Seed Plants - Angiosperms

Angiosperm

Most diverse group, classified under phylum “Anthophyta”

Angiosperm Lifecycle

-Alternation of generations between multicellular haploid and multicellular diploid

-Sporophyte is dominant, gametophyte is dependent and microscopic

Unique Features:

Flowers, Double Fertilization and usage of Fruits (synapomorphy of angiosperms)

Flowers - Leaf portion

Specialized shoot with 4 sets of modified leaves, bear structures with function in sexual reproduction

Attached to the ‘receptacle’

Sterile Parts:
-Sepals: help enclose and protect flower bud

-Petal: Attract insects and other pollinators

Flower - Reproductive Organs

Stamen - Microsporophyll, pollen producing, consists of a stalk and anther, flower often has many stamen that have 2-4 pollen sacs

Carpels - Megasporophyll, ovule-producing organs, consist of ovary, style and stigma

• Multiple carpel may be fused into a pistil

• Ovary contains many ovules

• Megaspore develops into female gametophyte via meiosis

Development of Male Gametophyte

-Pollen grain has 2-celled male gametophytes

-Pollen grain developed from microspores in the microsporangia

-Each microspore produces two-celled male gametophyte;

• Generative cell - develops into sperm nuclei

• Tube cell - forms pollen tube

Development of Female Gametophyte

-Ovule has 2 integuments, surrounded by megasporangium

-Four Megaspores developed from megasporocyte via meiosis in the megasporangium

-Only 1 functional megaspore forms

-A 7-celled female gametophyte is formed

-Polar Nuclei: central cell in the female gametophyte with 2 nuclei

Pollination in Angiosperm

-Pollen from an anther transferred to part of a seed plant containing ovules

-Tube cell produces pollen tube

-Generative cell produces 2 sperm nuclei via mitosis

-Pollen tube grows into the ovary, release sperm nuclei into ovule, leading to fertilization

Double Fertilization

Mechanism in which 2 sperm nuclei unite with 2 cells in female gametophyte to form a zygote

-One sperm fertilizes the egg to form a zygote

-Second sperm combines with a polar nuclei into a triploid cell, develops into endosperm to nourish the embryo

Seed Development (Post-Fertilization)

Ovule develops into a seed, ovary develops into a fruit

-Endosperm develops before embryo development

-Triploid endosperm grows into starchy food tissue for embryo via mitosis

-Zygote form elongated embryo containing cotyledons, shoots and roots

Fruit Development & It’s Types

Fruit formation takes place as sporophyte ovary thickens/matures

-Types of Fruit:

• Dry fruit, developed by drying of ovary

• Fleshy fruit, ovary becomes thick, soft and sweet at maturity

Fruits function to protect seeds and facilitate seed dispersal

Advantages of Angiosperm Fertilization

-Resources not wasted since endosperm develops after double fertilization - fruit development initiated by fertilization

-Size of female gametophyte is reduced, therefore less resources needed

-Life cycle can be completed in a short amount of time due to smaller size of female gametophyte and faster maturity rate

Coevolution

Joint evolution of 2 interacting species, each in response to selection imposed by the other

-Many flowering plants coevolve with specific pollinators:

• Shapes/sizes of flower correspond to pollen transporting parts of pollinating animals

• This is mutualistic; plants allocate less energy for dispersal, pollinators get nectar

Angiosperm Evolution

Factors contributing to adaptive radiation of angiosperms:

-Presence of modified xylem with vessel tissues, allowing for greater transportation of water

-Rapid speciation due to coevolution

-Rapid reproduction

Monocots

Group of angiosperms, form a monophyletic group

Characteristics:

-Flowers are ‘trimerous’ (multiples of 3)

-Presence of single cotyledon (embryonic leaf)

Eudicots

Diverse group of angiosperm, most angiosperms fall into this category

Characteristics

-Flowers are four-merous or five-merous (multiples of 4 or 5)

-Presence of two cotyledon (embryonic leaves)