Flowering plants dominate today’s flora. By evolutionary time-scales this has happened very quickly, and very recently. This aim of this article is to give you a little history of how and why flowering plants became so successful.
What was there before them?
Gymnosperms (naked seeds), such as Cycads, Gingko, Pines and Conifers, dominated the Earth’s flora along with Cryptograms (ferns and mosses), from the Carboniferous period up to the Jurassic period. The beginning of the Cretaceous period (125 million years ago, or so) saw the dawn of the flowering plants (Angiosperms) and the beginnings of their dominance of today’s flora. The astounding variety of flowering plants today (more than 250,000 species) is a bold contrast to the relative uniformity of gymnosperms.
When did Flowering Plants become so dominant?
Whilst the beginning of the angiosperm era was around 125 million years ago, about 65 million years ago saw the real boom of flowering plants.They exploded onto the scene almost instantly (in evolutionary terms) changing the way land looked. The first evidence of angiosperms is as far back as 125 million years ago.
The first angiosperms populated the drier upland regions of the earth. Frustratingly, these areas are less fossil-friendly, so evidence of early angiosperms in fossil form is rare. However, pollen is easily recognisable in fossil form, so although the actual plants cannot be found in fossils, there is a wealth of pollen evidence to show that angiosperms were making a move.
Why did Flowering Plants become so Dominant?
Theories abound as to why angiosperms appeared in such variety, so quickly. It is widely thought that geological changes throughout the world accelerated the spread of angiosperms. As tectonic plates shifted, continents developed and mountain ranges formed. Land masses and their occupants were moved from their safe, comfortable habitats to cooler or warmer environments where they were easily out-competed by the very adaptable angiosperms. The gymnosperms were very well adapted to moisture conservation in their relatively dry environment – a trait that became less competitive as time passed and conditions changed.
This global shift effectively loosened the gymnosperm grip and angiosperms took full advantage. When the earth became cooler and wetter, the water-retaining attributes of gymnosperms became useless and thus the gymnosperms were left standing still by the fast-moving angiosperms.
The alternative Alvarez theory is not completely unfounded. This cites an asteroid collision with Earth triggering calamitous global events and climate change. There could well have been a huge collision between the earth and an asteroid. This may have created dust clouds that blocked the sun, cooling the earth. Add this to the changing geology of the earth and the outcome is the same, with the adaptable, ‘fit’ angiosperms out-competing the gymnosperms.
Other factors that accelerate the angiosperm domination of the plant world include the fact that they are able to reproduce faster than most other living things. The ability of angiosperms to form fertile hybrids is well-known, and they can even immediately form new species through irregularities in chromosome development. This evolutionary flexibility allows angiosperms to diversify into many different habitats.
Insects have a great part to play in the development and progression of angiosperms. The rise of angiosperms could well have gone hand in hand with the rise of the insects. Flowering plants lure insects to their reproductive parts with sweet nectar and bright ‘landing strips’ on their petals. Whilst wind pollination would be great for a uniform field of grasses, it would be extremely inefficient in a woodland environment, where there are such a huge diversity of plant life. Insect pollination is critical in these habitats, with insects returning to flowers they have previously visited, or that they ‘prefer’ The insect will visit one flower, have a good feed, then move on to a similar flower for some more nutrition. It knows it will find food, so returns.
When it feeds on the second flower, it deposits its package of pollen on the female flower parts, leading to fertilisation. Specific insects visit specific plants, which means that co-existence of a diverse range of plants is possible.
Classes of Angiosperms
So, we’ve established that angiosperms, the flowering plants, make up the largest proportion of plant species found on our planet. Surely then, there would be a huge diversity of classes?
Not in the case of angiosperms. There are, surprisingly, only 2 – Dicotyledons and Monocotyledons.
Dicotyledons and Monocotyledons
The 65,000 species of Monocotyledons are represented by grasses, palms, lilies, sweetcorn and orchids among many others. Dicotyledons have around 170,000 species and includes many trees, vegetables, herbs, perennials and weeds.
It is thought that Dicots evolved from monocots, and they both follow very different evolutionary lines.
If you imagine a blade of grass (Monocotyledon), with the leaf veins running from base to tip, this easily contrasts to an Oak leaf (Dicotyledon), which has its veins branching through the leaf.
A Summary of Angiosperm Life History
Egg fertilisation and seed development occurs.
Seed germinates and grows into a mature plant.
The mature plant flowers. The gametophyte (genetic package) develops in the ovules (female) and anthers (male) of flowers.
In female flowers, a process occurs (meiosis) that produces a megaspore. This enters several rounds of mitosis, resulting in an embryo sac.
In male flowers, meiosis occurs in the anthers, producing a micro-spore that goes through mitosis, resulting in a pollen grain. The pollen grain has a tube cell and a generative cell.
After successful pollination, the pollen finds its way to the ovules, at which point the pollen tube’s growth is activated. The tube grows towards the ovules, whilst the generative cell’s nucleus divides into 2 sperm. When the sperms travels down the tube and reaches the embryo sac, one sperm fertilises the egg cell, with the other fertilising the bi-nucleate cell.
The resulting Zygote develops into a seed which contains the embryo, and is dispersed by the plant.
Hopefully the seed will germinate successfully and grow into a mature plant, so the reproductive cycle can continue.