Plant Kingdom of Angiospermae:
The vegetative cover of the earth including plants that grow under water is estimated to comprise about 393700 species.
The number of species under different major groups is roughly given below:
The above grouping of the plant kingdom represents the traditional classification.
A recent classification based on the study of phylogeny of plants has been proposed by Oswald Tippo and is as follows:
Tippo further arranges these groups of plant in a graphical form to show the relationship between them and the trend of evolution in the plant kingdom.
This is reproduced below:
It is evident from this that Tippo considers that the Filicinae, Gymnospermae and Angiospermae were derived from a common ancestral stock.
Most workers of phylogeny, however, agree that the seed-ferns (Pteridosperms) are the probable ancestors of the angiosperms. They are not the immediate ancestors but the Angiospermae is considered to have originated from, the seed-ferns through several evolutionary changes.
The Pteridosperms are a group of extinct Gymnospermae that flourished in the Carboniferous age and also occurred in the Jurassic. They had simple or branched stem and the branching was monopodial. Many species had cambium.
The large pinnately compound leaves were net-veined in some species. They had micro- and mega- sporophylls which were compound. In some cases these were born on the same plant. It is not unlikely that by repeated reductions the compound sporophylls developed into strobili.
The ovule of a seed-fern had a single integument like most other gymnosperms and one or more ovules were collectively subtended by a cupule. It is quite possible that in course of evolution each cupule subtended a single ovule and the cupule itself was modified into the second integument in Angiospermae.
How did then the primitive angiosperm look like? It is inferred that the hypothetical primitive angiosperm was an evergreen tree with alternate, simple, entire, stipulate, pinnately net-veined leaves. The leaves had ranunculaceous type of stomata. The nodes were unilacunar with 2 leaf-traces, or trilacunar with 3 leaf-traces. The cambium was well-developed and active but annual rings were not pronounced.
Vessels were absent in the wood. The long and slender tracheids had tapering ends and numerous scalariform bordered pits as met with in the ferns. The medullary rays were uni and multi-seriate. True sieve tubes were absent in phloem and so was sclerenchyma, and probably companion cells were also absent. There was no terminal sieve- plate but the sieve-elements were elongate and overlapping with lateral sieve areas.
The flowers were rather large, solitary and terminal on the leafy branches. The perianth was not differentiated into calyx and corolla and the numerous sepals were spirally arranged. Stamens were also numerous and spirally arranged. They were large and flat with the microsporangia embedded in the blade.
The pollen had one long groove and was uniaperturate. Carpels were many and free, distinctly stalked. Each carpel or carpellary leaf was folded along the midrib and the margins were just touching each other. There was a tangle of glandular hairs on the margins of a carpel and these effectively closed the carpel.
There were no style and stigma but the pollen grains germinated on these hairs when captured. The ovules were born on the inner (upper) surface of the carpellary leaf. These were anatropous and each had two integuments. Only one embryo-sac was produced and this was monosporic and eight-nucleate.
Double fertilization occurred. Copious triploid endosperm was formed. The embryo was small at the time of ripening of the capsular fruit. The cotyledons were 2 or more in the seed. The sepals, stamens and carpels were arranged in acropetal succession so that this apocarpus ovary was superior.
The flowers had no nectar, and pollination was by beetles which subsisted on the tender parts of the flower. The sepals, stamens and carpels probably had 3 leaf-traces and 3 primary veins each; from trilacunar nodes or some might have 2 leaf-traces from a unilacunar node.
Such were the characters of the most primitive taxon of angiosperm and origin of such a taxon from the seed ferns through several evolutionary changes is considered not improbable. Did such a taxon ever exist? May be yes or may be no.
We have not been able to discover any concrete evidence to prove that such an ancestor of the living angiosperms ever existed. But we find that several families of angiosperms have at least a few of such primitive characters along with some specialised or advanced characters. Magnoliaceae is one of those considered as a primitive family.
