Faculty of Science, School of Sciences, Natabua Campus Lautoka BIO706 Embryology Lectures: 12 Evolution of seed plants - I

Evolutionary Tree of Kingdom plantae

Overview of seed plant evolution There were three (3) major reproductive adaptations in the evolution of seed plants. Reduction of gametophyte (Gametophytes of seed plants are microscopic, so they can exist within the sporophyte). Seeds became an important means of dispersal. Pollen (Sperm cells) eliminated the liquid-water requirement for fertilization.

Delicate female gametophytes are protected from environmental stress. 1) Reduction of the gametophyte continued with the evolution of seed plants. In seed plants, the very small female (1N) gametophyte develops from spores retained in the sporangia of the (2N) sporophyte. Why?? Delicate female gametophytes are protected from environmental stress. Embryos that are produced are also protected from stress.                        

Three variations on gametophyte/sporophyte relationships

2) Seeds became an important means of dispersal.                         Seeds are resistant to environmental stress, and are multi-cellular complex structures that consist of a sporophyte embryo, a food supply, and a protective coat.   The embryo develops from the fertilized egg (derived from a spore) retained within the sporangium. The sperm (derived from a spore) is carried (produced by) a pollen grain released from a sporangium.

All seed plants are heterosporous. Female spores give rise to female gametes; male spores give rise to male gametes.                                     The megasporangia produce megaspores (female) and the microsporangia produce microspores (male).   The megasporangium is enveloped by layers of tissue called integuments.  The whole structure (integuments + megasporangium) is called the ovule.

From ovule to seed.

3) Pollen (with sperm cells) eliminated the liquid-water requirement for fertilization.                         Pollen travels by air or on animals.  It eliminates the need for water to be present during fertilization.

Hypothetical phylogeny of the seed plants. Note- two clades for seed plants.

Gymnosperms Gymnosperms are vascular plants that bear naked seeds – seeds not enclosed in specialized chambers.  Mesozoic era was the age of gymnosperms Gymnosperms were the most common plants during the Mesozoic era (the age of dinosaurs).  

Four phyla of extant gymnosperms   – Phylum Ginkgophyta: e.g., Ginkgo biloba. – Phylum Cycadophyta: e.g., cycads. – Phylum Gnetophyta: e.g., ephedra – Phylum Coniferophyta: e.g., pines, firs, spruces

Ginkgo biloba

cycads

ephedra

pines, firs, spruces

Pine nursery and plantation

Recently discovered Wollemi nobilis (Wollemi pine)

Life cycle of a pine demonstrates the key reproductive adaptations of seed plants                         1.  The tree is the sporophyte. 2.  Female gametophyte develops within the sporangium. 3.  Pollen cone has microsporangium that develops into pollen (male gametophyte). 4.  After fertilization, the embryo develops and is surrounded by food reserves and a seed coat. 5.  Embryo grows to produce a new sporophyte.

The life cycle of a pine.

Male Pine cone LS-Male Pine cone

Pine pollen grains

Female Pine cone LS-female Pine cone

Ovule development in female pine cone

Winged seed of a White Pine (Pinus strobus)

Questions are welcome