Let us make an in-depth study of Angiosperms. After reading this article you will learn about: 1. Evolution of Angiosperms 2. Concept of Angiosperms 3. Objectives 4. Angiosperms or Flowering Plants 5. Alternation of Generations: Life Cycle of Angiosperms 6. Origin of Angiosperms 7. Groups of Angiosperms 8. Principles of Taxonomy and Phylogeny 9. Oldest Angiosperms 10. Ancestry of Angiosperms.
Evolution of Angiosperms:
(i) Diversified habit and vegetative forms;
(ii) Higher degree of perfection of the vascular system; the xylem in addition to the tracheids, contains wood vessels, and the phloem possesses companion cells;
(iii) Successful invasion of all habitats;
(iv) Adaptation of flower to insect pollination;
(v) Bisexual flower, the bisexuality ensures self-pollination if cross-pollination fails, hence the seeds are formed in any case;
(vi) Development of ovules within the ovary which ensures proper protection to the developing ovules and seeds;
(vii) Efficient and effective dispersal by insects, birds, other animals, wind, water and by several other methods;
(viii) Efficient and varied means of vegetative propagation, which result in rapid multiplication;
(ix) Their utmost economic importance, and
(x) Besides the above listed reasons, there must be some hereditary causes such as better equipment of gene potential and useful gene mutations, which enable them to encounter variations in temperature and other environmental changes and led to their conquest over other plant groups.
Concept of Angiosperms:
What is Systematic Botany?
Systematic botany is the science which deals with the classification and naming of the plants. The science of classifying the plants is said to be plant taxonomy and lays emphasis upon phylogenetic relationships. The naming of the plants is known as nomenclature and provides each plant with a name. This way, the systematic botany consists of classifying and naming of the plants.
The important task included in the field of systematic botany is the collection of the plant specimens from all over the world and building the great herbaria. The plants are being classified chiefly on the basis of the study of their comparative morphology.
In modern days the study of systematic botany needs a good background in general botany, cytology, genetics, ecology, plant geography and paleobotany; otherwise, the origin of the plants cannot be traced out.
Nomenclature makes the important part of systematic botany. It deals with names which may or may not indicate relationships. The science of plant taxonomy classifies the plants on the basis of similarities and differences, which are now known as “phylogenetic relationships”. Without naming the plants, they cannot be classified and therefore, nomenclature makes a very important part of systematic botany.
Ecology and systematic botany are interrelated to each other. Certain species are definitely hydrophytic, xerophytic, mesophytic or halophytic. The soil characters are being indicated by the vegetation growing upon. The character of the soil may be indicated by its vegetation.
Scope of Systematic Botany:
(1) Plant taxonomy-which establishes the phylogenetic relationships that exist naturally between many groups of plants,
(2) Nomenclature, i.e., giving of names to all kinds of plants. There are millions of plants all over the world, and it is very clear, that no two individuals are exactly alike.
Most of the individuals are so different from each other that hardly they show any bond of relationship. On the basis of the comparative study of their morphology, the taxonomists have arranged the individual plants in a fairly orderly system. Thousands of specimens have been provided with their names and descriptions.
The group relationships, distribution, properties, etc., of the plants have been studied. This way, systematic botany prepares foundation for all sciences concerning plants.
The systematic botany is of great value in forestry. Every type of tree of the forest must be named and classified. At the same time, the characteristic features, distribution and abundance of the plants must be learned.
The study of systematic botany is equally essential in agriculture and horticulture. To get resistant varieties, etc., the crossing is needed, which is furnished by foreign seeds and plants. The foreign seeds and plants are introduced by the men who have sufficient knowledge of systematic botany.
The work of soil conservation needs the good study of systematic botany. The ecologists who are well trained in systematic botany choose the native grasses and plants which act as sand soil binders.
To study the plant ecology, the knowledge of systematic botany becomes essential. The plant ecologist must be knowing the names of the plants and their relationships to habitat and environment. The character of the soil of a land area may easily be judged by the vegetation growing there.
