Chapter 30: Plant Diversity II:

The Evolution of Seed Plants

 

3rd of the “Big Four”: Gymnosperms (Naked Seed Plants)

 

These species were evolving at the same time as the previous group (late Devonian on) Climatic changes towards end of Permian period (warmer, drier) made these much better adapted to survive the Permian Extinction (app. 250 m.y.a.) [Fascinating reading “The history of life is punctuated by mass extinctions” on pp. 490-492]

These same adaptations also allowed these plants to fill vacant niches during the Mesozoic Era = 'age of gymnosperms" (250-65 m.y.a.)

 

Adaptations of first vascular seed plants:

 

 1. reduction of the gametophyte (see Fig. 30.1 for overview*)

               no longer an independent generation

               retained w/in reproductive tissue of the sporophyte

               advantages--protection from desiccation, ionizing radiation

 

2. pollination replaces flagellated sperm

 

3. evolution of seed

               Advantages over spores:

               zygote develops into embryo protected w/in seed coat along with food supply

               protected from drought, freezing by protective seed coat

               more capable of wider dispersal by wind, water, animals

 

* [In nonvascular (moss) sporophyte is dependent on gametophyte.

     In seedless vascular (ferns) gametophyte is free-living.

     In seed plants gametophyte is dependent on sporophyte].

 

Four phyla/divisions:

Phylum/Division Coniferophyta (“cone-carrying”) See Fig. 30.8 for common species:

Pine, fir, cypress, juniper, yew, sequoia.

·        largest number of extant species (app. 550)

·        includes pines, spruces, larches, yews, junipers, cedars, cypresses, redwoods

·        nearly all evergreen (bald cypress is an exception, deciduous)--advantage growing at higher elevations and latitudes where growing season shorter – conifers have anti-freeze type substance to keep leaves from freezing over winter.

·        needle-shaped leaves, stomata in pits--adapted to drier conditions

·        thick cuticle covers needles, which are true megaphyll leaves

·        are monoecious (both sexes on same plant)

see Life Cycle of Pine (Fig. 30.9)

Tree = sporophyte; cones = compressed stalks holding sporangia

sporophyte forms a) pollen cones which form pollen within microsporangia at bases                                              of specialized scales (leaves) called microsporophylls

                              b) ovulate cones form megaspore mother cells (2n), megaspores                                              (1n), and eggs within the ovules (two per megasporangium)

                                             at bases of megasporophylls

 

·        pollen is spread by wind; pollen grain drawn into ovule via small opening (micropyle)

·        pollen takes app. 1 year to digest its way to ovum

·        in the meantime, megaspore mother cell makes megaspore via meiosis, which develops into female gametophyte with 2-3 archegonia, each with 1 egg

·        2 sperm cells reach end of germinating pollen tube; one sperm fertilizes egg nucleus, other disintegrates

·        one zygote per archegonium develops into embryo (new sporophyte); embryo is 2n, food reserve is 1n (from parent gametophyte; seed coat is 2n (from parent sporophyte)

·        takes 3 years from pollination to development of mature seed (cone spreads open and seeds shed on wind)

 

Phylum/Division Cycadophyta--Fig. 30.6

dioecious (“two houses”)--sexes on different plants

two years from pollination to mature seed

 

Phylum/Division Ginkgophyta--Fig. 30.5, can be seen at Mercer Arboretum

only one species (Ginkgo biloba) extant

also dioecious

Corny but effective:  “M” = Monoecious/Marriage = both sexes in same house

“D” = Dioecious/Dating = sexes in different houses/plants

 

Phylum/Division Gnetophyta – Fig. 30.7

Three genera, including Welwitschia – large, straplike leaves

 

4th of the “Big Four”: Flowering Plants (Angiosperms)

 

·        One Phylum/Division Anthophyta, with over 235,000 species extant

·        evolved abruptly during late Cretaceous (app. 130 m.y.a.), apparently successful  enough to out-compete Gymnosperms for niches in most habitats, especially during and following Cretaceous Extinction (app. 65 m.y.a.)

 

Major advances in this division:

1. efficient pollination - more and more by animal pollinators vs. wind (more directed, less random) Fig. 30.18

 

2. ovary to enclose the ovule, protect the developing embryo; forms the fruit for dispersal of seeds

 

3. flowers = compressed reproductive shoots, composed of 4 whorls of modified leaves: (Fig. 30.13)

1. sepals --green, enclose bud before opening

2. petals -- often brightly pigmented; may have UV-visible nectar guides to guide insect, bird pollinators

3. stamens = male reproductive parts

a. filament (stalk)

b. anther (terminal sac which produces pollen)

4. carpel (pistil) = female reproductive structures (Fig. 38.6)

a. sticky stigma to receive pollen

b. style (stalk), leading downward to

c. ovary which contains ovules, which form seeds after fertilization

 

Examples of floral diversity  Figs. 30.11 & 38.3

 

Two traditional classes listed below are now being reanalyzed (Fig. 38.11)

1. Monocots - named for the one embryonic leaf or cotyledon formed in seed

·        food reserves for seed mostly in 3n endosperm (see life cycle below) eg. corn

·        veins parallel along long axis of straplike leaves

 

2. Dicots - with two embryonic leaves per seed (New group eudicots includes most)

·        food reserves mainly in large cotyledons eg. beans

·        veins weblike throughout leaf

 

Evolutionary trends in angiosperms (many have to do with exploiting animals as pollinators):

·        reduction in number of floral parts

·        fusion of floral parts eg. compound carpels with multiple ovaries in same flower

·        symmetry evolving from radial to bilateral

·        ovary moving beneath level of sepals (more protected)

 

Fruits for seed protection and dispersal (Fig. 30.16)

               seed = mature ovule

               seed coat = ovule wall

               fruit = mature ovary, possibly + other floral parts (petals, sepals, etc.)

 

Fig. 30.15, Table 30.1

1. simple fruits = developed from 1 ovary of 1 flower eg peas, apples

2. aggregate fruits = several ovaries of the same flower eg citrus, raspberry, magnolia

3. multiple fruits from several flowers eg. pineapple

 

Life cycle of angiosperms (Fig. 30.17)

 

Angiosperms are heterosporous::

Megaspores make female gametophytes (embryo sacs) which make female gametes (ova)

Microspores make male gametophytes (pollen)which make male gametes (2 sperm nuclei)

               1 sperm nucleus fertilizes egg to form the zygote

               1 sperm nucleus unites with 2 central nuclei to form 3n endosperm (Fig. 38.9)

This process is called double fertilization (distinct from gymnosperms).

 

All major food crops world-wide are angiosperms, due in large part to the nutrient rich endosperm reserves (grains) and fleshy cotyledons (legumes) of these seed plants.

Their evolution therefore had dramatic effects on human evolution as well.