Have you ever looked at the underside of a fern leaf or a frond and found it covered with small yellowish, brown or maybe black clusters? They look like tiny piles of fine dirt and they come in various shapes organized into different patterns, although a circular shape is the most common. So what are they and why do ferns have them? If you watched my video on plant classification, and if you didn't, you should. You know that ferns are vascular plants.
So they have true roots and vascular tissues, allowing them to stay erect and grow tall. Just look at the size of this tree fern. But compared to the majority of land plants, there's something ferns are missing.
They have no flowers or seeds. However, they are able to reproduce. So how does the reproduction work?
That's the role of those strange dirt clusters. which are called psori. Psori is the plural, but if we talk about just one, it is a psoorus.
Each psoorus is made up of sporangia, which are these little balls that are clearly visible when I cut open one psoorus. In some ferns, sporangia clusters have induccia, which are these protective flaps that cover and protect each psoorus until the spores are ready to be released. Ferns as we know them Lush green plants with fronds are the sporophyte life stage in the fern life cycle.
That means they produce spores, as suggested by the name sporophyte, and their cells are diploid, having one set of chromosomes from each parent. Spores are produced inside sporangia via cell division called meiosis. During this process, the number of chromosomes in daughter cells is reduced by half, resulting in haploid spores.
When spores are ready to be released, sporangia need to open to release them, and there's a special mechanism for that. You can see in this magnified photo that each sporangium has this group or row of cells creating a seam. This is an annulus. Through the process involving water evaporation, the annulus cavitates and catapults the spores out of the sporangium.
When spores land on a suitable moist surface, they start to germinate, and via cell division called mitosis, grow into a prothallus. Remember, spores are haploid, and during mitosis, the number of chromosomes in daughter cells stays the same, so our prothallus is a haploid gametophyte. The prothallus is anchored to the soil by rhizoids.
Since it's very tiny, only a few millimeters long, We tend to not notice this stage of a fern life cycle in nature. However, if you ever find yourself around ferns, look closely on the ground around them and you might find some tiny gametophytes. They are mostly heart-shaped and look a lot like liverworts. As I already mentioned, the prothallus is the gametophyte stage in the fern life cycle, which means it produces gametes. Both male and female sexual structures Enteridia and erchigonia respectively are formed on a prothallus, so it is bisexual, although in some cases enteridia and erchigonia are found on separate gametophytes.
Other times, when both male and female sexual structures are located at the same prothallus, there are various strategies in place, such as pheromone production, that prevents self-fertilization. Enteridia produce male gametes, flagellated sperm, while Archegonia produce female gametes, eggs. Similarly to the sperm of mosses, fern sperm moves using flagella and it is fully dependent on water to get transported to an egg.
This dependence on water is one of the reasons you find ferns mostly in wet, shady habitats. Once sperm successfully fertilizes the egg, A zygote forms that later develops into an embryo. Both gametes, sperm and egg, are haploid, but as they fuse and the embryo is formed, the sporophyte part of the fern life cycle begins. As the embryo grows, a new sporophyte plant emerges out of the gametophyte, or prothallus.
This young sporophyte fern plant is attached to the gametophyte from which it takes all of its water and nutrients, Until it grows into a mature sporophyte plant, and the gametophyte dies off. Even though sexual reproduction is the most common way ferns reproduce, it is not the only way. Look at this fern, whose fronts are covered by what look like miniature replicas of the fern plant. This particular fern is known as hen and chicken's fern, or mother fern, and I got these photos in New Zealand, where it is native.
Hen and chicken's fern nicely shows vegetative reproduction by bulbils, which form on the front, and later fall off and grow into mature plants. The other and much more common form of asexual reproduction in ferns is cloning by rhizomes. Brenching rhizomes can often create large fern colonies.
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