Flowering plants (angiosperms) reproduce sexually via cells produced in their flowers. The environmental conditions that stimulate flowering in plants include a period of cold i.e. chill time that informs the plant what time of the year it is. It would be a waste to flower in autumn because the approaching winter would bring conditions not favourable to survival and dispersal of seeds from the flower.
Therefore, a chill period predicts that spring and summer are to follow, so flowering may commence. The process isn’t instantaneous, so the actual flowers finish developing once the climate is optimal rather than in the actual winter.
This response is called vernalisation (FLC stands for flowering locus C – a gene responsible for vernalisation; it is expressed highly in the plant before the chill time, during which it decreases, finally kickstarting flowering).
In addition to this, phytochrome (a plant receptor; more on this later) informs the plant on how much daylight is present. It directly responds to light, and controls the processes of growth and flowering.
Right, let’s delve into it!
A bit equivalently to mammalian sexual reproduction, sexual reproduction in plants involves complementary gamete cells that join to create a zygote that further develops into a new organism of that species.
In plants, pollen grains are equivalent to spermatozoa in mammals, while the embryo sac is equivalent to the ovum. Instead of forming a blastocyst where the embryo starts developing inside a developing placenta, they form a zygote inside a seed that also contains nutrients that the little zygote can use once it starts growing. Yes, we are about to learn about plant reproduction and the events that lead to seeds!
In terms of the formation of pollen grains and embryo sacs, the sequence of meiosis and mitosis between precursor stages of development is similar to that in mammals. Instead of mammalian testes and ovaries, plants have equivalent structures called anthers and ovules.
You might notice the ovule is inside a structure also called ovary. The flower ovary is what later develops into fruit. You know, apples, pears, etc. Their seeds are what is created as a result of the processes about to be outlined, inside the ovule. Another point to note is that both reproductive structures, anthers and ovules are present in the same flower. There are species that only have one or the other, but it’s common for them to have both. Hence, self-pollination versus cross-pollination – by the same flower, or between separate flowers.
Flowers have different appearance and features depending on whether they are pollinated by wind or by insects.
Flowers pollinated by wind must be able to easily release their pollen into the air while at the same time filter the air for incoming pollen from other plants. Flowers pollinated by insects must spread their pollen by attracting insects via multiple means, and have a mechanism through which pollen gets attached to the insect and then transmitted to another flower.
Therefore, a number of adaptations have evolved that differ between flowers depending on their mode of pollination. Wind pollinators have anthers that hang outside to optimise pollen dispersal; produce excess pollen to make up for wastage through the wind; have a feathery stigma to catch incoming pollen from the air; have no need for insect attraction features such as scent and nectaries; have smaller, dull petals; grow in dense groups over larger areas; have light pollen that may also have wings to aid dispersal; and have flowers grow in groups on the plant i.e. inflorescences.
On the other hand, insect pollinators produce attractive, large and brightly coloured petals, nectary glands that make nectar, and even flowers that mimic the look and smell of female wasps, or produce an intoxicating chemical that makes insects high in order to draw insects to the flower and make them carry pollen to other flowers on their way.
They grow individually rather than in dense groups; their stigma is relatively small as it is easily touched by the incoming insect; anthers lie inside the flower to make contact with the insect; and pollen is larger and may have projections that adhere to the incoming insect.
Finally, the fun part! The pollen grain makes its way towards the embryo sac down the style of the ovary. The tube nucleus develops into a pollen tube which with the help of digestive enzymes that can break away through the style travels down towards the embryo sac.
Once the two pollen gametes reach the embryo sac, one of them fertilises the egg to create the zygote which will develop into the actual new plant, while the other pollen nucleus has a threesome with the two polar nuclei to form the triploid endosperm which provides the nutrients for the zygote later in development.
And then this little thing is a seed! And there may be many of them within an ovary which then grows into apples and pears and etc.! Such fun.
Following fertilisation, the new seed is carried in fruit and can finally give rise to a new plant through germination at a location potentially further away than the parent plant. Fruit serves this spreading purpose by containing nutrients appealing to various animals which consume them and expel their seeds through defecation at a more distant place.
Seeds and fruit can develop into a multitude of shapes, sizes and arrangements in different flowering plants.
Broad beans have been cultivated for food for thousands of years, and are a great source of protein and folate.
In this case, the whole pod is the “fruit” while the bean is the seed. The testa is the outer coat that protects the embryo against dehydration and infection by outside agents.
The hilum is the scar left on the seed following its detachment from the wall of the ovary.
Germination (and flowering)
The micropyle was initially the opening through which the pollen entered the ovule for pollination, and now can serve to direct the emerging seedling out into the ground during germination. This can happen after a period of dormancy during which the seedling does not develop. This can help the plant save energy, and only develop in good conditions. Hydration of the seed can kickstart germination.
The plant hormone gibberellin has a role in ending dormancy, hence regulating germination. Gibberellins regulate seed germination, stem elongation, leaf growth, producing pollen and flowering.
Since many of these growth and flowering processes are light-dependent, their development is photomorphogenesis which means development of structure driven by light.
This is achieved via a plant receptor that is light-sensitive called phytochrome. It’s a protein-based molecule with a chromophore group at the centre called bilin. It absorbs light in the red and far-red end of the light spectrum, and hence acts as a detection mechanism which informs the plant of the duration of day and night to control flowering, as well as for the circadian rhythm.
The red (r) and far-red (fr) forms of phytochrome transition between states according to the presence of sunlight. When the far-red, active form becomes abundant, it initiates chemical reactions that start the processes of growth and flowering. The top half of phytochrome is the receptor part sensitive to light, while the bottom, rounded half is the part that catalyses chemical reactions to enact the changes based on the detection of light.
Light stimulates the action of gibberellin, which stimulates the breakdown of stored starch into active sugars which enable seedling growth and together with water encourage the continued expression of the relevant protein products in the seedling such as amylase which breaks down starch.
The radicle section of the embryo is first to emerge and start downwards growth, while the plumule continues its upward growth above soil.
Maize (corn) is the largest product in terms of quantity made, surpassing wheat and rice in terms of human crop and livestock production. Some is consumed directly, with large quantities going to animal feed and secondary products like corn starch and corn syrup.
Here, the kernels are the seeds with each one containing some familiar things: the large, nutritious endosperm, the seedling itself with a plumule and shoot and root meristems (the source of flowers and roots respectively) as well as new terms.
The cotyledon is the source of the leaf, while the aleurone is a protein store. All in all, they contribute different parts to the developing seedling during germination.
Maize, rice and wheat are key cereals that act as staple foods for humans on Earth. Staple foods constitute a basic food that is eaten daily by different populations. They provide most of the energy and nutrients. These three cereals constitute two-thirds of the total global food consumption, with rice alone being a staple for half of all people on Earth.
The security and sustainability of cereals are therefore critical. Cereal crops are vulnerable to increasingly unstable environmental conditions brought on by climate change. These include droughts, floods, storms, and greater temperatures ranges under which pests can thrive.
These plants must be made tougher by enhancing their genetic diversity and producing more varieties. These should be more resistant to environmental stress and diseases, while offering a greater yield and better nutrition.
Extraneous factors such as human disease can also impact crops and their prices. An example of this is the Ebola outbreak in West Africa, which threatened food security and the stability of food prices.
In Indonesia, relying on rice as a staple food is especially risky, as it is a very susceptible plant that has already suffered at the hand of droughts, floods and pests. Diversifying staple foods must be undertaken in order to mitigate these risks. Promising candidates for staple foods to match rice include cassava and sago.