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Seeds and pollen as an evolutionary adaptation to dry land

Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes), seeds contain a diploid embryo that will germinate into a sporophyte. Storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, and free reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy—induced by desiccation and the hormone abscisic acid—until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.

Pollen grains ( [link] ) are male gametophytes and are carried by wind, water, or a pollinator. The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain. The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko , the sperm still possess flagella that allow them to swim down the pollen tube    to the female gamete; however, they are enclosed in a pollen grain.

 The micrograph shows four different kinds of fossilized pollen. The pollen is oval or round in shape, with a bumpy texture.
This fossilized pollen is from a Buckbean fen core found in Yellowstone National Park, Wyoming. The pollen is magnified 1,054 times. (credit: R.G. Baker, USGS; scale-bar data from Matt Russell)

Evolution of angiosperms

Undisputed fossil records place the massive appearance and diversification of angiosperms in the middle to late Mesozoic era. Angiosperms (“seed in a vessel”) produce a flower containing male and/or female reproductive structures. Fossil evidence ( [link] ) indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants.

Although several hypotheses have been offered to explain this sudden profusion and variety of flowering plants, none have garnered the consensus of paleobotanists (scientists who study ancient plants). New data in comparative genomics and paleobotany have, however, shed some light on the evolution of angiosperms. Rather than being derived from gymnosperms, angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were basal angiosperms related to tropical grasses. Both views draw support from cladistics studies, and the so-called woody magnoliid hypothesis—which proposes that the early ancestors of angiosperms were shrubs—also offers molecular biological evidence.

Questions & Answers

what are the function of sympathetic nervous system and parasympathetic nervous system
Ubon Reply
what is preganglionic
Ubon
don't know
Sweety
what is reproduction
Aben
reproducing specifically : the process by which plants and animals give rise to offspring and which fundamentally consists of the segregation of a portion of the parental body by a sexual or an asexual process and its subsequent growth and differentiation into a new individual.
Black
is the process where by organism produce their new organism of the same species from those who a ready in existence.
Prudent
what is unicellular cell
Abigail
unicellular orgnism* meaning consisting of a single cell
Black
How does reproduction take place in human being
Aben
What is the Antibiotic
Tamara Reply
antibiotics is any medication that stop the growth of bacteria
onuoha
what is biomolecules
Lawerence Reply
This is a group of molecules produced by a living organism
Odion
thank alot,I had a hard time getting the answer
Lawerence
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It Reply
Now Mr. Niger Dac M;movement R;respiration N;nutrition G;growth E;excretion R;reproduction D;death A;adaptation C;competition
Odion
Sorry.... I;irritability
Odion
GOOD MORNING MY NAME IS MESUMBE PRECIOUS WHAT IS YOUR NAME
Mesumbe Reply
ccc
Peace
hi
Emmanuel
Emmanuel Daniel
Emmanuel
what is the meaning of Mr Niger?
Emmanuel
state all the elements and their symbols
Taiwo Reply
what types of muscles are found in the heart
Diamond Reply
Cardiac muscle
Divya
cardiac or myocardia muscle
onuoha
cardiac muscles
Elvis
cardiac muscle
Jemima
Cardiac muscle
Dr
Thank you dear!
yimam
please can you tell me the meaning of Mr Niger?
Emmanuel
It is an initial to represent life processes of organisms. M means movement, R for reproduction, N stands for nutrition, I for irritability or sensitivity,G means growth , E for excretion and R stands for respiration.
Quartey
cardiac muscle
Stanisla
what is digestion
Dolla Reply
it is the chemical break down of insoluble food substances such as fatty acid to soluble substances which are then used for body processes
Lawerence
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sadeeq Reply
levels of ecological study
sadeeq
no
Dolla
human sperms contains acid
Dolla
what is an artery
Dolla
It is one of the organs of the circulatory system that carries oxygenated blood from the heart to other parts of the body.
Quartey
artery is type of blood vessel which carries blood away from heart...
shams
how liver destroy red blood cells?
shams
big bang theory was discovered by
Sweety Reply
Georges Lemaître
Mr
the meaning of nucleus
Spring Reply
nucleus Is also known as the cell's headquarters
Feranmi
what is pathology?
Azi Reply
is the study of diseases, it's cause and development of a disease
Boadi
State the law of segregation
Theolla Reply
what is a cell
Bunmi Reply
A cell is a basic structural unit of an organism
Addo
what are molecules?
Veronica
molecule:a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.
Black
cell is basic structural and functional unit of life which can perform all the metabolic activities
Prakash
which organ function as modulator & effector in homeostasis glucose level ( Liver & islet of Langerhans ) & How ?
Dr
a cell is the basic unit of life
Taiwo
A cell is the basic unit of life
Odion
basic structural and functional unit of any living organism
Lawerence
comapare and contrast food chain
Chengo Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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