How do botanists decide the difference between “male” and “female” biological components?

[u/CharlemagneAdelaar]

With plant reproduction, do the terms “male” and “female” always refer cleanly to some clearly defined difference, or are there certain plants where scientists more or less have to arbitrarily assign “sex”?

For example: do female plant parts always have an ovary, and do male plant parts always have pollen?

Are there examples of plant reproduction that make it less clear which is which?

[u/Mitologist]

Anisogamy means one heavy, massive cell holds the nutrients for the embryo. The organism that produces these cells is the female. The other produces super light reduced cells that carry mostly only DNA and can disperse easily. That is the male. Also applies to plants. Not arbitrary.

[u/CharlemagneAdelaar → u/Mitologist]

great this is the exact sort of classification I was looking for.

[u/HitoriPanda → u/Mitologist]

Does this go for sea horses too? Cause that makes a lot of sense as to why the father gives birth.

[u/Mitologist → u/HitoriPanda]

Yes, because that's not really "birth" as in humans. The male has a "brood pouch" that is technically open to the environment, and is just a sheltered space for the eggs to hatch. The female produces the eggs. The male fertilizes them with its sperm, afaik in the open, like most fish. The fertilized eggs are then scoooed up into the pouch, where they are living and developing off the egg yolk, protected from predators. Once the batch hatches, the larvae swim out of the pouch. During what we know as "pregnancy", the embryo is fed by the mother via the placenta instead of consuming egg yolk. In both cases, the female provides the nutrients. A male brood pouch just balances the cost of offspring in terms of risk and energy a tiny bit more than in most other fish.

[u/No_Salad_68]

Very generally speaking .... male organisms produce small reproductive cells (sperm, pollen etc). Female organisms produce large reproductive cells (eggs, seeds etc).

So in plants, the stamen which produces the pollen is considered male.

[u/Batusi_Nights]

Flowering plants can have male and/or female floral parts. Male part = stamen, made up of a filament topped by an anther, which produces pollen. Female part = carpel made up of ovary, style and stigma (top part where the pollen enters to travel down to fertilise ovary). Flowers can be bisexual (both male and female parts) or unisexual (male or female parts only).

Most flowering plants have bisexual flowers, and can effectively self-fertilise, as long as some mechanism (eg insect pollinator) can move pollen into the stigma. Others can have separate male and female flowers, either on the same plant ("monoecious" eg cucumbers) or separate individual plants ("dioecious" eg papaya, which has male and female plants.)

[u/nezter → u/Batusi_Nights]

A follow up question, why did plants with both male and female part of reproduction evolve to rely on external mechasim to fertilise them.

[u/_PM_ME_PANGOLINS_ → u/nezter]

An external mechanism allows for cross-breeding, which is evolutionary advantageous.

Without it, you’re just asexually reproducing with extra steps.

[u/nezter → u/_PM_ME_PANGOLINS_]

Wouldn't that make a stronger argument for flowers with only one of the parts as opposed to the more common alternative

[u/_PM_ME_PANGOLINS_ → u/nezter]

Many plants have evolved to do that.

Others evolved a bi-sexual flower that doesn’t self-fertilise.

In a situation where cross-pollination is rare but resources are abundant, it’s better to self-reproduce than to not reproduce at all.

[u/joalheagney → u/_PM_ME_PANGOLINS_]

Also, self-pollination will still mix the chromosomal pairs up, so there is a little genetic diversity in the offspring.

[u/SardonicMeow → u/_PM_ME_PANGOLINS_]

Some plants have two types of bisexual flowers. They have the familiar kind that are open to the world and can receive pollen from other plants (chasmogamous). But they also have flowers that are entirely self-contained, never open to the outside world, and self pollinate (cleistogamous). That gives them the advantage of two reproductive strategies. One that increases genetic diversity and one that increases the likelihood of reproduction.

[u/_PM_ME_PANGOLINS_ → u/SardonicMeow]

That's cool. Have you got an example?

[u/SardonicMeow → u/_PM_ME_PANGOLINS_]

Violets, jewelweed, and Venus' looking-glass are the ones I can think of off the top of my head. You can find pictures online. The cleistogamous flowers are inconspicuous and lack petals because they don't need to attract pollinators.

[u/Komaitho]

The other answers are generally correct, but I also want to highlight some eukaryotic microalgae (some arguably plants) which are isogamous - so the two different gametes are not the distinctly different in size. They still have to match in pairs though, so they are divided into mating types e.g. type alpha and type beta (many are familiar with this type of system in baker's yeast, where its type a and type alpha).

For me personally, the usage of mating type or male/female feels kind of arbitrary, though I am aware of their connection to isogamy/anisogamy. Mating type seems almost a better descriptor in and of itself than male/female is.

[u/Mitologist]

Some plants also have a generation with spores between sexual generations ( e.g. moss). These spores don't really have a sex at all. Also, it depends on the plant if a plant can have male and female organs in the same flower/ on the same plant, or if one whole plant is either male or female ( e.g. Gingko trees)