Angiosperms and Gymnosperms
The great majority of plants are classified into two different groups, gymnosperms and angiosperms. The name gymnosperm comes from the greek words gymno (naked) and sperma (seed). Their name literally means “naked seeds” because gymnosperms are nonflowering plants that produce their seeds in the open spaces of cones. On the other hand, angiosperm is formed by the greek words angeion (vessels) and sperma (seed). With this said, angiosperms are flowering plants that are formed inside containers basically known as fruits. However, in order to comprehend in greater detail the differences between angiosperms and gymnosperms we must also take note of their forms of reproduction and ultimately, their life cycles.
Angiosperm and gymnosperm life cycles are similar to each other, in that they have a sporophyte to produce spores, a gametophyte to produce gametes, and use meisosis and mitosis to pass from one stage to the other. Nevertheless, their are major distinctions found in the details of each one of these processes.
Gymnosperms are more primitive than angiosperms and therefore, you could probably say that their reproduction and life cycles are much simpler than those of angiosperms. At their sporophyte stage (dominant stage of plant with which we are all familiar with), gymnosperms have two sets of chromosomes, which undergo meisosis and produce two kinds of diffrent spores in a process denominated as heterospory.
The two spores produced are megaspores and micropsores. Megaspores have one set of chromosomes. These are those of the female formed in the ovulate cone where they go through mitosis to later develop into a female gametophyte which will create unfertilized eggs. Micropsores have one set of chromosomes and belong to the male, where they grow in the pollen cone, go through mitosis, and later unfold into pollen grain.
The male gametophyte (pollen grain) will contain two nonflagellated sperm. This means that the sperm will not have flagella (tales) because they will not travel through water to reproduce. As a matter of fact, the nonflagellated sperm will be carried in the pollen grain propelled by the wind to pollinate an ovulate cone and fertilize a different plant’s egg to create genetic diversity. For this purpose, gymnosperms will normally produce exagerated amounts of pollen for them to reach at least one ovulate cone.
When the sperm and the egg join in fertilization, they will end up generating the next generation’s sporophyte. This will first occur through the creation of seeds. The seeds will possess three parts, the embryo (baby sporophyte), food for the embryo, and a seed coat to protect it from being damaged.
Angiosperms, just as gymnosperms, have a sporophyte phase which has two sets of chromosomes, undergoes meiosis, and creates two types of spores with one set of chromosomes: a megaspore and a microspore. The megaspore (of female) grows in an ovary, suffers mitosis and forms an embryo sac (female gametophyte). The microspore (of male) evolves in the Anther, submits to mitosis, and gives shape to pollen grain. The ovary will pass through mitosis and create eggs or female gametes and the pollen grain will also have mitosis and form two nonflagellated sperm or male gametes.
Some angiosperms like to use wind pollination, but many others stick to a more efficient route and utilize animals such as insects, birds, bats, and other sorts to disperse their pollen. Unlike gymnosperms, these guys encounter a double fertilization event, where one sperm fertilizes the egg and the other fertilizes a central cell. This central cell has two sets of chromosomes and the sperm has one. When they unite, they form a substance called an “endosperm” which is triploid or has three sets of chromosomes. The endosperm will be the food surce of the developing embryo. Let’s understand this process a bit better.
Flowers hold a female part recognized as carpel or pistil. This area within the flower consists of a stigma, a style, and an ovary that has many ovules. The stigma is a sticky place at the entrance of the pistil that catches pollen grains. Then, the pollen begins to institute a pollen tube into a long tube-like structure known as the style, to finally access the ovary of the carpel.
The ovary sustains many parts. First, it has the micropyle, which is in its essence a small opening that enables the pollen tube to enter an ovule. Once inside the ovule, the pollen will deliver its two nonflagellated sperm. Inside the ovule is the embryo sac made of 7 different cells: three antipodal cells at the hind of the ovule, one central cell, and three cells next to the micropyle. Those last ones are of great attention. They practically consist of the egg cell and two cynergids beside it that generate chemicals to make a concentration gradient that leads the pollen tube into the egg cell.
Once in the egg cell, one sperm will fertilize it and the other will head to the central cell with two polar nuclei, fertilize it, and create the endosperm. Hormones will then be produced and a seed will develop. To form the seed coat will be the ovule and the ovary will become the future fruit, whether edible or small enough to stick on an animals fur and deposite the seed somewhere else.
Finally, about 98% of angiosperms can be divided into dicots and monocots. Monocots usually have petals in multiples of three, vertical veins, adventitious roots that look like spread fibers, scattered vascular bundles in their stem, and one cotyledon (a seed leaf) inside the seed. Dicots have petals in multiples of four or five, are net veined, have a taproot (main or central root), have their vascular bundles at the edge of their stem, and have two cotyledons.
In conclusion, the basic difference between gymnosperms and angispoerms is that ones have naked seeds and the others are flowering plants. Notwithstanding, in their life cycles they are very similar, except when you go into detail about each’s reproductive organs and ways.