Chapter 30 The Evolution of Seed Plants

Biology 102Tri-County Technical CollegePendleton, SC

Some Key Terms

Seed is fertilized, ripened ovule of gymnosperm or angiospermSeed consists of an embryo, food supply, and protective seed coatDesigned for dispersal by various means: Wind Animals Water

Progressive Time Line

Several key adaptations were required in progression from nonseed to seed plantsTerrestrial habitat Support, water supply (access/conservation),

nutrients, and solving the “hanky-panky” problem

Size and dependency of generation Bryophytes: dominant gametophyte with reduced

sporophyte that is dependent of gametophyte

Progression, cont.

Ferns: dominant sporophyte with reduced gametophyte and both nutritionally independent of each other

Gymnosperms: dominant sporophyte and gametophyte reduced even further

Angiosperms: dominant sporophyte and even further reduced gametophyte

In seed plants, the gametophyte is nutritionally dependent on the sporophyte

Progression Visual

Homo versus Hetero

Homosporous plant produces single type spore that develops into gametophyte with both male and female reproductive organsHeterosporous plant = 2 distinct spores with megaspore becoming larger, female gametophyte (mega) and microspore becoming smaller, male gametophyte (micro)Bryophyta is homosporous; most ferns are homosporous (exceptions); seed plants (gymno/angio) always heterosporous

More on Spores

Spore (haploid reproductive structure) shed for dispersal by mosses and fernsSeed plants retain megaspore and female gametophyte develops within megasporangia and depends on sporophyte for food and waterMicrosporangium produces male gametophyte (pollen) which is distributed by wind, insect, bird, plant breeder, etc.Pollen grain reaches appropriate surface, it develops further

OMG..more on sperm and egg

Mosses and ferns still dependent of moisture for sperm to swim to eggSeed plants depend on pollen (male gametophyte) dispersal Appropriate location, pollen grain grows

pollen tube Tip of tube penetrates

megagametophyte, two sperm are released

Celebrate the Differences

Pollination is transfer of pollen to appropriate structure such as ovule, carpel, or pistilFertilization is union of sperm and egg to produce single cell called a zygoteTime for an Estesism….Yeehaw!!!!Cone is axis bearing tight cluster of scales or leaves specialized for reproduction

Continue the Celebration

Flower is total reproductive structure of angiosperms Dr. Richards: “modified leaf with

carpels”

Fruit is ripened and mature ovary (or group of ovaries) Sometimes applied to reproductive

structures of other groups of plants

Celebration III

Naked seed describes ovules and seeds not protected by flower or fruit tissueCovered seeds characteristic of the angiospermsOvary of flowering plant (with seeds it contains) develops into fruit after fertilization

Pine Life Cycle

Pollen grain falls onto ovulate cone and enters ovule through micropyleGerminates in ovule, forms pollen tube that digests way through nucellusMegaspore mother cell undergoes meiosis = 4 haploid megaspores (1 survives) It grows and divides repeatedly by mitosis =

immature female gametophyte

Pine Life Cycle, cont.

2 or 3 archegonia (each with egg) develop within multicellular gametophyteAfter 1 year, eggs ready for fertilization—2 sperm cells have developed and pollen tube has grown through nucellus to female gametophyteFertilization occurs when one sperm unites with egg nucleusAll eggs in ovule may be fertilized, but only ONE zygote develops into embryo (usually)

Pine Life Cycle, cont.

Pine embryo (new sporophyte) has rudimentary root and several embryonic leaves Embedded in female gametophyte Nourishes embryo until capable of

photosynthesis

Ovule has developed into pine seedEmbryo (2N), food source (N), surrounding seed coat (2N)Scales of ovulate cone separate (~another year) and winged seeds usually carried by wind to new location

Pine Life Cycle Visual

Pollen and Seed Cones

Multicellular gametophyte generation reduced and develops from haploid spores retained within sporangiaMale gametophyte is pollen grain (no anthridium)Female gametophyte consists of multicellular nutritive tissue and an archegonium that develops within an ovule

Pollen and Seed Cones, cont.

Conifer life cycles are heterosporousMale and female gametes develop from different types of spores produced by separate conesPollen cones have microsporangia Cells undergo meiosis = haploid microspores These small spores develop into pollen grains pollen grains = male gametophyte (not quite

true, but will do for this course)

Pollen and Seed Cones, cont.

Ovulate cones have megasporangia Cells here undergo meiosis = large

megaspores that develop into female gametophyte

Each ovule initially includes megasporangium (nucellus) enclosed in protective integumentsHas single opening called micropyle

XXX Rated Visual

It’s all about sex…

Perfect versus Imperfect

Flower containing both male and female reproductive parts called “perfect” Has both megasporangia and

microsporangia

Flower with ONLY megasporangia OR microsporangia called “imperfect” Either stamens or carpels nonfunctional

or absent in given flower

Monoecious/Dioecious

Monoecious means “one-house” Megasporangiate & microsporangiate

flowers occur on same plant (corn & beech) In same flower or separate flowers

Dioecious means “two-houses” Given plant produces flowers with stamens

or flowers with carpels (pistils) but NEVER BOTH

Willows, date palms, and wacky-backy