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Chapter: Plant Biology - Seed plants

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Evolution of vegetative structure, fruits and seeds

Evolution of vegetative structure: Flowering plants are enormously variable in vegetative structure, but the earliest were probably shrubs from which developed trees and herbaceous plants. Vessels appeared early but a few families have none. Dicot leaves are mainly net-veined, many monocot leaves parallel veined, but there are exceptions and some variable families. Evolution of fruits and seeds: Primitive fruits are likely to have been fleshy and seeds filled with endosperm. Many fruit and seed types have evolved in parallel in different families. Major trends have been towards reduction in size of fruit and seed number, and towards seeds with a large embryo and little or no endosperm at maturity.

Evolution of vegetative structure

There is a great range of vegetative form from tall long-lived trees (although none grows as tall or lives as long as certain conifers) to short-lived plants . They may float in fresh water without attachment to any soil or spread by underground rhizomes to cover a large area. Their leaves range from minute duckweeds measuring around 1 mm in diameter, to some palm leaves that can exceed 15 m in length, or they may be reduced to spines or absent. Most woody plants are dicots and have secondary thickening . Palms, bamboos and some other monocots have numerous separate vascular bundles and most only start to produce a trunk when the bud has grown broad at ground level; their trunk often tapers only slightly or not at all. In a few monocot trees, the vascular bundles line up and they have some secondary thickening. Evidence for the direction of evolution in these plants is thin, and vegetative form is flexible, many families containing a wide variety of form and leaf shape. The earliest flowering plants and their living representatives are mainly shrubs or trees and all living gymnosperms are woody. It is likely that herbaceous plants are derived from woody forms originally, but some woody plants, including monocot trees, may be secondarily evolved from herbaceous plants. Short-lived plants are the end-points of the reduction.

Xylem vessels evolved early on, but a few angiosperms including Amborella and the Winteraceae, a family with flowers retaining many primitive features,have none and in water-lilies the xylem cells are somewhat intermediate between tracheids and vessels.

Dicot leaves are mostly net-veined, the veins joining together in many places. Among monocots there is great variation, although many have spear-shaped leaves with parallel veins. Some monocot families, especially the arum family, have enormous variation in leaf shape, with net venation like the dicots and some even developing holes in the leaf such as the ‘swiss-cheese plant’, Monstera, popular as a house plant.

 

Evolution of fruits and seeds

There is some dispute over which fruit type is primitive; fossil evidence gives few clues, but it is likely that the early fruits were fleshy as many of the representatives of primitive groups have fleshy fruits, sometimes with outgrowths

from the seed (an aril). Trends in the evolution of fruits have gone in many directions, towards larger or smaller fruits or seeds, and towards dry dehiscent fruits, but one main trend appears to have been for a reduction in

size and in number of seeds per fruit. There has been a parallel evolution of fruit types in many plant families, i.e. evolution along similar paths in unrelated groups.

Primitive seeds are likely to have been fairly large and rich in endosperm, features retained in some specialized large-seeded plants such as the coconuts. The small seeds, mainly filled by the embryo, that are seen in many families are likely to be derived. Some of the most successful families such as the grass and daisy families have one-seeded fruits, but another large family, the orchids, have many hundreds or thousands of tiny seeds per pod.

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