Chapter 30

Plant Diversity II – The Seed Plants

The rise of the seed plants

• Recall that our discussions on terrestrial plants up until this point have focused on a transition from the aquatic green algae to the non-vascular bryophytes to the seedless vascular plants

• We have seen the evolutionary trend of the removal of water from the plants life cycle (though not necessarily the removal of the needs of water to sustain life in plants)

• Recall that the bryophytes were dominated by the gametophyte portion of the life cycle and the seedless plants were dominated by the sporophyte—HOWEVER both groups were HOMOSPOROUS

• A shift from homospory to heterospory led to the development of the seed, as two different spore types produce two different gametophytes– Microspores microgametophytes– Megaspores megagametophytes

• Though the microgametophytes are smaller than the megagametophytes—THEY are BOTH smaller than the gametophytes of the bryophytes and seedless vascular plants

• This miniaturization allows for important evolutionary innovations in the seed plants:– The gametophytes can be protected by the sporophyte’s tissues

from environmental stress– We can remove water from the life cycle

• Within the sporophyte– Ovule – Integument – surrounds and protects ;opening (micropyle)– Megasporangium meiosis megaspores megagametophyte– Microsporangium meiosis microspores microganmetophyte (pollen grain)– Fertilization – sperm (usu. non-flagellated) + egg = zygote (2n) embryo– Integument seed coat– Ovule seed!!!!

There is an evolutionary advantage to reproduction with seeds:

• Seed is protected by seed coat• Embryo inside of seed is supplied nutrients

– From female gametophyte tissue or endosperm (flowering plants)

• Seeds are able to cope with harsh environmental conditions…can withstand a period of hot, dry, cold

• Seeds may be dispersed by several agents away from parent plant

Two groups of seed plants

Gymnosperms – “naked seed”Angiosperms – “enclosed seed”

Gymnosperms

• Phylum Cycadophyta – the cycads– 130 spp.– Large cones (modified leaves that bear the sporangia)– “palm-like” leaves

Gymnosperms

• Phylum Ginkgophyta– Only 1 extant species, Ginkgo biloba (the maidenhair tree)– Fan-like leaves– Popular ornamental in cities – tolerates air pollution well

Gymnosperms

• Phylum Gnetophyta– 75 spp.– Only three genera– In North America – Ephedra, mormon tea produces ephedrine

Gymnosperms

• Phylum Coniferophyta– The conifers – all produce cones (modified leaves that bear

sporangia)– Most diverse phylum – 600 spp.

Gymnosperms• Phylum Coniferophyta

– Most conifers have modified leaves – needles• Water conservation measure• Live in cooler climates• Evergreen• Some of the largest (> 110m) and oldest (4,600 years old) living things today

are conifers

Life cycle of typical conifer, Pinus

Angiosperms• A better lifestyle

– Modified leaves to produce flowers (still contain sporagnia!!)

• Ovule seed• Ovary fruit• Fruits aids in the dispersal of the seeds (which they

contain)– Dispersal mechanisms

• Wind• Animals – seed coat protects seed from digestive tract

– Some seeds need to have the seed coat scarified (weakened) to allow for germination

– Some seeds are only allowed to germinate after they have passed through the digestive tract of an animal!!!

Life cycle of typical flowering plantAngiosperms

Life cycle of typical flowering plant• Important key points

– Double fertilization• Sperm + egg = zygote (2n)• Sperm + polar nuclei = endosperm (3n)

– Pollination – transfer of pollen grains to stigma of pistil

• Wind• Animals• Reward for animals’ efforts

– Pollen, nectar, floral parts are food sources– Is there another way to reward an animal????

Angiosperms

Many plants and their animal pollinators have evolved together and are dependant

on one another (coevolution)

Classification of Angiosperms

• Phylum Anthophyta– >250,000 spp.

• Two classes– Class Monocotyledones – MONOCOTS– Class Eudicotyledones – EUDICOTS

• Most successful group of plants– WHY????

Benefits of flowering plants

• Food• Medical uses• Lumber/timber• Clothing• Recreation