Spermatophyta

Gymnosperms are Seed Plants

Gymnosperms have seeds but not fruits or flowers. Gymnos means naked, sperm means seed: gymnosperm = naked seeds. Gymnosperms developed during the Paleozoic Era and became dominant during the early Mesozoic Era. There are 700 living species placed into four divisions: conifers, cycads, ginkgos, and gnetales (such as Ephedra).

Cycads

Cycads retain some fern-like features, notably pinnate leaves and circinate vernation. However, they (usually) produce cones of nonphotosynthetic reproductive structures. These cones are unisexual, in fact the plants are dioecious, having separate male and female plants. Cycads were much more prominent in the forsts of the Mesozoic than they are today. Presently, they are restricted to the tropics. Zamia floridana is the only cycad occurring natively in the continental United States. Several species of Cycas, notably C. revoluta, are commonly encountered cultivated plants in warm, moist areas. Cycas revoluta leaves are often used in Palm Sunday services.

Ginkgos

The ginkgos also were a much more prominent group in the past than they are today. The sole survivor of this once robust and diverse group is Ginkgo biloba, the maidenhair tree. Extensively used as an ornamental plant, Ginkgo was thought extinct in the wild until it was discovered growing natively in a remote area of China. Ginkos are dioecious, with separate male and female plants. The males are more commonly planted since the females produce seeds that have a nasty odor. Pollination is by wind. Recently, Ginkgo has become the current herbal rave.

Conifers

The conifers remain a major group of gymnosperms that include the pines, spruce, fir, bald cypress and Norfolk Island Pine (Araucaria).

Pines

Auracarias

Members of this group of conifers have numerous small, scale-like leaves spiralling around their stems. Araucaria, a major genus that gives its name to the group, is a common ornamental because of the symmetry and beauty of its growth form. The monkey puzzle tree is a species of Araucaria.

Gnetales

The Gnetales are an odd group: they have some angiosperm-like features but are not themselves angiosperms. Cladistic analyses support placement of the gtnetales (or some portion of them) as outgroups for the flowering plants. Three distinctive genera comprise this group: Welwitschia, Gnetum, and Ephedra. Ephedra occurs in the western United States where it has the common name "Mormon tea". It is a natural source for the chemical ephedrine, although there is no evidence the Mormons in Utah (where the plant is extremely common) ever used it for tea. Welwitschia is limited to coastal deserts in Southe Africa, although fossil leaf, cuticle and pollen evidence indicates plants of this type were widespread during the Mesozoic Era. Welwitschia is noted for its two long, prominent leaves.Gnetales has leaves that look remarkably like those in angiosperms, as well as vessels in the xylem, generally considered an angiosperm characteristic.

Angiosperms are Flowering Plants

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearing over 140 million years ago during the later part of the of the Age of Dinosaurs (the beginning of the Cretaceous, 140 million years ago). All flowering plants produce flowers. Within the female parts of the flower angiosperms produce a diploid zygote and triploid endosperm. Fertilization is accomplished by a variety of pollinators, including wind, animals, and water. Two sperm are released into the female gametophyte: one fuses with the egg to produce the zygote, the other helps form the nutritive tissue known as endosperm.

The angiosperms (angios = hidden) produce modified leaves grouped into flowers that in turn develop fruits and seeds. There are presently 235,000 known living species. Most angiosperms also have larger xylem cells known as vessels that improve the efficiency of their vascular systems.

The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers. Click here to view an illustration of suggested paths of floral evolution. However, this view has recently been contradicted by the oldest fossil yet found, a 140 million year old plant found by David Dilcher and his associates.

The angiosperms underwent an adaptive radiation during the Cretaceous, and for the most part escaped the major extinctions at the end of the Cretaceous.

Flowers

Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium).

Angiosperm Life Cycle

Flowering plants also exhibit the typical plant alternation of generations. The dominant phase is the sporophyte, with the gametophyte being much reduced in size and wholly dependant on the sporopohyte for nutrition. The is not a unique angiosperm condition, but occurs in all seed plants as well. What makes the angiosperms unique is their flowers and the "double fertilization" that occurs. Technically this is not double fertilization, but rather a single egg-sperm fusion (fertilization proper) plus a fusion of the second of two sperm cells with two haploid cells in the female gametophyte to p[roduce triploid (3n) endosperm, a nutritive tissue for the developing embryo.

Life cycle of corn, a typical monocot angiosperm. Note the formation of endosperm by "double fertilization". Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Trends in Plant Evolution

Developing from green algal ancestors, plants show a trend for reduction of the complexity, size, and dominance of the gametophyte generation. Plants also developed and refined the root-shoot-leaf axis with its specialized conduction cells. A third trend is the development of the seed to promote the dormancy