Exam 2 seeds Flashcards
Intro to seed plants
A seed consists of an embryo and nutrients surrounded by a protective coat
- Seed plants originated about 360 MYA
- Seeds enabled their bearers to become dominant producers in most terrestrial ecosystems
- Domestication of seed plants had begun by 8,000 years ago and allowed for permanent settlements
Traits of seed plants
- Seeds and pollen key adaptations to life on
land - In addition to seeds, the following are common to all
seed plants
– Reduced gametophytes
– Heterospory
– Ovules
– Pollen
Reduced gametophytes
Gametophytes of seed plants are typically microscopic
- Remain within the sporangia of parental sporophyte
- Depend on sporophyte for nutrition
- Protection from UV and drying
Heterospory
Most (not all) seedless plants are homosporous
- Seed plants are heterosporous
– Megasporangium -> megaspore -> ♀ gametophyte
– Microsporangium -> microspores -> ♂ gametophytes
Gametophytic relations
Female gametophyte develops within an ovule
- Male gametophyte develops within a pollen grain
- Fertilization of ovule with pollen grain leads to the development of a seed
Ovules
- An ovule consists of a megasporangium, megaspore, and one or more protective
integuments - Gymnosperm megaspores have one integument
- Angiosperm megaspores usually have two integuments
fig
Pollen and Production of Sperm
Microspores develop into pollen grains, which contain male gametophytes
Pollination
is transfer of pollen to part of a seed plant containing ovules
seeds
A seed develops from the whole ovule
- A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat
Seed plants:
Gymnosperms and angiosperms
Gymnosperms means “naked seeds”
- The seeds are exposed on sporophylls that form cones (strobili)
- Angiosperm seeds are found in fruits, which are mature ovaries
Gymnosperm origin & evolution
Progymnosperms acquire some adaptations that characterize seed plants (but not others)
Angiosperms began to replace gymnosperm
- Angiosperms now dominate more terrestrial ecosystems
- Today, cone-bearing gymnosperms called conifers dominate in the northern latitudes
Diversity of gymnosperms
- Four extant phyla
– Cycadophyta (cycads)– Ginkgophyta (one living species: Ginkgo biloba)– Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)– Coniferophyta (conifers, such as pine, fir, and redwood)
Phylum Cycadophyta
Large cones and palmlike leaves
- Thrived during the Mesozoic, but relatively few species exist today
Unlike most seed plants, cycads have flagellated sperm
- Individuals have large cones and palmlike leaves
Phylum Ginkgophyta
Consists of a single living species, Ginkgo biloba
- Like cycads, has flagellated sperm
- It has a high tolerance to air pollution and is a popular ornamental tree
Phylum Gnetophyta
- This phylum comprises three
genera: Gnetum, Ephedra and
Welwitschia - Species vary in appearance, and some are tropical whereas others live in deserts
Genus Welwitschia
Only one species
-Desert plant
-SW Africa
-Leaves among the largest of all plants
-dioecious
Genus Gnetum
~35 species
-Tropics
-Trees, shrubs, vines
-Look similar to
flowering plants
–seeds resemble fruits
Genus Ephedra
40 species
-Arid regions
-Global
-Ephidrine
Phylum Coniferophyta
By far the largest of the gymnosperm phyla
– ~3/4 of gymnosperm spp.
- Most conifers are evergreens and can carry out photosynthesis year round
Three key features of the gymnosperm life cycle:
-Dominance of the sporophyte generation
– Development of seeds from fertilized ovules
– The transfer of sperm to ovules by pollen
Pine life cycle 30.5
The pine tree is the sporophyte and produces sporangia in male and female cones
- Small cones produce microspores called pollen
grains, each of which contains a male gametophyte - The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes
- It takes well over two years from cone production to mature seed
Phylum Anthophyta
All angiosperms are in this phylum
- Antho = flower, phyta = plant
- Angio = container, sperm = seed
Flowers fig 30.8
An angiosperm structure specialized for sexual
reproduction
Flower = specialized shoot with up to 4 types of modified leaves
Complete and incomplete flowers
Complete flowers have
– Stamens
– Carpel
– Petals
– Sepals
perfect, or bi flowers have male and female parts (stamens and carpel
Incomplete flowers are missing one or more of the four
-grasses dont have petals
Angiosperm-pollinator
coevolution
- Coevolution is the evolution of interacting species in response to changes in each other
- Many flowering plants have coevolved with specific
pollinators - For example, Darwin correctly predicted a moth with a 28 cm long tongue based on the
morphology of a particular flower
Fruits
Typically consists of a mature ovary but can also include other flower parts
- Protect seeds and aid in dispersal
- Mature fruits can be either fleshy or dry– Wall of ovary (pericarp) becomes soft = fleshy
– Doesn’t = dry - Beans
- Nuts
- Grains
The Angiosperm Life Cycle 38.4
- Sporophyte flowers are composed of both male and
female structures - Male gametophytes are contained within pollen
grains produced by the
microsporangia of anthers - Female gametophyte, or embryo sac, develops within an ovule contained within an ovary at the base of a stigma
- Most flowers have mechanisms to ensure crosspollination between flowers from different plants of
the same species
Archaefructus sinensis
Chinese fossils of 125-million-year-old angiosperms share some traits with living angiosperms but lack others
- May belong to the earliest-diverging group of angiosperms
E.g., Archaefructus sinensis has anthers and seeds within closed carpels but lacks petals and sepals
Angiosperm phylogeny
The ancestors of angiosperms and gymnosperms diverged about 305 million years ago
