Note: I am unsure if I am not forcing people to guess the password. If you find this style okay, the next post will be built similarly.
As we have already seen, it’s a different matter to do anything significant to support a free-living gametophyte than one contained within the sporophyte body (the way seed-bearers do). It is certainly more difficult, but is it impossible?
To start with, let us see exactly what groups of seedless plants, minus mosses, we still have today. Here is a (pruned and decorated) tree of evolution of land plants from Pryer et al.1
The earliest, lowest (‘most basal’) branch is lycopods, who contributed a great lot to the forests of the Carboniferous, but today are quite rare and much smaller.
After lycopods branched off, evolution introduced true leaves—fleshy outgrows of sprouts with many veins in them.
Then, ancestors of ferns in the broadest sense and ancestors of seed plants in the broadest sense parted ways and began diversifying. If you haven’t worked with phylogenies, the picture makes it seem, at first glance, that all groups of ferns just kind of sorted things out more-or-less simultaneously, but it is far from truth. There are some pictures below, to give you a sense of what they look like and try to guess who is older and who is younger—your very first priors for these relationships. Pryer et. al.’s article provides estimates for when these groups did separate.
Don’t Google just yet. Let’s have some fun guessing what properties these plants might have, based on some hints I’ll give you and whatever you remember from other sources.
So.
In general, a life cycle goes like this: sporophyte (diploid, as in two chromosome sets) produces spores (haploid, since they underwent meiosis) that are released (singly or in fours or, in some cases, not released at all but kept where they were formed, in their sporangia). Spores germinate into (haploid) gametophytes that have archegonia (female reproductive organs making eggs) and/or antheridia (male ones, making sperm). Sperm swims to egg and fertilizes it, so that the resulting zygote again has two sets of chromosomes and the embryo develops into a sporophyte. It matures and sheds spores. All done.
What qualifiers can you imagine to make the cycle less general?
Seriously, take five minutes to tweak it. Maybe you can think up some broad restrictions posed by environment. Or a shortcut to success (be radical). Or a stability-oriented strategy. Or the relative advantages of being mobile or sessile (challenge what you are used to think about the issue here). Or being the pioneer of your species in a new locality. Or struggling to keep up with constant disruption of your habitat or even your body. Or not having the resources to produce spores regularly. Or not having to do it at all to maintain your existence for centuries or more. Or being a ruthless user of others (for a given resource). Or putting protective layers around your kids and yourself, lack of seed coat notwithstanding. Or being able to grow only on alkaline substrates. Or irregular meiosis, so that the spores have just as many chromosomes as the parent sporophyte.
Okay? Now look at the adult plants and seek out those who might stand up to what you have thought up. Comment on what fits your ideas and what you think is not presented at all. The first comment is a poll of your estimates:)
Pictures and data from Wiki, unless otherwise specified.
Lycopods.
Lycopodium obscurum (a clubmoss). It has a branching subterranean rhizome. In its roots it has symbiosis with a fungus (mycorrhiza). Its gametophytes are disk-shaped, about 1,5 cm in diameter. First year shoots [of young sporophytes] are unbranched and rarely penetrate soil surface.
What is your probability that
* gametophytes development takes less than a year;
given that,
* it is due to mycorrhiza that young sporophytes can support themselves for another season ununderground;
given that,
* if all shoots are destroyed two years in a row, the population can not recover?
Isoetes lacustris (a quillwort). It does not have traditional rootsa, but instead some of its leaves are modified to act like roots. As Čtvrtlíková et. al. have found2, quillwort germination may also be constrained during the growth season by its relatively high minimum temperature (no less than 12 °C) threshold for macrospore germination.
What is your probability that
* it grows in alpine climate;
given that,
* it is a species of rubbly slopes, adapted to breaking off leaves;
given that,
* the leaves are capable of rooting and establishing new plants;
given that,
* these new plants can shed macrospores that same year, if the weather is mild enough?
Polypodiophyta (leptosporangiate ferns).
Asplenium ceterach. This fern is well known as a resurrection plant due to its ability to withstand desiccation and subsequently recover on rewetting. Can be found growing up to 2700 metres above the sea level.
What is your probability that
* it can also grow on buildings;
given that,
* it is rather common within its range;
given that,
* it is difficult to study its historical spread, because outbreeding and multiple colonizations even out inter-populational differences?
Salvinia natans has two nickel-sized leaves lying flat against the surface of the water, and a third submerged leaf which functions as a root. Flotation is made possible by pouches of air within the leaves. Cuticular papillae on the leaves’ surface keep water from interfering with the leaves’ functioning, and serve to protect them from decay. Spore cases form at the plant’s base for reproduction.
What is your probability that
* it can have many generations during a season;
given that,
* competition between sporophytes of different generations peaks in late summer;
given that,
* older sporophytes depleting the habitat of nutrients restrict the growth of younger sporophytes through negative feedback loop?
Now, what plant was easiest for you to formulate an hypothesis about? The poll is in the first comment.
a—whatever that’s supposed to mean. There are lots of other plants without ‘traditional roots’.
1. American Journal of Botany 91(10): 1582–1598. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. K. Pryer, E. Schuettpelz, P. G. Wolf, H. Schneider, A. R. Smith, R. Cranfill.
2. Preslia 86: 279–292, 2014. The effect of temperature on the phenology of germination of Isoëtes lacustris. M. Čtvrlíková, P. Znachor, J. Vrba.
