Angiosperms - Flowering Plants

•	Charles Darwin called the evolution of the angiosperms “an abominable mystery”
•	The evolution of angiosperms remains a mystery to this day

•	Flowering plants evolved sometime during the Cretaceous, while the dinosaurs were at their peak
•	Water lilies one of the first clades to evolve
•	They quickly became the dominant plants, although gymnosperms continue to rule in cold, dry, or sandy habitats 

•	Oldest known angiosperm (recently discovered) is Archaefructus, ~ 122-145 my (upper Jurassic), discovered in mainland China in 2002

•	Most primitive living angiosperm (also recently discovered) is Amborella
•	Grows as a shrub or small tree on South Pacific island of New Caledonia

Angiosperms - Division Anthophyta

•	More than 255,000 species of flowering plants grouped in over 300 families

•	Flowering plants are superior competitors
>	Able to survive in a greater variety of habitats
>	Mature more quickly
>	Produce greater number of seeds

•	Flowering plants are superior competitors
>	Fruit for seed dispersal
>	Wider-bore vessels to conduct water
>	Animals aid in pollination - can survive as small scattered populations, whereas wind-pollinated species need dense populations

•	More diverse and specialized group
>	Gymnosperms are woody perennials, angiosperms can be perennials or annuals
>	Leaves are thin blades, diversity of shapes
>	Woody tissues are more complex and highly  specialized

Angiosperms - Economic Importance

•	Food - fruits, vegetables, grains, nuts, spices
•	Wood - homes, tools, ships… 
•	Oils and waxes - olive oil, perfumes, soap…
•	Drugs - coffee, chocolate, wine, beer…
•	Medicines - quinine, digitalis, codeine…

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All angiosperms have flowers

•	Flowers are reproductive structures that are formed from four sets of modified leaves
•	Amazing diversity of floral structure
•	Linnaeus used these differences in his first attempt at classifying plants

•	Angeios = Greek for vessel (container) 
•	Ovules are encased in an ovary, not lying naked on the scales of a strobilus, as they are in gymnosperms

•	Ovules in the ovary develop into seeds
•	The ovary walls form a fruit to help disperse the seeds
•	Fruit is a marvelous example of coevolution between plants and animals 

•	Coevolution occurs when an evolutionary change in one organism leads to an evolutionary change in another organism that interacts with it
•	Flowering plants show two great examples of coevolution
>	Evolution of fruit dispersal
>	Evolution of animal pollination

•	Fruits function to disperse seeds
>	Animals eat fruit, but don’t digest seeds
>	Tiny hooks and spines to attach to animal
>	Also dispersed by wind, water (coconuts)

•	Flowers that rely on wind pollination are tiny and inconspicuous (like oak trees, maple trees, corn, grasses)

•	Flowers that are pollinated by animals have showy petals to attract the pollinators
•	Flowers advertise their reward of nectar, sugar water to attract pollinators

•	Nectar contains sugar (sucrose, fructose or glucose), amino acids, vitamins, oils etc.
•	Made by glands called nectaries, usually near the base of the ovary
•	Nectar sipped by bees is the sugar source for honey!
 
•	Flowering plants go to great lengths to avoid pollinating themselves
>	Chemical – Pollen and ovule are chemically incompatible
>	Architectural – Stamens and pistils are arranged to avoid contact
>	Temporal – Pollen and pistil mature at different times 

•	Flowers consist of four sets of modified leaves on a shortened stem
>	Sepals - protect floral parts in the bud
>	Petals - attract pollinators
>	Stamens - anthers and filaments
>	Carpels - form the pistil (stigma, style, ovary)

•	What the heck is a carpel??
•	Cut into the pistil and you will see one or more tiny chambers, each chamber holding one or more sporangia on tiny stalks
•	These sporangia are the ovules - each carpel can hold one or several ovules

•	Goethe, German writer, philosopher, and (in his spare time) noted botanist, proposed that carpels evolved from leaves
•	Chambers in the pistil were probably formed from a sporophyll