The study of systematic botany is of great use in tracing the origin of plants. The fossils, scattered here and there in fragments are found to be very useful in tracing out the characters of the ancestral forms of the plants. The study of these fossils requires the sound knowledge of systematic botany.
Objectives of Systematic Botany:
They are as follows:
1. The first objective of systematic botany is to know the various kinds of plants on the surface of earth with their names, affinities, geographical distribution, habit characteristics and their economic importance.
2. The second objective of this science is to have a reference system for plants, where the scientists can work with named entities such as species, genus and family.
3. The third objective is to demonstrate the manifold diversities of plant kingdom and their relation to evolution of plants. A systematic reconstruction of plant kingdom can be made only after the complete knowledge of the individual plants. After making an inventory of the components of plant kingdom, the different facts of evolutionary knowledge with an accurate phylogenetic scheme can be obtained.
4. The fourth objective is to ascertain nomenclature. To every plant a binomial name is given, e.g., Solanum tuberosum Linn. The first name refers to its genus, the second to its species and Linn., or L. for Linnaeus, who first observed and reported the plant.
Angiosperms or Flowering Plants:
Angiosperms or flowering plants form the largest group of plant kingdom, including about 300 families (411 families, Hutchinson), 8,000 genera and 300,000 species. They are considered to be highest evolved plants on the surface of the earth. From Cretaceous age, the angiosperms eclipsed all other vegetation and now they are dominant. They are found almost everywhere in each possible type of habitat and climate.
They occur in deep lakes, deserts, in beds of seas and even on high peaks of mountains. The species of Opuntia (Cactaceae) can survive without water in acute desert conditions, whereas on the other hand the species of Hydrilla (aquatic plant) are extremely sensitive to drought conditions.
Some species are found on rocks, some in waterfalls and also some are marine. The species of Rhizophora, popularly known as ‘mangrove vegetation’ are found near the water of the sea. The epiphytes, parasites, saprophytes, symbionts and even insectivorous plants are also not uncommon.
They may be annual, biennial or perennial herbs, shrubs, trees, climbers, twiners and lianes. On one hand the angiosperms may be as minute in size as a pin head, e.g., Wolffia microscopica, on the other extremity like eucleptiles of Australia may reach upto 300 feet in height.
Alternation of Generations: Life Cycle of Angiosperms:
In the life-cycle of angiosperms, there is alternation of nutritionally independent and more complex sporophyte with the inconspicuous, reduced and parasitic gametophytes. The sporophyte, which may be a herb, shrub or a tree is differentiated into roots, stem and leaves each with a vascular tissue with the highest degree of perfection.
In addition, it bears floral leaves or sporophylls organised into a structure called the flower. The sporophylls are of two kinds microsporophylls and megasporophylls. Each microsporophyll bears four microsporangia. The latter contains microspores which are differentiated by meiosis from the diploid microspore mother cells.
The megasporophyll in a flower part, which is called the ovary, contains one or more ovules. The ovule consists of one or two integuments enclosing the nucellus or megasporangium.
Towards the micropylar end of the megasporangium is differentiated, by meiosis, a single functional megaspore which is haploid. The microspores and megaspores are the first structures of the gametophyte generation.
The sporophyte generation in the angiosperms, thus consists of the sporophyte plant, flowers, microsporophylls and megasporophylls, microsporangia, megasporangia, microspore mother cells and the megaspore mother cells.
With meiosis the sporophyte phase switches on the gametophyte phase. The first structures of the gametophyte phase are the microspores and the megaspores. On germination, they produce the alternate structures in the life-cycle which are the male and the female gametophytes and not the sporophyte plant. The male and the female gametophytes are extremely reduced and parasitic.
The female gametophyte consists only of six cells and two nuclei. Three cells at the micropylar end form the egg apparatus. The other three at the chalazal end form the antipodal group. There are two polar nuclei in the centre. The male gametophyte consists of a pollen tube containing the tube nucleus and two male nuclei.
The eggs and the male nuclei represent the gametes which are the last structures of the gametophyte phase. With the fusion of one of the male nuclei with the egg, the gametophyte phase ends. Fertilization thus is the second critical point in the life cycle.
