School of Sciences, Lautoka Campus BIO509 Botany Lecture 15: Gymnosperms

SEEDLESS VASCULAR PLANTS SEED PLANTS

Seeds Seeds are small embryonic plants enclosed in seed coats, usually with some stored food. Seeds were dramatic innovations during the evolution of vascular plants. Significant adaptations for plants that invaded the land.

Gymnosperms Gymnosperm refers to the exposed nature of the seeds. Gymnos –naked Sperma – seed Four living phyla.

Four phyla of Gymnosperms Phylum Pinophyta Phylum Ginkgophyta Phylum Cycadophyta Phylum Gnetophyta

Learning outcomes Know the difference between seeds and spores, their significance and evolutionary importance. Know the distinguishing features of the four phyla of living gymnosperms. Know the lifecycle of a pine. Know the pollination and fertilization mechanisms in gymnosperms. Know the human and ecological relevance of gymnosperms.

Seeds facilitated invasion of plants on land Seeds have protective seed coat and a supply of food (endosperm) for embryos. Encasing allow embryos to withstand the extremes of the terrestrial environment such as long periods of drought, freezing and even fires.

General characteristics of Gymnosperms Exposed nature of seeds. Seeds are produced on the surface of sporophylls or similar structures instead of being enclosed within a fruit as in flowering plants. Seed bearing sporophylls are spirally arranged in strobilli ( seed cones). Smaller pollen bearing strobilli produce pollen grains.

Female gametophyte is produced inside an ovule that contains a nucellus, enclosed in integument. Integument becomes a seed coat after fertilization. Sporophytes are mostly trees. Gametophytes are reduced in cell number and develop within the sporophyte.

Phylum Pinophyta – the Conifers Largest genus of conifers and Pinus species (Pines). Over 100 living species. Include world’s oldest known living organisms (Bristlecone Pines).

Bristlecone Pines, worlds oldest known living organisms

Recently discovered Wollemi nobilis (Wollemi pine)

Rediscovered in 1994 by David Noble (Wildlife Officer) in a deep gorge only 150km from Sydney. Previously found throughout Australia, New Zealand and Antarctica Oldest fossils are c. 90 million years old (remnant from the age of dinosaurs) “Living Fossil” Less than 100 adult trees remain (additional population found in 2000) Exact location is kept secret. Seedlings are now readily available commercially David Noble planting the Wollembi pine

Pine structure and form Pine leaves are needlelike and are arranged in clusters of bundles of two to five leaves. Each cluster (fascicle) forms a cylindrical rod if the leaves are held together. Fascicles are short shoots with restricted growth.

Pines leaves have several modifications to survive in harsh conditions. Hypodermis is located below the epidermis. Resin canals develop in the mesophyll. Central vascular cylinder surrounded by a layer called the endodermis. Between the two are the mesophyll cells that are tightly packed, unlike those of flowering plants. Conspicuous resin canals found in the mesophyll. These produce resin that protects the tree.

Reproduction Conifers form separate male and female reproductive structures called sporangia Usually on the same plant. Heterosporous, with two kinds of microsporangia Female sporangia are called megasporangia (= female cone). Male sporangia are called microsporangia (= male or pollen cone

Female cone Male cone

Megasporangia Megasporangia borne in pairs at the base of the scales (sporophylls) of the cone. Comprises megasporocyte surrounded by nutritive tissue (the nucellus). Ovule comprises the megasporangium and nucellus surrounded by the seed coat (integument). Ovule = megasporangium (megasporocyte + nucellus) + integument, all diploid like the tree itself.

Diploid megasporocyte (=megaspore mother cell) undergoes meiosis to produce 4 haploid megaspores. One forms the female gametophyte, the other three disintegrate. Mature megagametophyte has 2–6 archegonia (1 egg each). Diploid nucellus and integument surround haploid megagametophyte. Micropyle (pore) opens to pollen chamber

Microsporangia Male cone is much smaller than female cone. Microsporangia are borne in pairs at the base of papery scales. Contain many diploid microsporocytes (=microspore mother cells) that form the pollen.

Microsporocyte (2n) undergoes meiosis to produce 4 microspores (n). Microspores undergo mitosis to produce a generative cell and a tube cell, and a pair of wing-like air sacs develop on the surface, forming the pollen grain. Pollen is often produced in astronomical amounts

POLLEN GRAINS Pollen grain is the microgametophyte and consists of 2 cells. Facilitates transport of sperm to egg and eliminates need for water.

Questions??