Terminology: haploid plant = gametophyte (gameto, gamete + phyte, plant)
diploid plant = sporophyte (sporo, spore + phyte, plant)
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NZ Plants
Concepts: Reproduction in Plants
The alternation of generations: concepts
All plants have two forms or 'generations':
- there is a diploid and a haploid plant
- haploid gametes are formed on the haploid plant by mitosis
- fertilisation creates a diploid plant
- haploid spores are formed on the diploid plant by meiosis
- spores germinate and restore the haploid plant form
The alternation of generations: variations
The alternation of generations is expressed differently in various groups of plants. This is shown in the following three illustrations.
Bryophytes are the only plant group in which the dominant form (the familiar moss plant) is a haploid gametophyte. Water is required in order for the flagellated sperm to reach the egg. The sporophyte is tiny and attached to the moss plant.
In the ferns, the dominant form is the diploid sporophyte (the familiar fern plant). Water is required in order for spores to germinate and form a tiny thumbnail-sized gametophyte on the soil surface. Water is required in order for the flagellated sperm to locate an egg.
The familiar seed plant is diploid and spores germinate within the sporangia to form gametophytes. This results in the female gametophyte being protected and nourished by the seed plant. Water is not required for spore germination or for fertilisation.
The concept of the seed
All seed plants have two sizes of spores:
- small spores (microspores) that will germinate into male gametophytes
- large spores (megaspores) that will germinate into female gametophytes
Both types of gametophytes are responsible for the changes that take place as an ovule becomes a seed; this is shown in the following three illustrations.
(1) Young ovule with megaspore © Pearson Education, Inc (p 599, Biology, 6th ed, Campbell, N and Reece, J)
(2) Ovule with female gametophyte. © Pearson Education, Inc (p 599, Biology, 6th ed, Campbell, N and Reece, J)
The megaspore develops into a multicellular tissue (female gametophyte) containing one or more egg cells. A pollen grain enters through the micropyle and forms a pollen tube that delivers the sperm cells to the egg.
(3) Ovule that has developed into a seed © Pearson Education, Inc (p 599, Biology, 6th ed, Campbell, N and Reece, J)
The fertilised egg forms an embryo which is surrounded by a nutrient-rich tissue. The integument forms a tough, protective, seed coat.
Evolutionary trends among plants
Plant reproduction provides clues about the evolutionary trends among plants.
| Features | Green algae | Bryophytes |
Ferns and fern allies |
Gymnosperms |
Angiosperms |
|---|---|---|---|---|---|
| (n) or (2n) dominant | n |
n |
2n |
2n |
2n |
| homospory & or heterospory | homospory | homospory | homospory (heterospory in a few) | heterospory | heterospory |
| Male gametes | flagellated (in some) | flagellated | flagellated | no flagella in most |
no flagella in all |
| Seeds absent or present | absent | absent | absent | present | present |
| Dispersal agent of male gamete | water | water | water | wind & pollinators | wind & pollinators |