With it the gametophyte generation switches on to the sporophyte generation. The fertilized egg or oospore is the first structure of the next sporophyte. By segmentation it produces the embryo (i.e., the baby plant in the seed); the ovule to the seed and the ovary as a whole to the fruit.
The embryo lies dormant in the seed and the latter lies embedded in the fruit. The seed and the fruit give adequate protection to the embryo, store up food material for it, and are often well adapted for dispersal. Sooner or later as the seed germinates the embryo grows into a seedling which gradually grows into a mature plant.
Taxa or Taxon:
Taxonomic group of any rank or unit, viz., family, class, order, genus, species, etc., is called taxon (plural: taxa).
For example, the genus Solanum (the name of the genus begins always with a capital letter) consists of many species (the name of the species begins always with a small letter) such as tuberosum, melongena, nigrum, etc., but certain common characters of the genus Solanum can separate this genus from all other genera of the family Solanaceae.
Higher to genus is the family, where one or many genera are correlated between greater number of morphological characters is grouped together. The name of the family ends in aceae, e.g., Brassicaceae, Poaceae, etc.
The family represents a more natural taxon or unit than any of the higher categories, e.g., Brassicaceae, Palmaceae, Solanaceae, Rosaceae are identified as natural taxa by certain characters peculiar to them. The angiospermic families may be separated from one another by floral characters, inflorescence, type, nature of perianth, ovary position, number of carpels, placentation, etc.
The families which are allied to one another are grouped into an order which makes a natural taxon. For example, the order Ranales comprises of Ranunculaceae, Nymphaeceae, etc. Here the families are related to each other in general floral constitution.
What is a Species?
According to new definition the species are biological units that have evolved from a series of often identifiable ancestors, by which the smaller taxa of plants (genera, species, and their subdivisions) may be circumscribed by a combination of genetical, morphological and ecological criteria. According to this view, a species is considered as an objective definitive unit.
Taxonomy and Its Significance:
The study of taxonomy has among its objectives the learning of the kinds of plants on earth and their names, of their distinctions and their affinities, their distributions and characteristics of habitats, and the correlation of these facets of knowledge with pertinent scientific data. A secondary objective of taxonomy is the assemblage of knowledge gained. Floras are published to account for the plants of a given area.
All the products of taxonomic research add to the resources available to the scientist. They are essential to any study of the natural resources of an area, to studies of land potentials, to evaluation of resources of raw materials possible suited to man’s needs (for example, forest products, medicines, food, agricultural crops, ornamentals and industry).
A third important and scientific’ objective is the demonstration of the tremendous diversity of the plants and their relation to man’s understanding of evolution.
Interrelationships with allied sciences:
Taxonomy is dependent on many other sciences and they in turn are equally dependent on it. One who studies taxonomy must have a knowledge of morphology, embryology, floral anatomy, ontogenetical development and teratological variations of the plants with which he works. Modem taxonomists place considerable value on the importance of cytogenetic findings as criteria in delimiting the species and its elements.
In addition to an appreciation and understanding of the contributory value of morphological, anatomical and cytogenetical findings, modern taxonomic studies reflect the significance of distributional patterns and of more detailed data of the extent of normal variation and its causes.
All these viewpoints demonstrate the increasing dependence of taxonomy on the findings of related sciences; the product of modern taxonomic research is rapidly becoming one of synthesis rather than of individual conclusions.
Problems in taxonomy:
During the eighteenth and early nineteenth centuries the study of taxonomy dominated the field of botanical activity. Today the taxonomist is interested in the problems associated with the distribution of the plants. Knowledge of plant distribution is relevant to the determination of geographic areas of origins of species, of genera, and often of families.
These studies in distribution and geography bring taxonomy into the field of phytogeography, the inquiry into why a group occupies that area, how long it has been there, how rapidly it is migrating, and what evolutionary trends it is showing. Studies with this wider viewpoint represent a synthesis of ecologic, genetic and taxonomic aspects leading to a better understanding of a series of common problems.
