Heterospory and the seed habit

One kind of spore, or two

Most pteridophytes make spores that are all alike. They are called homosporous. Each spore can grow into a prothallus that bears both male and female sex organs.

A few genera, such as Selaginella and Salvinia, make two kinds of spores. They are called heterosporous.

• The large spores are megaspores. They grow into female gametophytes.
• The small spores are microspores. They grow into male gametophytes.
• A microsporangium holds many microspore mother cells, so it makes many microspores.
• A megasporangium in Selaginella usually has one megaspore mother cell, so it makes only four megaspores.

The female gametophyte stays home

In heterosporous plants, the female gametophyte is kept on the parent sporophyte for some time. It is not left on its own on the ground, as a prothallus is.

The egg is fertilised there. Then the zygote grows into a young embryo inside the female gametophyte.

Why does this matter? In a seed plant, the same three things happen.

1. Two kinds of spores are made.
2. The female gametophyte never leaves the parent plant.
3. The embryo grows inside it, protected and fed.

So heterospory, with the embryo growing inside a female gametophyte that stays on the parent, is a forerunner of the seed habit. It is an important step in the evolution of plants.

Comparing the two groups

• Main plant body: the gametophyte in bryophytes; the sporophyte in pteridophytes.
• Vascular tissue: none in bryophytes; xylem and phloem in pteridophytes.
• Roots: rhizoids in bryophytes; true roots in pteridophytes.
• Sporophyte: lives on the gametophyte in bryophytes; independent in pteridophytes.
• Water for fertilisation: needed in both.

Two generations

In both groups, a plant passes through two generations.

• The gametophyte is haploid ($n$). It makes the gametes.
• The sporophyte is diploid ($2n$). It grows from the zygote and makes spores by meiosis.

In bryophytes, the gametophyte is the big, green, lasting plant. In pteridophytes, the sporophyte is. You will see how these life cycles compare with those of the algae and seed plants later in this chapter.

You should find that $20$ mother cells give $80$ microspores against only $4$ megaspores. A megaspore $400$ µm wide has the volume of more than $2000$ microspores $30$ µm wide. It carries a large store of food.

Try it in Python

The program counts the two kinds of spores in one cone.

You should see $960$ microspores, $12$ megaspores, and $80$ small spores for each large one. With $5$ megasporangia there are $20$ megaspores, and $48$ microspores for each.

Practise with new numbers

Common mistakes

• Thinking every pteridophyte is heterosporous. Most are homosporous. Selaginella and Salvinia are the usual examples of heterospory.
• Mixing up the spores and the gametophytes. Megaspores grow into female gametophytes; microspores grow into male ones.
• Saying that Selaginella makes seeds. It does not. Keeping the embryo inside the female gametophyte is only a step towards seeds.
• Thinking spores are diploid. Spores are made by meiosis, so they are haploid.

Remember these

• Homosporous: one kind of spore. Heterosporous: microspores and megaspores.
• Megaspores grow into female gametophytes; microspores into male ones.
• In Selaginella and Salvinia, the female gametophyte stays on the parent, and the embryo starts to grow inside it.
• This is a forerunner of the seed habit.
• Gametophyte: haploid. Sporophyte: diploid, makes spores by meiosis.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does a megaspore grow into?

2. Name two heterosporous pteridophytes.

3. Why is heterospory called a step towards the seed habit?

4. Which generation is dominant in a moss, and which in a fern?

5. A microsporangium has $18$ microspore mother cells. How many microspores does it make?