1. Reproduction in flowering plants

2. Differences between gymnosperms and angiosperms in reproductive structures
Ovule
Gymnosperms, like the conifers, have ovules which are borne naked on the seed bearing leaf, the sporophyll.. In the flowering plants, the angiosperms, the ovules are protected inside a protective structure from the host plant, and are part of a complex structure, the flower, with male and female reproductive organs, attractive leaves, and protective leaves. We need to understand how this structure might have evolved.
Seed cones and
pollen cones

3. Evolution of the flower
A stem (receptacle) developed with 4 whorls of leaves tightly clustered at the end.
1. Outside leaves are protective (sepals)
2. Inside this are leaves which are usually developed for attraction (petals)
3. Inside these are the male sporophylls (Stamens composed of anther and filament)
4. Innermost leaves are the female sporophylls (carpels)
Most plants produce flowers that are hermaphrodite, that is they have both male and female parts. The flower consists of whorls of leaves borne on top of a piece of stem (the receptacle). The sepals and petals are clearly leaves, but closer examination of the anther shows that it does have vascular tissue passing through the centre suggesting development from a leaf, and each female reproductive structure, the carpel, also has vascular tissue.

4. Evolution of the carpel
TOP
Open leaf
with ovules
at the edges
Leaf
folds
over
Edges of
the leaf
meet and
fuse
If we look through the fossil record, we can see forms of ‘flowers’ that are consistent with this series. One look at a pea pod will confirm the rich network of veins on the pod. The carpel that is formed consists of 3 parts. A flattened receptive surface on top, the stigma, for pollen to land and germinate, a long neck, the style through which the pollen tube can grow, and a large ovary containing the ovules. This ovary helps to form part of the fruit.
Section

5. Flower variations
Monoecious e.g. maize - separate male and female flowers on the same plant (c.f. conifers)
Dioecious e.g. holly - separate male and female plants
Self-fertilisation can be useful in harsh habitats where the chance of pollen arriving from another plant may be remote, and where any selective advantages that the plant possesses need to be reinforced rather than diluted by genes arriving from outside the habitat. For many species, the enrichment in the genes produced by cross-fertilisation outweighs the advantages of self-fertilisation, and they have evolved strategies to facilitate this. There are a number of self-incompatibility strategies that are used in hermaphrodite flowers like having the stigma and anthers at different levels, having them ripen at different times, or using complex self-incompatibility genes to prevent pollen germinating. One the simplest and most effective is to get rid of hermaphrodite flowers altogether. Monoecious plants which have separate male and female flowers on the same plant encourages cross-fertilisation, but still permits self-fertilisation if cross-fertilisation doesn’t happen. Dioecious plants prevent self-fertilisation altogether by having an entire plant with only male flowers, or only female flowers.

6. Wind versus vectored pollination
not scented
petals small and inconspicuous
nectaries poorly developed
Vectored
scented
petals large and coloured
well developed nectaries
Vectored pollination where a vector such as an insect carries pollen from one flower to another is a much more efficient method of pollination than wind pollination where the pollen is scattered to the winds, and arrives at the stigma purely by random chance. The rapid evolution of the flowring plants (more than 300,000 known species) is paralleled by that of the insects, and it seems likely that in many cases the pollinator and pollinated co-evolved.

7. Ovule development Megaspore (n) divides mitotically 3 times to
give 8 nuclei:
3 antipodal cells
2 polar nuclei
In gymnosperms, a single egg cell is fertilised. In the flowering plants, we need a double fertilisation, one with the egg, and another with the 2 polar nuclei to give a triploid cell (3n) which will divide to give the endosperm. The synergids are thought to play a role in attracting the pollen tube to the micropyle.
Egg with 2 synergids

8. Pollen development Microspore (n) divides to give 2 nuclei.
One of these (vegetative nucleus) looks after the house-keeping functions.
The other (generative nucleus) divides to give 2 sperm nuclei.

9. Pollination and fertilisation
Pollen lands on the stigma (pollination) where it germinates. It sends out a pollen tube down the style into the ovary where it is attracted to the micropyle of an ovule.
2 nuclei are released, one to fertilise the egg (fertilisation), one to combine with the 2 polar nuclei. This gives a triploid tissue, the endosperm which acts as a storage reserve in the seed.

10. Embryo development ‘Torpedo- Division to give shaped, embryo
embryo and
suspensor
‘Heart
shaped’
embryo
Testa
(seed
coat)
‘Globular
embryo’
Cotyledons
The embryo is shown here in orange, the endosperm in red. On every see, it is possible to deduce the origins of the various tissues. There is always a small pore (micropyle) at one end, and a scar nearby (Hilium). The root apex always comes out at the micropyle end.
Shoot apex
Hilium
(scar where
the ovule
was attached
to the ovary)
Root apex
Suspensor
Micropyle

11. Simple fruits are formed from one or fused ovaries
Peach
In the flowering plants, the ovary wall, and sometimes the receptacle can develop to form the fruit. The ovary wall can either be hard (nuts and grains) or fleshy (tomato). Fruits can be either simple (one ovary), or compound (multiple ovaries)
The pea-pod is a simple fruit (consists of a single carpel). In the case of pea, one of the rows of ovules degenerates leaving a single row of peas. In young pea pods, you can clearly see the midrib at the bottom of the pod giving a rich supply of veins all over the pod, and with the stigma and style still present. In older pea pods, these shrivel leaving just the pod. In tomatoes, there is a fleshy ovary wall composed of fused carpels. The ovary is approximately spherical. If the ovary is elongated, longer fruits are seen (e.g. cucumber).
3 carpels
fused at their
edges e.g.
Tomato
Pea pod
Apple - ovaries are overgrown
by a fleshy receptacle

12. Aggregate fruits are formed from separate ovaries from one flower
Compound fruits have more than one separate ovary. There are two types, aggregate fruits and compound fruits. Aggregate fruits are where separate ovaries from the one flower develop such as strawberry and blackberry. Compound fruits are where multiple fruits develop from separate flowers clustered together e.g. pineapple.
Blackberry
Strawberry

13. Asexual reproduction (vegetative propagation)
Runners e.g. strawberry
Rhizomes e.g. violet
Tubers e.g potato
Root suckers e.g. apple
Grafting
Cuttings
Micropropagation
Above ground horizontal stems
Below ground horizontal stems
swollen underground stems
shoots formed from roots

14. Germination Above ground 2 cotyledons expand to form ‘seed leaves’
Epicotyl
Hypogeal-
hypocotyl
elongates
Seed
The epicotyl is the bit between the seed and the shoot apex.
The hypocotyl is the bit between the seed and the root apex.
The lateral roots have been omitted to make the diagram clearer.
In hypogeal germination (e.g. bean), the hypocotyl elongates pushing the seed and the epicotyl out of the ground. There is a bend in the hypocotyl so that the seed is dragged out of the ground above the epicotyl and delicate shoot apex. The 2 cotyledons are then above ground and can green and open out like leaves, in fact these are very often called ‘seed leaves’.
In epigeal germination (e.g. Pea) the epicotyl elongates leaving the seed and the hypocotyl in the ground. The developing shoot (plumule) forms a plumular hook that protects the shoot apex.
Below
Ground
Hypocotyl
Seed just about
to grow out
Epigeal-
epicotyl
elongates