Origin of Angiosperms:
The angiosperms appeared suddenly in Cretaceous age about 65 million years back. Charles Darwin described this sudden appearance of angiosperms in lower or upper Cretaceous as an ‘abominable mystery. When angiosperms appeared for the first time in lower or upper Cretaceous, they were full fledged like the trees and the herbs of today.
In support of this view Prof. Knowlton advocates in his ‘Plant of the past’, “from the time of their appearance they did not progress at all due to their full-fledged appearance in the Cretaceous”.
The fossil records of the angiosperms also support their appearance full-fledged in lower or upper Cretaceous. The fossils of that age are so characteristic and modem in appearance that most of them can be referred unmistakably to living families, general and even to some species.
Prof. Knowlton stated in his book, ‘Plant of the past’ “if a student of present day trees and shrubs, could have wandered over the hills and vales in those days, he would have found himself quite at home among the trees and shrubs growing there”.
The forms of cycads and conifers, which long dominated the universe were already pushed background and the earth had become infact the earth of flowering plants. Charles Darwin has called this sudden appearance of angiosperms as an “abominable mystery”.
However, some workers do not agree with the doctrine of ‘abominable mystery’. According to H.H. Thomas (1936), the angiosperms of the past replaced many of older gymnosperms in asturine and marshy waters. Graud Eury (1906) believes that the angiosperms came into existence through mutation. Guppy (1919) however, supported the view of mutation.
Prof. Bertrand is of opinion that all the great groups of vascular plants (Pteridophyta, Gymnospermae and Angiosperms) not only arose quite independently of each other but also they originated simultaneously as far back in the Archian period (2000 million years old-oldest).
There is a very considerable but scattered literature on the origin and phylogeny of angiosperms. The palaeobotanical evidence shows that there seems three possibilities as regards the origin of angiosperms.
These possibilities are:
1. That the angiosperms are monophyletic in their origin but have had a very much longer history than at present known, perhaps stretching back into Palaeozoic times and with a whole series of missing links;
2. That the angiosperms are monophyletic but that the first and at present unknown group diverged quickly in terms of geological time, into a considerable number of different groups;
3. That the angiosperms are polyphyletic.
According to Campbell, “both comparative morphology and the geological record indicate that the existing angiosperms represent a number of distinct phyla which cannot be traced back to a single ancestral type”. This statement shows that he does not believe in monophyletic origin of angiosperms.
According to Thomas, the evolutionary tendencies detected in the three groups, i.e., Caytoniales, Bennettitales and Pteriodsperms furnish reasonable grounds for the idea that the angiosperms were derived from various pteridosperms early in the Mesozoic period.
Parkin also argues for the monophyletic origin of the angiosperms.
In conclusion we can say, that the history of the angiosperms is still almost as great a mystery as it was in the time of Darwin. We do not know, when, where or from what the presumably most recent and now dominant large group of existing terrestrial plants arose.
Lotsy says, that hybridization is the key to evolution of angiosperms. This view has been supported by Anderson. He has suggested on the basis of cytological investigations that the angiosperms may have arisen as a result of hybridization between two gymnosperms.
According to Hagerup, the origin of some angiosperms took place from the Coniferales through Gnetales.
According to Eames, Sinnott and Bailey the more primitive angiosperms were arboreal in habit and the herbaceous angiosperms have been evolved from them.
According to Hutchinson, in certain groups, trees and shrubs are probably more primitive than herbs.
Evidently views in this respect are divergent and speculative, the available data being meagre, fragmentary and isolated.
Some of the theories proposed from time to time in this connection, are as follows:
1. Bennettitales-Ranales Theory:
(a) Hallier’s view (1906) regarding the origin of angiosperms is that Ranales, (e.g., Magnolia) seems to be related to Bennettitales and may have been derived from Cycadeoidea, and that the monocotyledons are an offshoot of the dicotyledons.
The elongated floral axis of Ranales with spirally arranged male and female sporophylls and the cone of Cycadeoidea are definitely alike. Both the groups were also abundant in the Cretaceous.
