In this article we will discuss about the contribution of various botanists towards plant classification.
1. Sexual (or Artificial) System of Linnaeus:
Carolus Linnaeus (1707-1778), a Swedish naturalist published a sexual system of classification in Hortus Uplandicus (1730) and elaborated it in his Genera Plantarum (1737). The latter work is of importance in modern taxonomy as a source of description of 935 genera. The “Species Plantarum” published by Linnaeus in 1753 has been chosen by modern taxonomists as a starting point of present day nomenclature.
Linnaeus classified all plants into 24 classes as follows:
1. Monandria-stamen one e.g. Lemna.
2. Diandria-stamens two e.g. Veronica.
3. Triandria-stamens three e.g. Iris.
4. Tetrandria-stamens four e.g. Mentha.
5. Pentandria-stamens five e.g. Primula.
6. Hexandria-stamens six e.g. Rumex.
7. Heptandria-stamens seven.
8. Octandria-stamens eight.
9. Enneandria-stamens nine.
10. Decandria-stamens ten.
11. Dodecandria-stamens 11.
12. Icosandria-stamens more than twelve, attached to calyx, e.g. Rosa.
13. Polyandria-stamens more than 12 attached to the receptale e.g. Papaver.
14. Didynamia-stamens didynamous (2 long, 2 short), e.g. Linaria.
15. Tetradynamia-stamens tetradynamous (4 long, 2 short), e.g. members of Brassicaceae (Cruciferae).
16. Monadelphia-stamens monadelphous, (in one bundle) e.g. Malvaceae.
17. Diadelphia-stamens diadelphous (in 2 bundles) e.g. Lathyrus.
18. Polyadelphia-stamens polyadelphous (in several bundles).
19. Syngenesia-stamens syngenesious (with united antherlobes), e.g. Asteraceae (Compositae).
20. Gynandria-stamens gynandrous (adnate to the pistil) e.g. Aristolochia.
21. Monoecia-plant monoecious (unisexual flowers on the same plant), e.g. Typha.
22. Dioecia-plant dioecious (unisexual flowers on the separate plants), e.g. Salix.
23. Polygamia-plants polygamous.
24. Cryptogamia-flowers concealed (the cryptogams-algae, fungi, mosses and ferns).
Michel Adanson (1727-1806) in his classical work: “Families des plantes” (1763) published for the first time a natural system of classification and described taxa more or less equivalent to modern orders and families.
Jean. B. de Lamark (1744-1829) was a French biologist. In 1778 he published his classical work “Flore Francoise”.
In this work he laid down the principles of a natural classification:
(1) the determination of which a plant precedes another in a natural series;
(2) the rules for natural grouping of species and
(3) the treatment of order and sub-families. Lamark is well known for his theory – the Lamarckism.
De Jussieu (1699-1777), a French contemporary of Linnaeus. He did not publish his results, but his nephew, Antoine Laurent de Jussieu (1748-1836) published his uncle’s plan along with his own in “Genera Plantarum Secundum Ordines Naturals Disposita” (1789).
His natural classification included a hundred orders (now mostly recognised as families) classified in fifteen classes as follows:
1. Acotyledones-cotyledones absent, cryptogams and some aquatic flowering plants.
2. Monocotyledones, stamens hypogynous.
3. Monocotyledones, stamens perigynous.
4. Monocotyledones, stamens epigynous.
5. Dicotyledones, Apetalae – stamens epigynous.
6. Dicotyledones, Apetalae stamens perigynous.
7. Dicotyledones, Monopetalae – stamens hypogynous.
8. Dicotyledones, Monopetalae – cocolla hypogynous.
9. Dicotyledones, Monopetalae – corolla perigynous.
10. Dicotyledones, Monopetalae-corolla epigynous, anthers united.
11. Dicotyledones, Monopetalae-corolla epigynous, anthers distinct.
12. Dicotyledones, Polypetalae-stamens epigynous.
13. Dicotyledones, Polypetalae-stamens hypogynous.
14. Dicotyledones, Polypetalae-stamens perigynous.
15. Diclines, irregulares-Gymnosperms, Urticaceae, Euphorbias.
Jussieu’s conception of perigyny is different from that of the present day.
Augustin Pyrame de Candolle (1778-1841). His views were expressed in his Theorie elementaire de la botanique (1813). He laid much stress on morphological characters and also recognised the significance of vestigial organs.
