Flower anatomy

So I'm going to chicken out here, and only cover floral anatomy. Mostly because I'm a little tired and pressed for time at the moment. Will put up the mechanics of fertilisation in the next post probably.

Basically a flower is there to ensure pollination, so a fertilised seed can be produced.
There are many types of floral anatomy, and it's too complicated to go through them all, however most plants conform to a single type, and have basic variations on that theme.

I'll run through flowers that have both the male and female parts, although as mentioned in the last post, not all flowers do.

Right. Diagram time (wooo paint, because I don't have photoshop on this computer!):

(not bad for a paint job, hey?!)
The parts of the flower and their function are outlined below:

Petal: Can be bright and colourful to attract pollinators. Can vary in size, shape and colour depending on what the plant is trying to attract, if anything at all. Usually in plants that are not trying to attract anything, they're reduced in size and dull in colour, if they're even present at all.

Sepal: (The green bits at the bottom of the Petals) Together they make up the 'Calyx', functionally they may be there purely to protect the flower (while in bud), or they may be specialised and look like petals. For example most orchids have a specialised Calyx that looks like petals. Dasies are another example - the center of the daisy is actually hundreds of tiny flowers, while the 'petals' are actually a specialised Calyx.

Receptacle: Is basically the base of the flower, it is the part where most of the other components are anchored.

Peduncle: The stem of the flower, from the base of the Receptacle right down until it attaches to the branch it's growing on.

 Now the reproductive bits:

Female parts:
Stigma: The part that catches pollen for fertilisation, and part of the Pistil of the flower. Usually it's sticky and pollen-catching only when receptive to fertilisation. Because of this, if the plant splits up when the male and female parts of the flower are mature, the plant can often avoid many of the problems of self-fertilisation.

Style: The stem the Stigma sits on. This can be long or quite short, together with Stigma and Ovary comprises part two of the Pistil.

Ovary: Found at the base of the style, the Ovary contains the female reproductive parts of the plant, the Ovules. The whole structure from Stigma to Ovary makes up the Pistil. The structure of the Ovary can be quite important for identifying plants; it can have one to many chambers, it can sit above the place where the Petals/Sepals/Stames attach to the Receptacle (superior Ovary), halfway down where they attach (semi-inferior) or below where they attach to the Receptacle (inferior Ovary - perhaps I'll put diagrams up sometime to make that clearer!)

Ovules: Again, there may be one or many hundreds of thousands of these, each having the potential to become a new seed. Not all will be successfully fertilised, think of a pea pod. Sometimes you get those tiny little not-developed peas in there? They're probably aborted seeds, or those Ovules that simply haven't managed to be fertilised.

Male parts:
Anther: The pollen-containing part of the flower. Usually the Anther will split open on maturity, releasing the pollen to allow it to be either carried away by the environment or an animal. What axis the Anther splits open on, or if the pollen just moves through pores etc, is often family or genus specific and can be taxonomically useful.

Filament: The stem that the Anther is found on. Together they make up the Stamen, and this can vary in size, number, shape and position in the flower. Plants that want to avoid fertilising themselves might have the Anthers below the Stigma, to stop pollen from the same flower getting to the Stigma. Or they might have Anthers that sit above the Stigma, but mature either before or after the Stigma becomes receptive to pollen.


So that is basic flower anatomy. Pollen is released by Anthers, aiming to get stuck on Stigmas. But you may have noticed that the Stigma is a long way from the Ovules in the Ovary? How does the pollen get from being stuck on top of the Stigma to fertilise the Ovule in the Ovary? I'll tell you in the next post ;)

Seed plant reproduction

So post #2 in the series about plant reproduction is here!

So in the previous post, alternation of generations was (sort of) explained.
Here we're mostly talking about the sporophyte, which is the tree you see when you walk outside, and how it manages to produce gametophytes, where these occur and how the whole thing works. Starting with seed plants, and touching on flowering plants slightly (although there will be more of those in the next post).

So imagine a pine tree. You know in spring when all the ponds and creeks get covered in yellow grime? Yep, that's pollen. Seed plants like pines are wind-pollinated, resulting in a whole lot of pollen moving around the landscape. But then, that's a price you pay with wind-pollination. It tends to me more hit-and-miss than animal pollination, so more pollen is usually produced to compensate (more on this in a later post, promise!).
The pollen is produced in the male cones, known as microsporangium, and is the entire microgametophyte phase of this plant. Remembering from the last post, where mosses and ferns may have almost a 50/50 split or at least a far more prevalent gametophyte stage, in seed and flowering plants the gametophyte is much smaller, only hangs around for a very short length of time, and is less obvious. The male cones are usually found on the outside of the tree, at the ends of the branches, and this allows for more easy pollen dispersal on the wind.

The female part of the lifecycle occurs completely within the female cone, which is the megasporangium. Under the scales of the cone, there are megagametophytes. These, along with other associated tissues, make up the ova (egg). Once pollen lands on the right part of the girl cone, fertilisation may occur. In this instance, the male pollen (microgametophyte) will fuse with the femle egg (megagametophyte) to form a new embryo (zygote, and it is the next sporophyte phase). This develops as a seed, and once it reaches maturity, can (ideally) be dispersed. So really, 3/4 of the process happens in the female cone (megasporangium) as once the pollen has been developed and dispersed, the microsporangium has little to do with it ;)

So in summary:
- Male cone (microsporangium): Produces pollen (microgametophyte), disperses via wind
- Female cone (megasporangium): Produces the egg (containing megagametophyte), is where fertilisation takes place and where the seed (new sporophyte) develops, protected, until dispersal can occur.

