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Magnolias at the Brooklyn Botanic Garden

Spring has come early this year and the beautiful tulip-like petals of New York City’s magnolia trees are already beginning to fall into great drifts of white and pink.  If you stop and pick up one of the pretty petals from such a pile you will be surprised by the leathery resilience of the delicate-looking petals.  The durability of the petals of magnolia flowers is not coincidental—the flowers are different from other common flowering trees because Magnoliidae trees were among the first flowering trees to evolve.  The earliest known fossils of such flowers date from the upper Cretaceous period around 130 million years ago. Magnoliidae petals are tough because they were originally meant to attract the attention of beetles rather than bees (which do not appear in the fossil record until 100 million years ago).  Since there were no insects specially adapted to live as pollinators when magnolia-like trees first appeared, the petals and reproductive structures of these first flowering trees had to be robust to survive attention from the hungry clumsy beetles (toughness which has passed on to the modern ornamental trees).

Paeleobotanists have not yet unraveled the entire history of the evolution of flowering plants (indeed, Charles Darwin called the abrupt appearance of flowers in the fossil record “the abominable mystery”) however magnolia-like trees appeared long before the great radiation of angiosperms which occurred approximately 100 million years ago.  The first magnoliid trees must have seemed tremendously strange–explosions of color and shape surrounded by great uniformly green forests of gymnosperm trees (like the familiar conifers). Magnolia blossoms betray evidence of their ancient lineage through several “primitive” features: the petals are nearly indistinguishable from the sepals; each flower has many stamens which are arranged in spiral rows; there are multiple pistils; and all of the stamens and pistils are supported by a “fingerlike receptacle.”

By attracting the attention of animals (either through the colorful appearance and appealing scent of flowers, or by the edible nectar and fruit) flowering plants were better able to reproduce themselves.  Magnolias spread around the temperate world and began the complicated interdependent relationship which all sorts of animals (including humans) have with flowering plants.

A life cycle diagram is a stylized pictorial representation of the path an organism must undergo in order to renew itself and continue living for multiple generations.  For example here is a very simple life cycle diagram of a human.

A few details might have been glossed over...

Straight forward enough:  the person begins where the two red lines intersect as a little zygote—with genes from both mother and father.  The zygote matures in the mother’s womb, is born, grows up to sexual maturity (notice the awkward puberty phase), and then contributes a single haploid reproductive cell to combine with the haploid reproductive cell of the opposite gender mate.  Voila: another zygote which becomes a fetus and begins the cycle again!  A more creative artist could have provided a bit more context (for example see the adorable penguin life cycle below), but, minus some snappy duds and sharp patter, humankind’s path to continued existence is pretty much all there.

Aw! It's like they're wearing little tuxedos!

The story starts getting more convoluted as we examine organisms that are less familiar.  Here for example is the life cycle of a hornwort.

Argh! What is going on here? In fact, what is a hornwort anyway?

To start with, hornworts are ancient land plants which trace their origin back to before the Devonian (416 million years ago).  These non-vascular plants were one of the first organisms to colonize the barren continents, back when life was mostly an ocean-only affair.  To allude to a different post, they were one of the original invasive species on land.

The hornwort’s life cycle is alien to us because the plant, like almost all plants and fungi (and like some protists), utilizes alternation of generations to reproduce.  Looking at the diagram it is hard to choose a place to start from: spore, egg, gametophyte?  But if our human diagram weren’t so familiar, it would be difficult to find the starting place on it too.  Life cycle diagrams have a “chicken or the egg?” paradox built in to them.

For the hornwort, let’s start with the spore.  There it is in the upper right corner like a little cube of cheese.  It is a haploid cell–having only one set of chromosomes just like a mammal’s sperm or egg.  But it is not like a sperm or egg!  Look at the diagram:  by itself, it turns into a protonema, the little transitional sprout which develops into the adult gametophyte.  The gametophyte is what we think of as the independent living hornwort plant.  Once this (still haploid) plant has grown to adult size it produces haploid sex cells in special structures called the antheridium (male cells) and the archegonium (female cells).

For the next stage, water is required so that the sperm can swim to the egg which remains stationary in the archegonium.  This is why hornworts could not leave damp watery areas.  The sperm and egg combine to form a diploid zygote (with two sets of chomosomes just like in the human zygote and the human adult!) which develops into a sporophyte.  The sporophyte remains dependent on the gametophyte out of which it sprouts.  The life cycle diagram magnifies a section of the sphorophyte–which is portrayed as a complicated green tube along the right of the picture.  Inside the sphorophyte, within sporogenous cells, meiosis takes place and new haploid spores are created.  When they are ready, the sporophyte capsule breaks open and the spores are released to in turn become protonema and begin the process again.

Although I have chosen the unfamiliar hornwort as an example, the underlying life cycle of familiar angiosperm (flowering) plants such as my beloved roses is not dissimilar.

Hmm, the terminology and morphology are pretty different though...

To quote Wikipedia, “alternation of generations implies that both the diploid and haploid stages are multicellular.”  This is important: if we thought of the unicellular sperm (or the egg) from the diagram at the top as a separate entity from ourselves, humans would reproduce by a sort of separation of generations, but we don’t really think of our conception that way!

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