The three generations of de Candolle collaborated for the preparations of the monumental work “Prodromus systematis naturalis ragni vegetabilis” (1824-1823) in which every species then known to science was described and classified.
2. Natural System of A.P. de Candolle:
Synopsis of A.P. de Candolle’s system of classification.
I. Vascular Plants with Cotyledon:
(I) Exogens or Dicotyledons; vessel arranged in a ring and embryo with two cotyledons.
(A) flowers with perianths differentiated into calyx and corolla, i.e., perianth leaves in two series.
(a) Thalamiflorae-Petals free on the receptacle.
Cohort 1 – Carpels many and stamens opposite to the petals.
Natural orders 1-8, Ranunculaceae, etc.
Cohort 2 – parietal placentation, carpels solitary or many, connate.
Natural orders – Cruciferae, Violaceae etc.
Cohort 3 – Ovary single.
Natural orders – Caryophyllaceae, Rutaceae etc.
Cohort 4 – Fruit gynobasic.
Natural order – Simarubeae etc.
(b) Calyciflorae – Flowers perigynous, petals free or fused, ovary inferior or superior.
Natural orders – epigynous flowers with gamopetalous corolla, e.g. Rubiaceae, Compositae.
(c) Corolliflorae – Petals united, ovary superior.
Natural order – Acanthaceae, Asclepiadaceae etc.
(B) Monochlamydeae – Perianth in one whorl.
(II) Plants with scattered vascular bundles, cotyledon one. It includes Monocotyledons and Cycadaceae with 27 orders.
II. Plants without Cotyledons:
(a) Foliaceous – Mosses and Liverworts.
(b) Non-foliaceous of unknown sexuality. Fungi, Lichens, etc.
Robert Brown (1773-1858), a Scottish botanist. He published no system of his own but proved that the Gymnosperms were a discrete group with naked ovules and seeds.
Adolphe Theodore Brongniart (1801-1876), a French botanist treated Apetalae as a reduced members of a polypetalae.
3. Natural System Classification of Bentham and Hooker:
George Bentham (1800-1884) and Sir Joseph Hooker (1817-1911) – British botanists associated with Royal Botanic Gardens, Kew, contributed an important system of classification, as a 3 volume work in their Genera Plantarum (1862-1883). In this some 202 orders (now called families) were described.
Summary of Bentham and Hooker’s System of Classification (1862-93):
Embryo with two cotyledons; stem with open bundles; usually reticulate venation; flowers usually tetra-or pentamerous.
Group I. Polypetalae:
Flowers usually with two whorls of perianth, inner whorl or corolla free.
Series I. Thalamiflorae:
Sepals usually distinct and separate, free from ovary; petals 1, 2 or indefinite; stamens hypogynous; ovary superior.
It includes six orders from Ranales to Malvales.
Series 2. Disciflorae:
Sepals distinct or united, free from ovary; disk usually conspicuous as a ring cushion, or spread over the base of the calyx tube, or confluent with the base of the ovary; stamens definite; ovary superior. It includes 4 orders from Geraniales to Sapindales.
Series 3. Calyciflorae:
Sepals united, rarely free, often adnate to ovary; petals uniseriate, peri-or epi-gynous; disk adnate to base of calyx, rarely raised into a torus or gynophore; stamens perigynous, ovary often inferior. It includes 5 orders from Rosales to Umbellales.
Group II. Gamopetalae:
Petals are united.
Series 1. Inferae:
Ovary inferior; stamens usually many or as many as corolla lobes. It includes three orders i.e. Rubiales, Asterales, Campanulales.
Series 2. Heteromerae:
Ovary usually superior; stamens epipetalous or free from corolla, opposite or alternate to its segments or twice as many; carpels more than two. It includes three orders from Ericales to Ebenales.
Series 3. Bicarpellatae:
Ovary usually superior; stamens as many as or less than corolla lobes, alternate to them, carpels two rarely one or three. It includes four order from Gentianales to Lamiales.
Group III. Monochlamydeae or Incompletae:
Flowers usually with one whorl of perianth, commonly sepaloid or none.
Series 1. Curvembryae:
Terrestrial plants with usually hermaphrodite flowers; stamens generally equal to perianth segments; ovule usually solitary; embryo curved. It includes seven families.
Series 2. Multiovulatae Aquaticae:
Aquatic plants with syncarpous ovary and indefinite ovules.