This is a family of trees and shrubs with alternate, usually entire, pinnately-veined, simple leaves. Young buds are enclosed by large deciduous stipules. The flowers are rather large, usually solitary and terminal and bisexual. The receptacle is elongate. The calyx and corolla are distinguishable but often very similar.
The stamens are free and numerous and spirally arranged. Two pairs of linear microsporangia arc deeply immersed in a laminar structure which is not differentiated into filament and connective. The sterile portion is much produced above the microsporangia.
The stamens are trinerved and have 3 leaf-traces. The number of traces varies from 3-7 in some cases and rarely there is only one. The pollen is monocolpate and the sporoderm is of the type of Bennettitales, Cycads and Ginkgo.
The carpels are usually numerous, free and spirally arranged. In the 2 genera Elmerriella and Manglietia the margins of conduplicate carpels are only slightly fused and in Michelia they are fused only in the basal half at anthesis.
The fruit is also of the most primitive type, viz. an aggregate of follicles. The seed contains copious endosperm and a small embryo. The testa in some cases is also of primitive type having a somewhat fleshy external layer.
The stem anatomy also shows primitive characters. The vessels here usually have scalariform perforations. The medullary rays are usually composed of heterogeneous elements. The notes are multi-lacunar and with 6-7 leaf-traces. Palaeobotanical evidence shows that the family was widely distributed in the Late Cretaceous and in the Tertiary. The primitiveness of the family is also proved by cytological characters.
Winteraceae is another family possessing many primitive characters. This is a family of trees and shrubs with simple, entire, pinnately-veined, alternate leaves. The flowers are bisexual or unisexual. Here the calyx and corolla are well differentiated but the stamens are many and in 2-5 spiral turns.
The stamens show specialisation but in Belliolum they are comparatively primitive with a long-produced connective. The carpels although in one’ series, are usually free. In some cases they resemble con-duplicate young leaves. In some species of Drimys, Bubbia and Exospermum the margins of a folded carpellary leaf are not at all fused.
Along the margins of the inner surface there is a broad zone covered by papillose hairs. The pollens fall on the papillose hairs where they germinate and the pollen-tubes pass through the interlocking papillose hairs.
The ovules are situated on either sides of the midrib or median vein away from the margin. The placentation is therefore parietal and not marginal. The flowers are insect pollinated. The fruits are aggregate of follicles or berry-like.
The stem has trilacunar nodes and the xylem is without vessels. The tracheids are long and thick walled. The medullary rays are heterogenous. The wood parenchyma is diffuse. Degeneriaceae is another primitive family which consists of a single genus Degeneria with a single species D. vitiensis, endemic in the Fiji Islands.
Here too the leaves are simple, pinnately net-veined and alternate. The flowers are solitary, supra-axillary, bisexual and spirocyclic.
The calyx and corolla are quite distinct but the stamens and carpels are of very primitive nature. Stamens are many, free and spirally arranged with a few sterile ones above. The stamens are laminar. Two pairs of long micro-sporangia are immersed in sporophyll tissue. Pollens are monocolpate. The gynoecium consists of only one carpel or rarely 2 free carpels.
The carpel is conduplicately folded; the margins are completely free and before anthesis are noticeably distant from each other. Broad lines of interlocking papillose glandular hairs on the ventral as well as dorsal surfaces of the carpels along both the margins from the so called elongate sessile stigma for receiving the pollens.
The ovules are many, arranged in two rows parallel to and remote from the margins. The seed contains a small embryo and 3-4 cotyledons.
The wood is also of a primitive type having thin-walled vessel elements which are angular in outline and have scalariform perforation plates and scalariform pittings on the wall. The medullary rays are uni- or multiseriate. The nodes of the stem are pentalacunar.
Himantandraceae and Eupomatiaceae are two small families allied to Magnoliaceae and Degeneriaceae, and each consisting of only one genus, e.g. Himantandra and Eupomatia. In Himantandra the leaves are simple, entire, pinnately veined and alternate. There are numerous stamens in a flower, and these are laminar and spirally arranged.