Ranales is regarded as the earliest stock from which the polycarpic families of dicotyledons have arisen, and also the monocotyledons as an offshoot. But the differences in the anatomical structure of the wood, types of sporophylls, nature and position of ovules, etc., in the two groups, i.e., Bennettitales and Ranales, have led to difficulties in accepting this view. It is more probable that both the groups have evolved from a common ancestry and developed in unrelated parallel lines.
(b) Arber and Parkin (1907), proposed the existence of an imaginary group (Hemiangiospermae) having cycadeoid type of flowers linking the above two groups. From this imaginary group the Ranalian type of flowers might have originated, and from this Ranalian type all other angiosperms had sprung up.
They were of the opinion that the earliest monocotyledons were more primitive than the dicotyledons. They also supported the Hallier’s view.
(c) Hutchinson (1925) considered the origin of angiosperms as monophyletic, and supported the views of Hallier (1906) and Arber and Parkin (1907). He believed in the Bennettitalean origin of angiosperms and stressed on two parallel evolutionary lines for the primitive dicotyledons a woody (arborescent) line, called Lignosae, starting from Magnoliales, and a herbaceous line, called Herbaceae starting from Ranales.
He further held the view that the monocotyledons were derived from a primitive dicotyledonous order. The Ranales.
2. Coniferae-Amentiferae Theory:
Engler and Prantl (1924) rejected the Cycadeoidean origin of angiosperms, as proposed by Hallier (1906) earlier. They held the view that dicotyledons and monocotyledons had arisen independently from a hypothetical group of extinct gymnosperms (allied coniferae) with unisexual strobilus which developed in the Mesozoic.
Thus, according to them, the angiosperms had a polyphyletic origin, and evolution took place on several parallel lines from the beginning. They considered the monocotyledons to be more primitive than the dicotyledons.
The unisexual naked (without perianth) condition of the angiospermic flowers, such as Pandanales (monocotyledons) and Amentiferae (catkin-bearing dicotyledons, e.g., Casuarina. Salix, Betula, etc.) was most primitive. But according to the modem classification these orders are regarded more advanced.
3. Gnetales-Casuarinales Theory:
Wettstein (1935) held the view that angiosperms of Casuarina type evolved from Gnetales, (particularly Ephedra), a highly advanced group of gymnosperms, which branched off from the main gymnospermic line. However, there are some angiospermic features in Gnetales but this group has meagre fossil records, and not gone earlier than Tertiary.
Fagerland (1947) was of the opinion that both Gnetales and Proangiosperms had a common ancestor and the modern angiosperms evolved from the Proangiosperms in Polyphyletic lines.
4. Caytoniales-Angiosperm Theory:
Thomas (1925) suggested that Caytoniales, a small group of angiosperm-like plants, discovered in the Jurassic rocks of Yorkshire (England), might be the ancestor of angiosperms. However, Harris (1932-33) opposed this view.
Knowlton (1927) expressed the view in his book Plants of the Past that both Caytoniales and angiosperms evolved from the large extinct Palaeozoic group of Pteridosperms. Arnold (1948), expressed the view that Caytoniales were allied to the Pteridosperms rather than to the angiosperms.
5. Pteridosperm-Angiosperm Theory:
Andrews (1947) was of opinion that seed ferns or Pteridosperms, an ancient group of the Palaeozoic, might be the starting point for the angiospermic plants.
There are two large groups of angiospertns:
(1) Dicotyledons and
(2) Monocotyledons.
Groups of Origin of Angiosperms:
Origin of Dicotyledons:
The dicots are more important and they are supposed to have originated before the monocotyledons. It is thought that for the first time dicotyledons appeared in the early Mesozoic era, or perhaps the late Palaeozoic. The oldest known fossils of dicots are from the lower Cretaceous rocks.
It is generally believed and agreed that the dicotyledons have been originated from gymnosperms of a type somewhat different from present day forms of gymnosperms. According to some similarities noted in these two groups, the development seems to be parallel, or in some respects convergent development of the two groups.
Origin of Monocotyledons:
This is the subject of great controversy. For some time this was thought that the monocotyledons were more primitive than the dicotyledons and probably they have given rise to the dicotyledons. Now, this belief has been totally given up. It is now generally thought that the dicotyledons are more primitive and they have given rise to the monocotyledons.
