Everybody In The Gene Pool - Plants That Swim

In the LOTR The Two Towers we find the tree herders that can move on their own despite being plants. Today’s exception is a version of this, if on a much smaller scale. There are a few types of trees that have motile parts; they don’t rely on wind, gravity, insects or anything else to move from one place to another.

Plants are divided up into many categories, and few people agree completely on the groupings. I’ve got a new grouping – swimming plants versus non-swimming plants! It’s as good as anyone else has come up with – let’s investigate the plants that can move on their own.

Plants are most often called by their binomial system names (genus, species), but this is just naming, not categorizing in a large way. Animals, protists, fungi, etc. are all divided into different phyla based on their similarities ad differences. But for some reason (I hope there’s a reason) botany uses divisions instead of phyla.

There are 10-12 divisions of plants (we’ll use 10), covering everything from the mosses to the flowers. Compare that to 21 phyla of animals and it seems like the plants will be easier to classify – but not so fast.

Group plants according to various characteristics and you start to muddy the waters. You might divide them up according to whether they have vascular tissue or not. Non-vascular plants are short because they don’t have vessels to move water very high – these are the Anthocerotophyta (hornworts), the Bryophyta (mosses), and the Marchantiophyta(liverworts). Together they are called the Bryophytes.

The vascular plants are all seven of the other divisions – the Lycopodiophyta (the spikemosses and clubmosses), Pteridophyta (ferns and horsetails), Coniferophyta (conifers), Cycadophyta (cycads), Ginkgophyta (just one species, Ginkgo biloba), the Gnetophyta (a weird group), and Angiospermae (flowering plants).  The conifers, cycads, ginkgo and gnetophytes are often grouped together as the gymnosperms – you’ve probably heard of them.

On the other hand, you might divide the plants up into the seed plants and non-seed plants. In that case, you lump the club mosses and the ferns with the bryophytes, since they all reproduce using spores, not seeds.

Every plant alternates between versions of itself that are diploid (sporophyte) and haploid (gametophyte). Different types of plants spend different amounts of time as one or the other. Mosses and other bryophytes are almost always in their gametophyte state, while trees are sporophytes with microgametophytes (antheridium and archegonium) located in their flowers only.

Lest you think that all those divisions that are seed plants are the same, you can divide them all differently based on whether their seeds are naked (the gymnosperms) or those with fruit (parts of the ovary or accessory organs that overgrow – all in the angiosperms).  Or you could divide them on the basis of how many leaves the embryonic plants have; monocots have one while dicots have two. Angiosperms play by these rules, but gymnosperm seeds don’t, their embryos may have none, one, two, or dozens of cotyledons (embryonic leaves).

Here’s one classification method you may not have heard before – gametophyte dominant vs. sporophyte dominant plants. This has to do with the cycle of life of plants.

Every plant has two lives. Part of its life is spent as a haploid gametophyte (produces haploid gametes) while another plant of the species is a diploid sporophyte.  The sporophyte produces spores that grow into the gametophyte, then the gametophyte produces gametes that join together during fertilization to become a new sporophyte.

Some plant types (like bryophytes) exist mostly in the gametophyte stage and are therefore called gametophyte dominant. Other plants (like trees and flowers) spend all there time as sporophytes and only small parts of them become gametophytes (like pollen or cones).

One way you shouldn’t classify plants is based on their movement. Sure, some plants can grow in a certain direction, toward or away from some stimulus (tropisms, see this post), but plants aren’t motile. They don’t pick up and move themselves from one place to another under their own power.

The Himalayan Balsam is an invasive viny flower that has become a problem in Europe and is invading the US as well. Their seed pods contain fins under great strain when fill with water. The slightest provocation will cause them to explode, sending seeds more than 25 ft (7.5 m).