Before the seed. I. Guesswork
Note: I am unsure if I am not forcing people to guess the password. If you find this style okay, the next post will be built similarly.
As we have already seen, it’s a different matter to do anything significant to support a free-living gametophyte than one contained within the sporophyte body (the way seed-bearers do). It is certainly more difficult, but is it impossible?
To start with, let us see exactly what groups of seedless plants, minus mosses, we still have today. Here is a (pruned and decorated) tree of evolution of land plants from Pryer et al.1
The earliest, lowest (‘most basal’) branch is lycopods, who contributed a great lot to the forests of the Carboniferous, but today are quite rare and much smaller.
After lycopods branched off, evolution introduced true leaves—fleshy outgrows of sprouts with many veins in them.
Then, ancestors of ferns in the broadest sense and ancestors of seed plants in the broadest sense parted ways and began diversifying. If you haven’t worked with phylogenies, the picture makes it seem, at first glance, that all groups of ferns just kind of sorted things out more-or-less simultaneously, but it is far from truth. There are some pictures below, to give you a sense of what they look like and try to guess who is older and who is younger—your very first priors for these relationships. Pryer et. al.’s article provides estimates for when these groups did separate.
Don’t Google just yet. Let’s have some fun guessing what properties these plants might have, based on some hints I’ll give you and whatever you remember from other sources.
So.
In general, a life cycle goes like this: sporophyte (diploid, as in two chromosome sets) produces spores (haploid, since they underwent meiosis) that are released (singly or in fours or, in some cases, not released at all but kept where they were formed, in their sporangia). Spores germinate into (haploid) gametophytes that have archegonia (female reproductive organs making eggs) and/or antheridia (male ones, making sperm). Sperm swims to egg and fertilizes it, so that the resulting zygote again has two sets of chromosomes and the embryo develops into a sporophyte. It matures and sheds spores. All done.
What qualifiers can you imagine to make the cycle less general?
Seriously, take five minutes to tweak it. Maybe you can think up some broad restrictions posed by environment. Or a shortcut to success (be radical). Or a stability-oriented strategy. Or the relative advantages of being mobile or sessile (challenge what you are used to think about the issue here). Or being the pioneer of your species in a new locality. Or struggling to keep up with constant disruption of your habitat or even your body. Or not having the resources to produce spores regularly. Or not having to do it at all to maintain your existence for centuries or more. Or being a ruthless user of others (for a given resource). Or putting protective layers around your kids and yourself, lack of seed coat notwithstanding. Or being able to grow only on alkaline substrates. Or irregular meiosis, so that the spores have just as many chromosomes as the parent sporophyte.
Okay? Now look at the adult plants and seek out those who might stand up to what you have thought up. Comment on what fits your ideas and what you think is not presented at all. The first comment is a poll of your estimates:)
Pictures and data from Wiki, unless otherwise specified.
Lycopods.
Lycopodium obscurum (a clubmoss). It has a branching subterranean rhizome. In its roots it has symbiosis with a fungus (mycorrhiza). Its gametophytes are disk-shaped, about 1,5 cm in diameter. First year shoots [of young sporophytes] are unbranched and rarely penetrate soil surface.
What is your probability that
* gametophytes development takes less than a year;
given that,
* it is due to mycorrhiza that young sporophytes can support themselves for another season ununderground;
given that,
* if all shoots are destroyed two years in a row, the population can not recover?
Isoetes lacustris (a quillwort). It does not have traditional rootsa, but instead some of its leaves are modified to act like roots. As Čtvrtlíková et. al. have found2, quillwort germination may also be constrained during the growth season by its relatively high minimum temperature (no less than 12 °C) threshold for macrospore germination.
What is your probability that
* it grows in alpine climate;
given that,
* it is a species of rubbly slopes, adapted to breaking off leaves;
given that,
* the leaves are capable of rooting and establishing new plants;
given that,
* these new plants can shed macrospores that same year, if the weather is mild enough?
Polypodiophyta (leptosporangiate ferns).
Asplenium ceterach. This fern is well known as a resurrection plant due to its ability to withstand desiccation and subsequently recover on rewetting. Can be found growing up to 2700 metres above the sea level.
What is your probability that
* it can also grow on buildings;
given that,
* it is rather common within its range;
given that,
* it is difficult to study its historical spread, because outbreeding and multiple colonizations even out inter-populational differences?
Salvinia natans has two nickel-sized leaves lying flat against the surface of the water, and a third submerged leaf which functions as a root. Flotation is made possible by pouches of air within the leaves. Cuticular papillae on the leaves’ surface keep water from interfering with the leaves’ functioning, and serve to protect them from decay. Spore cases form at the plant’s base for reproduction.
What is your probability that
* it can have many generations during a season;
given that,
* competition between sporophytes of different generations peaks in late summer;
given that,
* older sporophytes depleting the habitat of nutrients restrict the growth of younger sporophytes through negative feedback loop?
Now, what plant was easiest for you to formulate an hypothesis about? The poll is in the first comment.
a—whatever that’s supposed to mean. There are lots of other plants without ‘traditional roots’.
1. American Journal of Botany 91(10): 1582–1598. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. K. Pryer, E. Schuettpelz, P. G. Wolf, H. Schneider, A. R. Smith, R. Cranfill.
2. Preslia 86: 279–292, 2014. The effect of temperature on the phenology of germination of Isoëtes lacustris. M. Čtvrlíková, P. Znachor, J. Vrba.