Series 3. Multiovulatae Terrestres:
Terrestrial plants with syncarpous ovary and indefinite ovules.
Series 4. Microembryae:
Ovary syncarpous or apocarpous; ovules solitary or few; embryo very small surrounded by endosperm.
Series 5. Daphnales:
Ovary usually of one carpel, ovules solitary or few; perianth perfect, sepaloid; stamens perigynous.
Series 6. Achlamydosporeae:
Ovary unilocular, one to three ovules; seed without testa.
Series 7. Unisexuales:
Flowers unisexual; ovary syncarpous or of one carpel; perianth much reduced or absent.
Series 8. Anomalous Families:
Unisexual families of doubtful or unknown affinities.
Embryo with one cotyledon; stem with closed vascular bundles; leaves with parallel venation; flowers usually trimerous.
Series 1. Microspermae:
Inner perianth petaloid; ovary inferior with parietal placentation; seed minute, exalbuminous.
Series 2. Epigynae:
Perianth partly petaloid; ovary usually inferior; endosperm abundant.
Series 3. Coronarieae:
Inner perianth petaloid; ovary usually free, superior; endosperm abundant.
Series 4. Calycinae:
Perianth sepaloid, herbaceous, condition of ovary as in series 3.
Series 5. Nudifiorae:
Perianth none or represented by hairs or scales; carpels one or several, syncarpous; ovary superior; ovules indefinite.
Series 6. Apocarpae:
Perianth in one or two whorls, or none, ovary superior, apocarpous; endosperm absent.
Series 7. Glumaceae:
Flowers solitary, sessile in the axils of bracts and arranged in heads or spikelets with bracts; perianth of scales or none; ovary unilocular; one ovule; endosperm present. e.g. Gramineae, Cyperaceae.
4. Phylogenetic Systems of Eichler:
August Wilhelem Eichler (1839-1887), a professor of botany in Germany. He modified the system of Bentham and Hooker by placing gymnosperms in their proper sequence.
The outline of his system is as follows:
5. Phylogenetic System of Engler and Prantl:
Adolf Engler (1844-1930), has adopted the main features of the Eichler’s classification. Engler in association with Karl Prantl (1849-1893) published “Die Naturlichen Pflanzenfamilien” (1887-1915). In this monumental work were given details on the basis of morphology, anatomy and economic aspects of various families, profuse illustrations and keys to the genera dealt within a phylogenetic manner.
Engler’s system of classifications is based upon the trends of evolution in existing families.
In this system plant kingdom is divided into 14 divisions viz.:
(14) Embryophyta or seed plants.
The division Embryophyta is sub-divided into two subdivisions – the Gymnospermae and Angiospermae. The subdivision Angiospermae is further divided as follows:
Sub-division 2. Angiospermae:
Ovules enclosed in the ovary and tracheae or true vessels are present.
Class 1. Monocotyledoneae:
Cotyledon one; leaves with parallel venation; flowers trimerous; stem with closed vascular bundles. It is divided into 11 orders.
Class 2. Dicotyledoneae:
Cotyledons two; leaves with reticulate venation; flower tetra- or pentamerous; stem with open vascular bundles. It is divided into sub-classes:
Sub-class I. Archichlamydeae:
Petals always free. It includes 33 orders.
Sub-class II. Metachlamydeae. (Sympetalae):
Petals always fused. It includes 11 orders.
Critical Assessment of Engler’s System:
Engler’s system is based upon his theoretical concept of a primitive flower. It supposed that the angiosperm flower was derived from a gymnosperm strobilus. The ancestral strobilus was unisexual either megasporangiate or microsporangiate along with their sporophylls.
It produced the catkin type of inflorescence, in angiosperms which are either staminate or pistillate. The bisexual flower was derived from this unisexual type. Hence Engler placed the Amentiferae in the beginning of Archichlamydeae. Here the flowers were either naked or having a monochlamydous bract like perianth.
The dichlamydous and sympetalous (gamopetalous) families were considered more evolved. Engler considered that the gymnosperms gave rise to the Amentiferae on the one hand and the monocots on the other. Angiosperms were therefore polyphyletic in origin. In fact Engler never believed in a linear system of evolution and therefore never tried to draw an evolutionary tree.
The primitive position of Amentiferae has been criticized by a number of botanists who have laid evidences that the anatomy and other features of this group can be best interpreted as a consequence of reduction rather than as a primitive condition.