This is also believed, that the monocotyledons were an offshoot of the primitive dicotyledons back in the early Mesozoic era. They are thought to be monophyletic {i.e., of one origin) in their origin. According to another conception the monocotyledons are not monophyletic but polyphyletic in their origin.
Principles of Taxonomy and Phylogeny of Angiosperms:
The problem of classifying the angiospermic plants in a systematic way may either be termed as, ‘taxonomy of angiosperitis’ or ‘systematic botany’. This is a functional science and deals with identification, nomenclature and classification of the plants found all over the world.
The angiosperms are widely distributed with so many morphological variations that sometimes it seems almost impossible to arrange them in systematic order.
Since the prehistoric times, people were interested in placing the plants in a systematic way and for the first time a few plants were classified according to their medicinal and food value and thus the study of taxonomy of flowering plants began. In nineteenth century, the formation of the principles of taxonomy began and several principles were formed, which are still very helpful in arranging the plants in a systematic order.
Swingle has proposed the principles, which have been uniformly accepted by the botanists. They are as follows:
1. Plant relationships are up and down genetic lines and these must constitute the framework of phylogenetic taxonomy. This will naturally form a branching but not reticulate structure.
2. Some evolutionary processes are progressive while others are regressive.
3. Evolution does not necessarily involve all organs of the plant at one time or in the same direction. One organ may be advancing while another is stationary or retrogressing
4. Evolution has generally been consistent and when a particular progression or regression has set in, it is persisted into the end of the phylum.
5. In any phylum the chlorophyll bearing plants precede the chlorophyll less ones. Saprophytes are derived from independent forms and parasites usually from the saprophytes among the lower plants, and from independent forms among the flowering plants.
6. Usually structures with many similar parts are more primitive, and those with fewer and dissimilar parts are more advanced.
7. Among seed plants the stem structure with collateral bundles arranged in a cylinder is more primitive than that with scattered bundles.
8. In most groups of seed plants woody members have preceded the herbaceous ones.
9. In most groups of seed plants erect members have preceded the vines.
10. Perennials are more primitive than biennials and biennials are usually more primitive than annuals.
11. Scalariform vessels are more primitive than vessels with round pits.
12. The spiral arrangement of the leaves on the stem and of the floral leaves precedes that of opposite and whorled types.
13. Simple leaves are more primitive than compound leaves.
14. Historically leaves were first persistent (evergreen) and later deciduous. This way, the deciduous leaves show advanced character.
15. Among the seed plants the netted venation of leaves is more primitive than the parallel venation.
16. The many-parted flower is the more primitive, the type with few parts being derived from it, and the change is accompanied by a progressive sterilization of sporophylls.
17. A condition in which the perianth consists of like segments is more primitive than one in which sepals and petals are unlike each other.
18. Flowers with petals preceded apetalous ones, the latter being derived by reduction.
19. Polypetalous flowers are more primitive than gamopetalous ones, the latter being derived from the former by symphysis.
20. Regular flowers are more primitive than irregular ones.
21. Spirally imbricate floral parts are more primitive than those that are whorled and valvate.
22. Hypogyny is the primitive structure and from it perigyny and epigyny have been derived.
23. Numerous carpels represent a more primitive condition (than united carpels).
24. Separate carpels represent a more primitive condition than united carpels.
25. Axile placentation preceded parietal and central placentation of ovaries.
26. The presence of numerous stamens indicates a more primitive condition than that of a few stamens.
27. Separate stamens preceded united stamens.
28. Evolution in angiosperms is believed to have proceeded from seeds with two seed coats to those with only one.
29. The primitive seed contains endosperm and a small embryo, the advanced type has little or no endosperm, with the food stored in a large embryo.
30. A straight embryo is usually more primitive than a curved one.
31. The solitary flower is more primitive than the inflorescence.
32. Bisexual flowers preceded unisexual flowers.
33. The monoecious condition is primitive than dioecious.
34. Simple and aggregate fruits preceded multiple fruits.
35. The same evolutionary phenomena have often been repeated as separate occurrences in different parts of the plant kingdom, e.g., loss of chlorophyll, loss of petals, stamens and carpels, acquisition of fleshy texture in fruits and of various types of thorns, change from simple to compound leaves, from erect to prostrate habit and from hypogynous to perigynous or epigynous insertion of floral parts, and lateral union (symphysis) of petals, stamens and carpels.
