (Orchidales). Bessey placed the families of the Scitaminales under the Iridales. Most taxonomists have divided the order into four distinct families: Musaceae, Zingiberaceae, Cannaceae, Marantaceae.
Hutchinson went to the extent of creating a new order, the Zingiberales, and raising the number of families to six by including the Strelitziaceae and Lowiaceae besides the usual four. Cronquist, Takhtajan, Stebbins and Dahlgren used the ordinal name Zingiberales for the Scitamineae and accommodated eight families (Strelitziaceae, Lowiaceae, Heliconiaceae, Musaceae, Zingiberaceae, Costaceae, Cannaceae, Marantaceae) within the order. Dahlgren and Clifford set up the superorder Zingiberiflorae to place the Zingiberales.
The Scitaminales is a natural taxon. This is reflected in the habit, leaf structure, epigyny. seeds with perisperm and pollen grains. Furthermore, one can trace an increasing floral complexity in conjunction with the entomophilous or orninthophilous habit. “This development finds expression in the reduction of stamens to petaloid structures to which the attractive function becomes more or less relegated”.
As a matter of fact, only 5 stamens are generally fertile in the Musaceae, the sixth being absent or represented by a structure which is not petaloid. Only a single entire stamen is fertile in the Zingiberaceae. But in the Cannaceae and Marantaceae, only half an anther is fertile, the rest of the stamen being somewhat petaloid in form.
Cronquist (1968) pointed out that “… some of the differences between individual families, appear to be intimately related to the mechanism of pollination. A detailed correlation of pollinators with the families and genera of the order remains to the undertaken, however. The adaptive significance of many of the characters of presently unknown or doubtful significance is the arrangement of leaves, and the presence or absence of raphide sacs, laticifers, mucilage ducts and oil cells”.
The usual four families of the Scitaminales may be characterised below:
Order 11. Orchidales:
In the scale of monocot evolution, the Orchidales is the most highly developed group; this is evident in the elaboration, complexity and reduction of floral members. Engler, Rendle, Lawrence, Core, Mitra, Cronquist and Stebbins considered the order as the last evolved and most advanced among the monocots.
However, Wettstein did not concur with this view and regarded his Spadiciflorae and Pandanales as more advanced. Hutchinson, Takhtajan, Ehrendorfer as well as Dahlgren and Clifford treated the orchids as phylogenetically more primitive than the grasses.
The order is characterised by the very numerous minute seeds with an undifferentiated embryo and little or no endosperm.
“The lack of differentiation of the embryo is doubtless at least in part a consequence of mycotrophy, but the number and size of seeds and the loss of endosperm are presumably due to other factors…. The plants are physiologically dependent on their fungal symbionts, sometimes even for food, sometimes only for other factors as yet not fully understood, but in any case they can grow only where their symbiont finds the conditions suitable”.
The order contains two families which may be distinguished as follows:
(1) Flowers generally actinomorphic; seeds with endosperm – Burmanniaceae
(1) Flowers zygomorphic; seeds without endosperm – Orchidaceae.
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