Amentiferae would thus be a highly specialized group derived from dichlamydous and bisexual flowers. But as recently as 1957 Helsop-Harrison has mentioned that all unisexual angiosperms need not have arisen from bisexual types and it could even be a primitive condition. Thus this point remains unsolved.
Richard von Wettstein (1862-1931), was an Austrian systematist. His classification was similar to that of Engler’s in most respects.
He considered the dicotyledons to be primitive and the monocotyledons to be derived from them through Ranales. In general his system of classification was a much better phylogenetic classification than that of the Engler’s.
6. Phylogenetic System of Bessey:
Charles Edwin Bessey (1845-1915), was the first American to make a major contribution to the knowledge of plant relationship and classification. His system now known as Besseyan system, was evolved in “The Phylogenetic Taxonomy of Flowering Plants”.
The system was based upon a series of ‘dicta’ or statements of guiding principles used to determine the degree of primitiveness or advancement, of the plant groups. Bessey took Ranales as the basal group from which both monocotyledons and other groups of dicotyledons have evolved, but also believed that all flowering plants originated from strobiliferous, cycad ancestors.
Hans Hallier (1868-1938), was a German botanist. His system closely resembled that of Bessey. He made much use of ovule structure and position than other workers.
Alfred Barton Rendle (1865-1938), Director of British Museum London. In his two volumes “Classification of Flowering Plants” he classified plants chiefly according to the Engler and Prantl’s system of classification.
7. Phylogenetic System of Hutchinson:
John Hutchinson of Royal Botanic Gardens, Kew has proposed a system which first appeared in Kew bulletins and later in his “Families of Flowering Plants (1926, 34) and (1959) Vol. I Dicotyledons and Vol. II Monocotyledons”. His system is somewhat like that of Bessey.
Hutchinson’s revised classification has been based on 24 principles which are similar to Bessey’s dicta and are as follows:
1. Evolution is both upwards and downwards.
2. Evolution does not necessarily involve all organs of plant at a time.
3. Evolution has generally been consistent.
4. The trees and shrubs are more primitive than herbs in certain groups.
5. In one family or genus, the trees and shrubs are older than climbers.
6. Perennials are older than biennials and from biennials the annuals are derived.
7. Aquatic flowering plants are derived from terrestrial ones.
8. Collateral vascular bundles arranged in a ring is a primitive condition e.g. dicots.
9. Spiral arrangement of leaves is a primitive character.
10. Simple leaves are primitive than compound leaves.
11. Bisexual flowers are primitive than unisexual.
12. Solitary flowers are more primitive than being arranged in an inflorescence.
13. Spiral arrangement of floral parts is a primitive condition.
14. Polymerous condition is primitive than definite numbers.
15. Apetalous flowers are derived from petaliferous flowers.
16. Polypetaly is primitive than gamopetaly.
17. Actinomorphic condition is primitive than zygomorphic.
18. Hypogyny is primitive than epigyny and perigyny.
19. Apocarpous condition is primitive than syncarpous.
20. Many carpelled condition is primitive than few or one carpel.
21. Endospermic seeds are primitive.
22. Indefinite number of stamens are in primitive flowers.
23. Separate stamens precede connate stamens.
24. Simple fruits are more primitive than aggregate fruits.
Critical assessment of Hutchinson’s System:
Hutchinson imagined the angiosperms to have originated from a Bennettitaliean stock like Cycadoidea along two lines. On one side the woody arborescent types arose and side by side the non-woody herbaceous types also arose. They were termed Lignosae and Herbaceae respectively.
The monochlamydeae was considered polyphyletic from these groups and hence this group was altogether omitted. The Magnoliales represented the most primitive of Ligonsae and Ranales the most primitive of Herbaceae.
This system separated families which were otherwise closely similar because they were either woody or herbaceous. Thus herbaceous Umbelliferae and woody Araliaceae or Cornaceae were separated even though their floral characters were very similar. Similarly the Labiatae, from the woody Verbenaceae or the Bignoniaceae from the Scrophulariaceae and Genseriaceae.
The main weakness of the system seems to be that all the emphasis has been placed only on the herbaceous and woody habit and neglecting other equally important floral characters.
But it must be said to the credit of Hutchinson that his description of families is very correct and that he provided a key which is extremely useful in identification of a family. Hutchinson’s system is not useful for herbarium keeping and therefore it has not been adopted in any herbaria.
