Chapter 12 Seed Plants (Gymnosperms)

Chapter 12 : Seed plants (Gymnosperms) Seed plants are well adapted to life on land : a) have pollen, carried by wind or animals, instead of sperm. b) do not have a separate gametophyte plant, instead the female gametophyte is protected inside the seed. c) seeds are much larger than spores, with big food reserves so they can live much longer before germinating. All are heterosporous ( pollen and egg cell ). They first evolved about 400 million years ago. Gymnosperms Gymnosperms have naked seeds ( not surrounded by the ovary ). They do not have any flowers, instead the seeds are produced in cones. They are the earliest seed plants that evolved. Gymnosperms (such as pine trees) are usually wind pollinated. Present day gymnosperms are divided into four phyla: 1) Phylum Gnetophyta Close relatives of flowering plants ( angiosperms ). Two main genera survive today : Ephedra and Welwitschia. Ephedra is a desert plant ( western US ) with tiny leaves. It produces nectar to attract insects for pollination. Common name : "Mormon tea", it is used medicinally by native Americans. Ephedra contains ephedrine, used to treat asthma, hay fever. Dietary supplements containing ephedra were banned in 2004, after they were linked to over 100 deaths. Pseudoephedrine is used in some cold medication (also used in making methamphetamine). Welwitschia is a desert plant ( Africa ) with only two leaves, and a long tap root. It gets most of its water from fog and dew that condense on the leaves at night. Photos. Fossils of Welwitschiaare almost 300 million years old. 2) Phylum Cycadophyta (cycads) Mainly tropical / subtropical. Often planted ornamentally ( Common name "sago palms" no, they are not related to palm trees, which are flowering plants!). Wind pollinated: the pollen tube grows close to the egg, then releases sperm. Examples : genus Cycas (Sago palm) Sago palm. Commonly planted in Florida, it is native to Asia. Zamia (Coontie, or Arrowroot). This is native to Florida. The Timucuan tribe of North Florida ground up the roots of this plant to make bread. 3) Phylum Ginkgophyta Only 1 species survives : Ginkgo biloba. Native to China, it has remained almost unchanged for almost 300 million years. It is dioecious and has triangular leaves are deciduous. Sperm are released from the end of the pollen tube (as in cycads). Ginkgo is resistant to air pollution, so is planted in many cities. The seeds smell very bad (like rotting meat), so typically only the male trees are planted. Ginkgo is claimed to be useful in treating Alzheimer's disease, although more testing is needed. 4) Phylum Coniferophyta (conifers) Over 700 species still exist. Conifers are often the dominant trees in cold climates eg Canada. The stem is strengthened with lignin, so they can grow very tall. Usually evergreen, drought tolerant: needle shaped leaves, thick cuticle, stomata sunk in pits. The stem gets wider ( secondary growth ) due to the cambium which produces xylem on the inside of the stem and phloem the outside. Wind pollinated ; pollen tube grows all the way to the egg. Usually bisexual ( or monoecious ): each tree produces small male cones and large, long lived female cones. Conifers are important commercially for wood, paper etc. Slash pine. In Florida almost half the state is forested, and in northeast Florida the paper and lumber industries have over $7 billion a year in output. Example : genus Pinus. Dendrochronology Dendrochronology involves dating past events by studying tree ring growth. Trees typically grow fast in spring, and growth stops in the fall, producing annual rings in the xylem. A sample ("core") of the wood is removed using a stem borer. Dendrochronology Conifers are often used, eg Pinus longaeva (Bristle cone pine). The oldest cone was 5,000 years old. Current records (using trees that died centuries ago) in Europe and US go back over 9,000 years. living bristle Dendrochronology is used in: a) dating archaeological sites. b) studying past climatic changes. A study in 2011 used dendrochronology to show that the Aztec empire in Mexico collapsed due to many years of drought. c) showing fire and insect damage in the past.