Edit: A picture! To hopefully make that clearer (apologies for bad quality and my terrible handwriting. Haploid stage in yellow, diploid is blue):

Flowering plants work in a very similar way, with only a couple of differences. Firstly, they still split the micro- and mega-gametophytes up, however now they have flowers too. In most flowers you can find both the male and female structures, however some plants have separate flowers for them. Also, some plants are either male or female too. In flowers with both male and female parts, sometimes the same plant can't fertilise itself, sometimes it can. Sometimes the male part sits above the female, sometimes it's the other way around. But the basic mechanics are the same: male pollen, produced in a microsporangium (in the flowers case it's called an anther) meets the female egg, produced in a megasporangium (in the flower it's the ovary), they fuse to form a zygote, which grows into an embryo and develops inside a seed.

The actual mechanics of fertilisation in flowering plants are interesting, but can be a bit complicated. I might do a post regarding basic flower anatomy, and also detail how fertilisation occurs. Then the long-anticipated post about pollination syndromes will come...

Time to give this blog a reboot I think. So a post about the alternation of generations in plants

So my posting went from semi-regular to irregular to nonexistent. Apologies.

So in order to give it a kick in the pants, I think I'll write about... pollination. Or perhaps first the different types of reproductive methods found in the plant kindgom, then end with a post regarding pollination. It's a little complicated, so might take a couple of posts, but at least I haven't covered that yet! I might actually start with the confusing part, which is a process known as alternation of generations, because understanding that will make understanding plant reproduction a whole lot easier.

So, there are several kinds of plants, which are generally thought of at differing evolutionary levels.
Going from the most 'primitive' to the most 'modern' you have the Non-Vascular plants, such as mosses (Bryophytes), the Seedless Vascular plants, such as ferns (Pterophytes), then there are the higher plants which are made up of Seed plants such as connifers (Gymosperms) and finally the Flowering plants (Angiosperms).

Each group of plants has evolved its own way of solving the problem of reproduction, with each level seemingly involving more and more complex structures in order to do so. But first I'll talk about alternation of generations, otherwise known as the alternation of phases or metagenesis. But I know it as alternation of generations ;)

So what does that even mean?
Well, in short it refers to which tissue type (or rather, what that tissue is derived from) makes up which part of a plant's lifecycle, and whether or not this can be easily separated into two separate generations. It was 'discovered' (read: described) by Wilhelm Hofmeister in the late 1800s, and shows clearly why the reproductive structures we see today are necessary for the funtion of the various reproductive methods.

Edit: First a couple of definitions to help out:
Haploid: Cells that contain a single set of chromosomes (n).
Diploid: Cells that contain two sets of the same chromosomes (2n), ie paired chromosomes
Meiosis: The division of a cell whereby the chromosomes do not replicate, resulting in one of each pair of chromosomes occuring in each of the new daughter cells (ie produces 2 haploid (n) cells)
Meitosis: The division of a cell whereby the chromosomes do replicate, and each daughter cell has a pair of each chromosome (ie produces 2 diploid (2n) cells).

There are two basic phases that plant tissues go through. The first is the Sporophyte phase, which is diploid (like us it has 2n chromosomes) and the second is the Gametophyte, which is haploid (has only n chromosomes). Depending on the organism, one phase is usually dependent on the other for surivival, however ins some plants (eg Ferns) the two stages may be capeable of living independently. Following so far? Good :)
The Sporophyte, perhaps unsurprisingly, produces spores, whereas the Gametophyte (also unsurprisingly) produces gametes.

So how does this happen? Well:

The easiest place to start with this is the Sporophyte.
Like I mentioned before, the Sporophyte is diploid (2n), and in most plants it forms the largest and most long-lived part of the lifecycle (for example every part of a tree you can see is the Sporophyte). In Bryophytes (mosses), it's the other way around, and the Gametophyte is dominant, with the Sporophyte only appearing in time to reproduce. In either case the reproduction is what we're interested in here. So selected cells in the Sporophyte undergoes meiosis, and split into 2 haploid (n) cells. Meiosis differs from meitosis in that there is no replication of the DNA within the cell, rather the chromosomes halve in number when the cell divides. So this process produces spores that are haploid, and that will form the basis for the next stage of the life cycle.

The spores then either disperse or don't (depends on the plant) and develop into a haploid Gametophyte stage. In Bryophytes they usually do disperse, and once they have done they grow into the new Gametophyte. The Gametophyte produces gametes through mitosis (remember, the Gametophyte is haploid so must undergo mitosis or the number of chromosomes would be halved again, which would be disasterous!) and depending on the the structure they're produced in, and the plant we're talking about, these are usually different sizes. The male is generally termed the 'microgametophyte' and is a motile sperm (ie it can swim towards the egg), and the female the 'megagametophyte' and is usually a sessile egg (ie it stays still).
Again, depending on the plants we're talking about, these can be produced in specialised structures (in mosses for example the male structure is the antheridium and the female is the archegonium). If fertilisation occurs, the mirco- and megagametophytes fuse and form a new diploid (2n) zygote, which will grow into the new Sporophyte generation and the whole cycle can continue again.

Edit: A picture to help out (because I finally drew one). Using Mosses (Bryophytes) as an example

 

Phew. So that in a nutshell is a brief (and I hope not too confusing) explaination of the alternation of generations in plants. I think that's probably enough to absorb right now, so I might explain how seed plants and flowering plants all fiddle with this in the next few posts ;) Also if I'm really motivated, I might even do some diagrams to try to help explain this a little better (edit: yay! Did that!).

Also if there is a topic you would really like to discuss, leave me a comment and I'll see what I can do about that. I promise nothing (hell, I'm not an expert in any particular field anyway!) but it never hurts to ask :)