Angiosperms

Angiosperms

1. Angiosperms: Evolution, Concept and Life Cycle |
Flowering Plants
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;

2. (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

3. 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.

4. • 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.

5. 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

6. 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.

7. • 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.

8. • 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

9. 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.

10. • 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

11. 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

12. 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

13. 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.

14. • 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.

15. ▪ 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.

16. 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.

17. 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

18. 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:

19. ▪ 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

20. 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.

21. 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.

22. 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.

23. 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.

24. 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

25. 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.

26. 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.”

27. ▪ 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.

28. 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

29. 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.

30. ▪ 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.

31. 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

32. 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.

33. ▪ 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.