Plants also have many ways of moving their seeds and this is sort of a way of plants moving, but I think it’s cheating a bit. Seed dispersal mechanisms can rely on the wind (maples, dandelions) or water (cranberries, coconuts). They can use animals that eat them (many), or just grab ride on them (devil’s claws), or they can burst out (peas) or be shaken out (poppies) and use gravity. But this isn’t really a plant moving by it’s own power.

Plants disperse seeds as new plants, but they also disperse their male gametes in order to find the egg on another plant of their species. You have to get the pollen of seed plants (containing male gametes) or the male gametes themselves to the egg. Like seeds, pollen grains can be moved by insect, by wind, by rain, etc. These are the ways most plants get their male gametes to the egg in order to create a new plant, either in a seed or without a seed.

But there are exceptions, and this is weird exception. Some plants have male gametes that are motile. They swim to the egg! No big deal for animals, they pretty much all have motile male gametes (we’ll look at the exceptions to that), but it’s quite the stunner in plants.

Who knew SpongeBob cartoons were science lesson. Plankton is green, a phytoplankton – an algae to be precise. Those two antennae? Probably his two flagella, the way he would swim around. The feet and the single eye-not so scientific.

Algae are probably the ancestors of all land plants, and we know their gametes have flagella for swimming from our post on them. But some land plants retain this method of male gamete dispersal, but they do include some weird twists. The plants that have motile male gametes cross many of the classes that we described above. There are some seed plants and some spore plants, some vascular and some non-vascular, some are gametophyte dominant and a few are sporophyte dominant – but no flowering plants do this.

Bryophyte males gametes are swimmers. The mosses, liverworts and hornworts  live close to the ground and must have standing water for the make gamete to reach the female gametophyte and egg. The haploid gametophytes are the moss that we usually see. The antheridium grow on the top of the male plants to produce male gametes, while the archegonium on the female plant tip produces the ovule with the egg. When the water is high enough the antheridium releases the male gametes and they swim to the egg using two flagella. The sporophyte (diploid) plant grows from the top of the female gametophyte.

Ferns, horsetails, and club mosses are taller than mosses because they are vascular, but they still require water for their male gametes to swim to the egg. The gametpophyte is a heart shaped leaf that lies near the ground. At one spot the archegonium grows the egg, while the male gametes in the antheridium grow nearby. Water resting on the leaf allows the male gametes to swim across the leaf to the egg (or from leaf to leaf). The sporophyte grows from the heart-shaped gametophyte and is the fern we usually think of.

Cycads (left) are gymnosperms whose trunk are formed from the bases of the leaves as they grow and are lost. There are about 300 species of cycads known, with several added every year. The Ginkgo biloba is the only extant species of the division. Because its wood is insect resistant, some trees may be over 2500 years old.

Angiosperms and other seed plants use pollen to send the male gametes to the ovule. When the pollen reaches the archegonium, a tube grows into the ovule and to the egg. The male gamete cells are carried along by the pollen tube right to the front door of the egg. This is when fertilization occurs.  But not allseed plants work this way. A few of the gymnosperms still use a modified version of swimming to the egg.

Cycads (about 300 species) and the lone extant ginkgo, Ginkgo biloba, do have pollen grains that represent the male gametophyte plant. They get blow or carried to the female gametophyte cone and then it gets weird.

Cycad and ginkgo male gametes move on their own, the only exceptions in the seed plants. They have hundreds to thousands of cilia, as opposed to flagella in bryophyte gametes, which pull the cell forward. Ginkgo male gametes are huge (0.3 mm), larger than an entire Wolffia globosa plant.

The male gametes have about a thousand of cilia (not flagella) that pull the cell through the watery environment inside the ovule toward the egg. Fusion and fertilization occurs when the male gametes find the egg – as always. The cycad and ginkgo male gametes swim, but they swim in the indoor pool, not out in the old swimmin’ hole like the bryophytes.

Next week – the base of the undulipodia has a special story all its own. Is it another instance of bacteria evolving into one of our organelles? And it has two very different jobs – which came first?