Karl Christian Mez (1866-1944), a German professor of botany, evolved the theory that the relationships between the larger groups of genetically related plants could be determined by study and analysis of their protein reactions. This immunological approach is known as serum diagonosis.
8. Oswald Tippo:
Oswald Tippo (born in 1911), formerly of the University of Illinois and now of the New York University of U.S.A. published an outline of a projected classification in 1942. He claims no originality for his system. Tippo’s system is a compilation of systems proposed by G. Smith (1938) for non-vascular plants and by A.J. Eames (1936) for vascular plants.
Linear Sequence of Order in Hutchinson’s Revised System of Classification (1959/60) (with phylogenetic affinities and origin).
Synopsis of Tippo’s System (1942):
Sub-kingdom, I. Thallophyta. Plant forming no embryos.
Phylum 1. Cyanophyta.
Phylum 2. Euglenophyta.
Phylum 3. Chlorophyta.
Phylum 4. Chrysophyta.
Phylum 5. Phaeophyta.
Phylum 6. Rhodophyta.
Phylum 7. Schizomycophyta.
Phylum 8. Myxomycophyta.
Phylum 9. Eumycophyta.
Sub-kingdom II. Embryophyta. Plants forming embryos.
Phylum 1. Bryophyta or Atracheata. Plants without vascular tissues.
Phylum 2. Tracheophyta or Tracheata. Plants with vascular tissues.
Sub-phylum 1. Psilopsida. Root-less and leafless vascular plants.
Sub-phylum 2. Lycopsida. Scale like leaves and simple vascular tissues in plants.
Sub-phylum 3. Sphenopsida. Jointed, ribbed stem; scale like leaves and simple vascular tissues in plants.
Sub-phylum 4. Pteropsida. Large leaves and complex vascular tissues in plants.
Class 1. Filicineae.
Class 2. Gymnospermae.
Class 3. Angiospermae.
Sub-class 1. Monocotyledonae.
Sub-class 2. Dicotyledonae.
Alfred Gunderson in 1950 introduced a phylogenetic system of classification which is based upon chromosome number, anatomical and morphological characters. He has divided dicots into 10 groups and 42 orders in his “Families of Dicotyledons”. He has not described the monocotyledons.
Knowledge of chromosomes has contributed much to the understanding of varieties, species and genera but has contributed little to the understanding of classification of families or orders.
9. Lyman Benson:
Lyman Benson, professor of Botany, California, proposed a new system of classification in 1957 in his book “Plant Classification”. He has divided the vascular plants into five divisions. Benson has mainly followed Bentham and Hooker’s system.
Synopsis of Benson’s System (1957):
Class – Angiospermae (covered seeds)
Sub-class I. Dicotyledoneae (2 seed leaves)
Group 1. Thalamiflorae (hypogynous, polypetalous). It includes 20 orders.
Group 2. Corolliflorae (hypogynous, gamopetalous). It includes 10 orders.
Group 3. Calyciflorae (peri-or epigynous, polypetalous). It includes 12 orders.
Group 4. Ovariflorae (epigynous, gamopetalous). It includes 4 orders.
Group 5. Amentiferae (catkin inflorescence; woody plants). It includes 10 orders.
Sub-class II. Monocotyledoneae (1 seed leaf). It includes 13 orders.
Cronquist, Takhtajan and Zimmermann (1966) proposed a new idea to classify the higher taxa of Embryohionta. They divided the Embryobionta into eight divisions and each in turn is divided into classes and sub-classes.
10. Armen L. Takhtajan (1980):
The latest classification of Takhtajan was published in 1980 with the heading “Outline of the classification of flowering plants (Magnoliophyta)” in the Botanical Review His first paper regarding classification was published on the structural types of gynoecium and placentation (1942). In which he gave a very preliminary phyletic diagram of angiosperms orders.
His actual system of classification was published in 1954 as an appendix in his book on the origin of angiosperms (English translation published in 1958). In 1959 his general system of the angiosperms classification was published with some important modifications.
In 1966 he gave a more detailed rationale for the delimitation and arrangement of families and orders than was in 1959. After 1966 the system of classification has been gradually under going some modification.
The main criteria used by Takhtajan in evaluating the relative degree of advancement of flowering plants were growth habit, leaves and leaf arrangement, stomatal apparatus, nodal structure, wood anatomy, inflorescences; general floral structure – androecium, pollen grains, carpels, gynoecium and placentation, ovules, pollination, gametophytes and fertilization, seeds and fruits.
