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