Flowering plants evolved distinctive characteristics that help to define this plant division. Angiosperms possess flowers, develop fruits, contain specialized conducting cells in their vascular tissues, develop a double-layered seed coat, exhibit a distinctive column-like structure in their pollen grain walls, and undergo double fertilization during their life cycle.

New Reproductive Stategies

New reproductive strategies helped angiosperms become a great success and diversify into the forms we know today. In angiosperms male and female structures develop within flowers. The pistil is the central, female organ of the flower and typically consists of an ovary with ovules, a style and stigma. The stamen is the male part of a flower and typically consists of a filament or stalk topped with pollen producing anthers.

When pollen comes into contact with a flower's stigma the growth of a pollen tube is activated. Each pollen grain carries two sperm. One sperm fertilizes an egg in the ovule; the other sperm unites with two haploid cells in the same ovule. This process is known as double fertilization and is an important adaptation found in angiosperms.

The fertilized egg will undergo cell division to become a zygote and then an embryo. The second fertilization results not in offspring, but rather the development of endosperm, which acts as a nutrient for the embryo. Cells in the endosperm have three sets of chromosomes. Endosperm not only serves as an important food source for the embryos of flowering plants it also is important to other animals. Humans depend upon the endosperm of rice, wheat, and corn. Recent research indicates the endosperm may also act as a fertilization sensor helping to abort embryos of incompatible crosses (Juniper & Mabberley 2006, p.27).

A seed is formed when the endosperm and the embryo become enveloped in a part of the ovule that hardens into the seed coat. The ovary or other parts of the flower in angiosperms develop into a fleshy fruit surrounding the seeds. Many organisms such as birds, bats, and insects have coevolved to help pollinate angiosperms. The fleshy fruits of angiosperms are an adaptation for seed dispersal. Many animals use the fruit as a food source, which results in the dispersal of seeds encapsulated within a natural fertilizer!

Monocots & Dicots

Traditionally angiosperms are divided into the monocotyledons and dicotyledons. Today angiosperms are divided into the monocots, eudicots, and magnoliids. Monocots and eudicots are monophyletic groups. Eudicots contain most of the dicots. It is useful to known the major differences between monocots and dicots (eudicots & magnoliids) when studying both extinct and extant plants.

Monocots have one cotyledon (seed leaf) at germination. Monocots usually have flower parts in threes, one aperture or furrow on their pollen, parallel leaf venation, a scattered arrangement of vascular bundles, and usually no secondary woody growth. Grasses and palms are well known examples of monocots. Petrified plam wood or Palmoxylon is the state stone for Texas and the state fossil for Louisiana. The state stone for Mississippi is petrified wood and much of the fossil wood found in the state is Palmoxylon.

Dicots have two cotyledons when they germinate. Today there are six times as many dicots as monocots. Dicots usually have flower parts in fours or fives, possess three apertures on their pollen (except the magnoliids, which have one), netlike leaf venation, vascular bundles arranged in rings, and commonly have secondary woody growth (Willis & McElwain, 2002, pp. 156-157). Woody dicots possess eustele stems; a central pith surrounded by secondary wood and bark. Woody deciduous trees such as oak, elm, and maple are good examples of dicots. When looking at permineralized wood in cross-section one can quickly distinguish between gymnosperms and angiosperms with a 10x loupe.

New Vascular Tissue

Most angiosperms have two cell types that are distinctly different in size. The large, water conducting cells, are called vessels; the smaller diameter, more abundant cells are fibers. Gymnosperm wood is made of small diameter tracheids. Tracheids are more easily seen with a 20x loupe. Angiosperms also have tracheids for water conduction. Among the angiosperms we can also distinguish between dicots and monocots. Dicots have their vessels and fibers arranged in rings while monocots have their vascular bundles scattered throughout the stem giving a speckled appearance even to the naked eye (Kenrick & Davis, 2004, p. 74).

A New Age of Plants

Sycamore Leaf
Platanus wyomingensis
Green River Formation
Eocene
Uintah County, Utah
11 cm wide x 10 cm tall

Poplar Leaf with Insect Damage
Populus wilmattae
Green River Formation
Eocene
Bonanza, Utah

Bibliography

Elias, T.S. (1980). The Complete Trees of North America. Van Nostrand Reinhold Company: New York.

Juniper, B.E. & Mabberley, D.J. (2006). The Story of the Apple. Timber Press, Oregon.