Takhtajan (1980) divided Division Magnoliophyta or angiospermae in two classes – Magnoliopsida or dicotyledones and Liliopsida or monocotyledones. He further divided magnoliopsida into 7 sub-classes, 20 super orders and 71 orders and Liliopsida or monocotyledones into 3 sub-classes, 8 superorders and 21 orders. In all he recognizes 92 orders and 410 families of angiosperms.
Merits of Takhtajan’s Classification:
1. Dicots (Magnoliopsida) are discussed before monocots (Liliophsida).
2. Dicots begin with the Magnoliales, which are most primitive living angiosperms.
3. The order Alismatales is the starting point of monocots, being most primitive living monocots.
11. Arthur Cronquist (1981):
Cronquist (1981) on the basis of a wide range of taxonomic characters of phylogenetic importance presented a phylogenetic classification. With the help of synoptic keys and detailed charts, he grouped the angiospermic families into orders and subclasses on a worldwide basis. He considered pteridosperms (seed ferns) as the probable ancestors of angiosperms.
In this system class Magnoliopsida (Dicots) has been divided into six subclasses i.e. Magnoliidae, Hamamelidae, Caryophyllidae, Dilleniidae, Rosidae and Asteridae. Of them, the Magnoliidae were thought to be the basal complex and all the remaining five subclasses were derived from it separately. Asteridae formed the most advanced group of dicots.
In this system Class Liliopsida (Monocots) has been divided into five subclasses, i.e. Alismatidae, Arecidae, Commelinidae, Zingiberidae, and Lillidae. Cronquist (1981) proposed that Liliopsida have been arised from aquatic ancestors.
Cronquist (1981) recognized 2 classes, 11 subclasses, 83 orders and 383 families and about 2,19,300 species among the angiosperms.
12. Rolf Dahlgren (1983):
Rolf Dahlgren, Professor at the Botanical Museum, University of Copenhagen, Gothersgrado 130. DK-1123 Copenhagen, K, Denmark, proposed a new classification of angiosperms on the basis of characters from developing fields of micro- and macro molecular chemistry, ultrastructure and micromorphology in combination with macromorphology, anatomy, embryology, paleontology, cytology, biological interaction, and distribution.
Methods used in its construction differ from most previous attempts at classification, in two ways:
1. That many modern groups of angiosperms are ancestors of other living groups. Thus the Magnoliales are not seen as an ancestral group, but as a group which has retained a high proportion of ancestral characters.
2. The characters used in the classification have been drawn consistently from a wide fields of biology.
13. Robert F. Thome (1983):
Thorne (1983), published a phylogenetic system of angiosperm’s classification on the basis of comparative morphology, embryology, cytology, palaeobotany, pollen and seed morphology, plant geography and host-parasite relationships. He believed in theory of monophyletic origin of angiosperms.
He divided class Annonopsida (Angiospermae) into two subclasses, 28 super orders, 54 orders, 75 sub-orders and 350 families. A detail statistical summary of his classification is also given in 2.9.2.
Statistical Summary of Robert.F. Thome’s (1998) Classification:
Thome’s Principles on Evolutionary Trends in Angiosperms:
1. Existing species have descended with change from pre-existing species.
2. Ancestral conditions and trends of specialization are often recognizable in the organs, tissues, and cells of living and fossil angiosperms.
3. The presence of vestigial rudiments of organs, or sometimes the presence of vestigial vascular supply to greatly modified or missing organs, often furnishes evidence of evolutionary reduction, loss, fusion, or other major modification of structures.
4. The prevalence of parallel and convergent evolution in habit, function, and structure is predictable consequence of the relatively limited means, angiosperms have for effective reproduction and for adaptation to available environment.
5. All parts of plants at all stages of their development may produce evidence that is valuable in establishing relationships.
6. Evolution may tend toward elaboration and diversity or toward reduction and simplicity.
7. The rate and direction of evolution may vary in the different organs and tissues of plants.
8. Most of the existing angiosperms are highly specialized and greatly modified from their primitive, generalized ancestors.
9. Evolutionary trends are sometimes reversible under the influence of changes in environmental factors.
10. Once lost, organs usually are not regained.
11. New angiospermous structures have evolved as modifications of or as outgrowths from pre-existing structures.