The microsporangia are immersed in the sporophyll tissue. The pollen is also of the primitive type. There are 6-10 conduplicate carpels spirally arranged; these are slightly connate at the base in flower but more united in fruit. The embryo is small and endosperm oily.
Eupomatia with only 2 species also show some very primitive characters. Here also the simple leaves are entire, pinnately veined and alternate. The flowers are comparatively large, solitary and terminal. Each flower has numerous laminar stamens with 3-7 traces. There are numerous carpels spirally arranged on the concave receptacle.
The carpels are united but they are slightly open and have decur-rent marginal stigmatic surfaces without any style. Each carpel has several traces. The seeds of Eupomatia have small embryo and ruminated endorsperm.
The leaves of Eupomatia have 7-11 traces. The wood is also of primitive type. The vessel elements are long and slender with oblique ends; perforation bars are numerous and scalariform. The medullary rays are heterogeneous.
Annonaceae is a large family of trees, shrubs and large climbers growing in tropical and subtropical regions. Here the leaves are simple, entire, pinnately veined and alternate. There are numerous stamens in a flower spirally arranged and many free carpels. The pollens are monocolpate. The seed contains a small embryo and copious ruminate endosperm. Here the wood structure is noticeably more advanced.
Several other families also are considered as primitive by characters manifested in different parts of the plants.
What do they indicate? From a study of these primitive families we can conclude that either there existed a hypothetical ancestor which had such primitive characters and which was derived from the seed-ferns or these primitive families originated from the seed-ferns by specialisation in course of evolution through the ages.
So it is the Pteridosperm that gave rise to the Angiosperms as they did to different groups of Gymnosperms also.
Origin of Angiosperms:
1. The Isoetes-Monocotyledon Theory of Campbell:
According to this theory the Monocotyledons were derived from the Isoetes which group bears some superficial resemblance to Monocotyledons. At present there is no supporter of this theory.
2. The Protangiosperm Theory:
According to this theory a group of plants existed in the Mesozoic era having bisporangiate flowers with or without a rudimentary perianth and which were animophilous. This group has been named as Protangiospermae. The embryo here had one or two cotyledons and the vascular bundles were of both types, open or closed.
The protangiosperms gave rise to different lines of Angiosperms. Engler and Prantl’s system of classification is based on this theory and the Pandanales of Monocotyledonae and Amentiferae of Dicotyledonae are considered as most primitive among the angiosperms and are derived from the Protangiospermae.
3. The Gymnosperm Theory:
Many botanists consider that the Angiospermae arose from the Gymnospermae either from a single group or separately from different groups. Thus Wettstein thought that the primitive angiosperms were derived from the Gnetales.
Markgraf, George, Manle and Fagerlind supported the Gnetalian origin of the angiosperms. Hagerup and Embergen stated that angiosperms evolved in several lines from different gymnosperm-groups. Anderson considers that the angiosperms may be hybrids of widely divergent groups of gymnosperms.
4. The Phyllospermae-Stachyospermae Theory:
Sahni was of the opinion that the angiosperms originated in 2 separate lines from the Pteridophytes. One group having foliar type of carpels is called Phyllospermae while the other has the carpels or mega- sporangia protected by sterile microsporangiophores. This theory was redefined and emended by Lam.
5. The Hemiangiosperm Theory:
Arber and Parkin postulated that a group of plants with cycadeoid flowers existed which gave rise to the modern flowering plants and called this hypothetical ancestor as Hemiangiospermae. Bessy, Hutchinson, Lotsy, and Hallier accepted this theory.
6. The Filicales-Cycadales and Lycopodiales-Cordaitales Theory:
Hagerup suggested a diphyletic origin of the angiosperms with one line extending from the Filicales through the Cycads to the Ranales and the second line from the Lycopods through the Cordaitales, Goniferales and Gnetales to such angiosperm taxa as the Centrospermae and Personatae.