•	Edges of the leaf folded over and fused together to form a protective chamber
•	Goethe’s “foliar theory of the carpel” is still the best hypothesis for explaining the evolution of the carpel

•	Sporophylls are leaves modified to hold spores
•	Carpels are leaves modified to hold seeds

•	What we are used to calling the pistil consists of the fusion of several carpels along the midrib of the modified leaves
•	The root “carp” means fruit – appropriate, this portion of the flower becomes the fruit

•	Sporophytes form sporangia
•	Stamens are highly modified sporophylls
•	Sporangia are located on the stamens, inside the anthers, atop a slender filament
•	Each anther holds four microsporangia - microspores develop in microsporangia

•	Microspore mother cell divides by meiosis to form four haploid microspores
•	Each microspore develops into a multicellular pollen grain
•	Pollen grains are the male gametophytes

•	Microspore divides into a tube cell (will form pollen tube) and a sperm cell (nucleus will act as sperm)
•	Mature male gametophyte is teeny-tiny, only a few cells

•	Cross walls between two pairs of microsporangia break down
•	Results in two long pollen sacs full of developing gametophytes
•	Pollen grain are wrapped in sporopollenin, the same biopolymer that protects spores

•	Surface of pollen grains can be very complex, species specific patterns - floral thumbprint

•	Pollen shapes are critical for flowers - lets them recognize pollen of their own species
•	Pollen in fossil record also allows us to reconstruct ancient environments

•	Meanwhile, inside the ovary….

•	Sporophytes form sporangia 
•	Megaspores develop in megasporangia
•	Megasporangia are located in the ovary, at the base of the pistil
•	Each pistil consists of one or more carpels (modified leaves - sporophylls)

•	Each carpel holds one or more ovules
•	Ovules = megasporangium + integument
•	Ovules are attached to the walls of the ovary by a short stalk (just like babies)
•	Each ovule (megasporangia) will ultimately contain a single egg

•	Megaspore mother cell divides by meiosis to form four haploid megaspores

•	Three megaspores degenerate, the fourth divides by mitosis three times to form 8 haploid nuclei
•	This large cell with 8 nuclei is the female gametophyte, called the embryo sac

•	Weird cellular square dance ensues…

•	One nucleus from each group of four now migrates to the center of the embryo sac
•	Remaining three nuclei in each group migrate to each pole

•	Cell walls form around the the three nuclei
•	Female gametophyte now consists of 7 cells, 3 at the top, 3 at the bottom, 1 large cell in the middle with two nuclei
•	One of the bottom 3 cells will act as the egg

•	Pollen grain reaches the stigma of the pistil, grows a long tube (pollen tube) to penetrate the neck of the pistil and reach the ovary
•	As the pollen tube approaches the embryo sac, the sperm nucleus divides in two, mature male gametophyte now contains three nuclei - two sperm nuclei, one tube

•	Pollen tube enters the micropyle, discharges the sperm nuclei into the embryo sac

•	Two central female nuclei fuse together to make a 2N nucleus
•	One sperm nucleus fuses with the 2N nucleus to make a 3N nucleus - the other sperm nucleus fertilizes the egg
•	Embryo sac now contains a diploid zygote, which will grow up into an adult sporophyte
•	Also contains a 3N nucleus, which begins to divide repeatedly, forms the nutrition for the embryo - called endosperm (seed within)

•	This process is called double fertilization, and it is unique to angiosperms
•	Ephedra has double fertilization, suggesting that gnetophytes are related to angiosperms, but Ephedra doesn’t form endosperm

•	Ovule = megasporangium + integuments
•	Megasporangium holds developing embryo
•	Integument develops into a hard protective jacket, the seed coat, which protects the embryo from dessication and mechanical damage
•	Walls of the ovary develop into the fruit
•	Each section of orange, each chamber in the tomato is a carpel, now filled with little embryos inside seeds