Evolution is not always upward, but often it involves degradation and degeneration.
In general, homogeneous structures (with many and similar parts) are lower, and heterogeneous structures (with fewer and dissimilar parts) are higher.
Evolution does not necessarily incvolve all organs of the plant equally in any particular period, and one organ may be advancing while another is retrograding.
Upward development is sometimes through an increase in complexity, and sometimes by a simplification of an organ or a set of organs.
Evolution has generally been consistent, and when a particular progression or retrogression has set in, it is persisted in to the end of the phylum.
In any phylum the holophytic (chlorophyll-green) plants precede the colourless (hysterophytic) plants, and the latter are derived from the former.
Plant relationships are up and down the genetic lines, and must constitute the framework of phylogenetic taxonomy.
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The stem structure with collateral vascular bundles arranged in a cylinder is more primitive than that with scattered bundles, and the latter are to be regarded as derived from the former.
Woody stems (as of trees) are more primitive than herbaceous stems, and herbs are held to have been derived from trees.
The simple, unbranched stem is an earlier type, from which branching stems have been derived.
Historically the arrangement of leaves in pairs on the stem is held to have preceded the spiral arrangement in which the leaves are solitary at the nodes.
Historically simple leaves preceded branched ("compound") leaves.
Historically leaves were first persistent ("evergreen") and later deciduous.
The reticulated venation of leaves is the normal structure, and the parallel venation of some leaves is derived from it.
The polymerous flower structure precedes, and the oligomerous structure follows from it, an this is accompanied by a progressive sterilization of sporophylls.
Petaly is the normal perianth structure, and apetaly is the result of perianth reduction (aphanisis).
The apochlamydeous perianth is earlier and the gamochlamydeous perianth is derived from it by symphysis of the members of perianth whorls.
Actinomorphy is an earlier structure than zygomorphy, and the latter results from a change from similar to dissimilar growth of the members of the perianth whorls.
Hypogyny is the more primitive structure, and from it epigyny was derived later.
Apocarpy is the primitive structure, and from it syncarpy was derived later.
Polycarpy is the earlier condition, and oligocarpy was derived from it later.
The endospermous seed is primitive and lower, while the seed without endosperm is derived and higher.
Consequently, the seed with a small embryo (in endosperm) is more primitive than the seed with a large embryo (in scanty or no endosperm).
In earlier (primitive) flowers, there are many stamens (polystemonous), while in later flowers there are fewer stamens (oligostemonous).
The stamens of primitive flowers are separate (apostemonous), while those of derived flowers are often united (synstemonous).
The condition of powdery pollen is more primitive than that with coherent or massed pollen.
Flowers with both stamens and carpels (monoclinous) precede those in which these occur in separate flowers (diclinous).
In diclinous plants the monoecious condition is the earlier, and the dioecious later.
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In 1915 C. E. Bessey published his ideas on flowering plant evolution in a paper, "The phylogenetic taxonomy of flowering plants" (Ann. Mo. Bot. Gard. 2: 108-164). He had trained under Asa Gray and then gone on to become Professor of Botany at the University of Nebraska. In earlier works he had followed the Bentham and Hooker system, but finally came to a synthesis of his own based on the dicta above, and in which the most primitive modern plants were the Ranunculales (the "Ranales" - as also in the Bentham and Hooker system). He believed that both the monocots and the other dicot groups arose from within the Ranunculales, along three major lines, as shown in a diagram in the 1915 paper that has come to be known as "Bessey's cactus" or as "Opuntia besseyi" (also, Judd et al. 2002, Fig. 3.2).
Bessey's ideas were based on a belief that flowering plants arose from the extinct, Mesozoic cycad-like Bennettitales (= cycadeoids, in some texts). In these plants the reproductive structures consisted of cone-like strobili with spirally arranged bracts, microsporophylls, and megasporophylls. Bessey supposed that these organs became transformed into, respectively, the perianth, stamens, and pistils of flowering plants. This hypothesis, and Bessey's system generally, was widely taught in North America where it was seen as providing a useful illustration of how evolution had presumably taken place (Benson 1957).
Outside of North America, however, the system of flowering plant classification devised by Adolf Engler and Karl Prantl in Berlin was followed much more widely. In this system the flowering plants were supposed to have originated along two independent lines from unknown, wind-pollinated gymnosperms similar to the modern conifers. One line led first to the most primitive modern dicots, the "Amentiferae," a group of wind-pollinated plants with small, apetalous flowers in unisexual inflorescences (e.g. Salicaceae, Betulaceae, Fagaceae). The other line led to the most primitive modern monocots, the Pandanales. Engler and Prantl supposed that the evolution of typical modern flowers involved the progressive development of a perianth, and the eventual expression of both genders within a single strobilus. The Engler and Prantl system was followed in most herbaria around the world, and in a great many of the floras written in the 20th century.
Benson, L. (1957). Plant Classification. Boston, D. C. Heath.
Morton, A. G. (1981). History of botanical science. London, Academic Press.
Porter, C. L. (1967) Taxonomy of flowering plants. San Francisco, W. H. Freeman.
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