36. In determining the closeness of relationship between two families or other groups, it is usually best to compare with each other the most primitive, or basal members of each group, rather than those that are simplified by reduction or those that are most highly specialized.
Principles proposed by Swingle:
The following parallel principles of taxonomy proposed by C.E. Bessey (1915) are of equal importance and are used by all modern taxonomists of twentieth century. These are known as Besseyan principles.
They are as follows:
1. There is progressive evolution, i.e., life advances from simplicity to complexity.
2. The existing simpler forms resemble more to their simple ancestors than to the present complex ones.
3. The evolved forms never become like ancestors.
4. The herbaceous habit of plants is more advanced than arboreal habit.
5. Annuals are more advanced than biennials and biennials are more advanced than perennials.
6. Hydrophytes, epiphytes, saprophytes and parasites are more advanced than ordinary terrestrial forms.
7. Compound leaves are more advanced than simple leaves.
8. Bisexual flowers are more primitive than unisexual ones.
9. Dioecious plants are more advanced than monoecious ones.
10. Inflorescence is more advanced than solitary flowers.
11. Gamopetalous flowers are more advanced than polypetalous flowers. The gamopetaly has been derived from polypetaly by the fusion of the petals.
12. Flowers lacking petals (apetaly) are more advanced.
13. Actinomorphic flowers are more primitive than zygomorphic ones.
14. Hypogyny precedes epigyny.
15. Apocarpous condition is more primitive than syncarpous condition.
16. Exalbuminous seeds are more advanced than albuminous seeds.
17. Polyandrous stamens precede jointed condition of stamens.
18. Simple fruits are more primitive than compound fruits.
Turril states, that “the angiosperms were the last of the great groups (Thallophyta, Bryophyta, Pteridophyta, Gymnosperms, Angiosperms) to appear. Hence, changes towards characters peculiar to the angiosperms may with some justification be read as progressive, the more so that they are now the dominant group ecologically over much of the land surface of the globe.”
The ‘conservative’ characters have been supposed by many taxonomists to be most useful and valuable in phylogenetic studies. For plants, the reproductive organs are supposed to be more conservative than the vegetative organs. For example, the morphogenetic evolutionary changes in carpels have been far greater than in any vegetative organ considered throughout Spermophyta.
According to Tuzson, the monocots form an older group than the dicots, because the smaller groups of monocots are separable into series and families which show greater gaps on the whole than those in dicots.
As a Gray is of opinion that a natural system of classification of plants aim to arrange all the known plants of the plant kingdom, in a series of grades according to their resemblances, in all respects, so that each species, genus, tribe, family, order, etc., shall stand next to those which it most resembles in all respects, or rather in the whole plan of structure.
For example, two plants may be very much alike in external appearance, yet very different in their principal structure.
According to Sprague the monophyletic groups are regarded as the only natural ones at and above the family level in existing angiosperms. On the other hand, Hutchinson in his phylogenetic scheme and classification definitely retains groups he considers polyphyletic, e.g., Asterales and Euphorbiales.
Oldest Angiosperms:
Caytoniales:
In Jurassic rocks have been found the oldest known plants which were angiospermous in the true sense of the term; that is, in having seed enclosed in a carpel. These plants represent two closely-related genera of the order Caytoniales.
The carpels were borne on sporophylls. Each sporophyll consisted of a central stalk with pinnately arranged short side branches each of which was terminated by a carpel or fruit. The carpel was completely closed, and the tip of the portion of the pinnule which bent over became the stigma.
Pollen grains have been found attached to the stigmas. The seed were borne within the closed carpel (Fig. 6.3). The integument of the seed is rather strikingly similar to that of certain seed of seed-ferns. The fruits were fleshy, so the seeds may have been scattered by animals that ate the fruit.
The anthers are found on sporophylls similar in general outline to those which bore the carpels. The branches of the sporophylls divided, and the tips of the divisions bore groups of stamens (Fig. 6.2). The anthers were sessile or at the ends of short filaments. They had a four- winged form, and each wing seems to have contained a pollen sac. Thus they had the same general form as the stamen of a modern angiosperm.
The leaves which appear to belong to the known Caytoniales are of a type which was very widespread during the Jurassic period and extended from Triassic to Cretaceous times. The venation was netted, but similar to that of Glossopteris which is generally regarded as a seed-fern. The general character of the leaves, sporophylls, and seed indicate that the Caytoniales were derived from the seed ferns.
In the Caytoniales from the Jurassic the pollen seems to have been caught on a stigma, and the ovules are enclosed in ovaries, but in a form from the upper Triassic pollen grains are found in the micropyles of the seed. In this form there appear to have been canals running through the “stigma,” and through these canals pollen grains reached the micropyles of the seed (Fig. 6.4).
This Triassic Caytonia is yet completely angiospermous. It has been evidently not suggested that the walls of the fruit of the Caytoniales had their origin in the fusion of cupules which surrounded the seed of Cycadofilicales.
Unfortunately we do not know how the sporophylls of the Caytoniales were borne, or what kind of plant bore them, and so the relationship of the Caytoniales to modern angiosperms is obscure.
Ancestry of Angiosperms:
The ancestry of angiosperms has long been a moot question. There is not enough evidence to reach a definite decision, and there is much disagreement as to theory.
Among living gymnosperms the group which is most similar to the angiosperms is the Gnetales. While there is great dissimilarity between the Gnetales and the angiosperms, still there is enough similarity to convince many that either the angiosperms are descended from the Gnetales, perhaps from extinct forms, or else the two groups are closely related and have a common ancestry.
Some regard the Gnetales as being intermediate between the conifers and angiosperms, and so are inclined to a belief in a coniferous ancestry for angiosperms. This belief is based in part on similarity in wood structure and on the fact that in conifers, Gnetales, and angiosperms the fertilization is by male nuclei and not by spermatozoids as in all other living groups of land plants.
A rather complete understanding of the “flower” of Cycadeoidea was followed by an extensive discussion of a cycadophyte ancestry for angiosperms; not from Cycadeoidea with its highly specialized stem, rather from one more nearly related to Williamsonia or Williamsoniella, but still more primitive.
It was pointed out that the bracts, stamens, and ovuliferous cone of Cycadeoidea were in the same relative positions as the perianth, stamens, and carpels of the angiosperms. Also, the embryo had two cotyledons, while the seeds were almost enclosed by the inter-seminal bracts.
The discovery of the Caytoniales, first in the Jurrasic and later in the late Triassic, has been taken by some as indicating that the angiosperms, through forms related to the Caytoniales, are descended direct from the seed-ferns.
A recent theory is that the carpels of angiosperms were derived through a modification and fusion of cupules which surrounded the seeds of Cycadofilicales, and that the ovaries of the Caytoniales represent an intermediate condition between the seed-ferns and modern angiosperms.
One feature which is common to practically all theories as to the origin of the angiosperms is that they go far back into early Mesozoic or latter Palaeozoic times; also that their ancestors were generalized forms and the not such specialized ones as modem conifers or cycads or the late Mesozoic cycadeoideas.
A conservative suggestion is that they were derived from somewhere in the general gymnospermous complex, from a line in which the marked peculiarities of more modern groups had not become so pronounced as they appear in the well-known specialized types.
Our ignorance as to the ancestry of the angiosperms is not surprising when we remember how scanty our knowledge of plant floras is. If, as seems probable, the angiosperms evolved in Arctic regions, much of the record may be thickly covered with an icy mantle and inaccessible to us.
Also, the record of former vegetation is largely that in and around swamps, about lakes, and in similar situations; and any trace of much of that which flourished on higher ground is forever lost.
However, in comparatively recent years a great deal of evidence as to the history and relationships of land plants has been discovered; so we may hope that perhaps someday we may have a fairly connected account of the development of the angiosperms.