The general life cycle of Gymnosperm||Life cycle of gymnosperm is haplontic or diplontic.

Life cycle of gymnosperm is haplontic or diplontic.If you are searching the general life cycle of gymnosperm then you are at the right place. Here you will find the life cycle of gymnosperm.

structure of megasporangium in gymnosperm illustration.

The general life cycle of Gymnosperm. 

Life cycle of Pine tree;reproduction of gymnosperm.

The general life cycle of gymnosperms and we're going to use conifers as our example recall that conifers are seed plants and part of the larger group called vascular plants. Because of this, the dominant stage of the life cycle is the sporophyte generation so the tree that we see is 2n gymnosperms organize their sporangia on Strobel I and on a conifer, there are two types of Strobel I or cones.  

Green pine tree and pine cones.

The first kind is the male cone which some people refer to as the pollen cone and the second one is the female cone although both of these cones bear sporangia. 

spruce cone and spruce branches.

They're both quite distinct from one another the pollen cones are male cones that tend to be soft and flexible because they are derived from leaves the female cones are hard and woody. Because they are derived from stem tissue like Brax we're going to start by having a closer look at the male cones and one thing you should notice is that the male cones are organized in a cluster each one of these is an individual cone and if we zoom in we would see a series of several stacked sporangia if I zoom inside a single sporangia --m what I see is a structure that looks like this that would be the micro sporangium and it's actually supported by some tissue that's part of the sporophyte so here's the micro sporangium being supported by some extra tissue from the sporophyte inside the micro sporangium is a series of cells you might be tempted to think that these cells are the spores but they're not these in fact are the micro spore Oh sites the micros porous sites are the cells that undergo meiosis to give us the microspores themselves so if we have a look at one of these after it undergoes meiosis you would see that it forms a diagnostic structure that's called a tetrad remember that the product of meiosis is always four things so we have a tetrad of microspores given that we've made spores each of these spores are then going to undergo mitosis to form the micro gametophyte the micro gametophyte is multicellular it's the product of the mitotic division of micro spores and we typically refer to the micro gametophyte as pollen it'simportant to keep in mind that each of these micro spora sites that you see is going to undergo division so these micro spore Oh sites undergo division to form a tetrad of microspores each one of which can divide into a pollen grain so though that goes a long way in explaining why there's so much pollen produced by conifers coupled with this is the fact that the pollen of conifers is oftentimes air dispersed and so there are accessory structures on the pollen grain called air sacs looking closer at the female now I'm going to zoom in on one scale of this cone when I do that what I see is the outer hard woody surface there's a little opening at the base an inside of that we have the megasporangium just to emphasize this I'm going to color in the mega spray in GM here in blue so that you can see its structure notice that the megasporangium is enclosed and protected by the sporophyte because in seed plants the spores never leave the parent plant they develop in place into either mega or micro gametophyte similar to the mail on the inside of the megasporangium there is a single cell and that single cell is called the megaspore o site the megaspore Oh site is the cell that is going to undergo division to give us our megaspores and so after we have meiosis we once again end up with that seam structure with the megasporangium internally but instead what we have are four megaspores there are not enough nutritive resources to allow for the development of for total megaspores and so there is some programmed cell death or apoptosis where by three of the megaspores die and one remains to undergo mitosis to give us our fully developed mega gametophyte so now we have the mitotic division of the mega spore which gives us rise to our fully mature mega gametophyte one thing to keep in mind is that there are several rounds of mitosis necessary to produce the fully developed mega gametophyte the mega gametophyte of gymnosperms in contrast to angiosperms is relatively large and it holds two archegonia. So once again I'm drawing the remains of my mega sporangium and inside we have two archegonia remember archegonia produce eggs so inside each archegonium there is an egg recall that the eggs are haploid so now what we need is for sperm to be delivered to produce a zygote recall that I told you that pollen of gymnosperms is oftentimes when dispersed so off of these pollen cones what we get is several grains of pollen that with the help of their air sacs are able to travel long distances through the air if you go outside during certain times of the year. You'll notice your car is completely covered by all kinds of yellow pollen especially if you live near a large tree unless pollen makes its way all the way to the scale of the cone the pollen works its way down and eventually ends up on the outer edge of the remains of the megasporangium.After which each pollen grain grows a pollen tube to deliver one sperm to each egg if you were to examine the scale of a pine tree or other conifer up close you would notice that at the base of each scale there are two small depressions or divots that at one time contain seeds.So once the egg is fertilized these two zygotes grow and develop into mature seeds of conifers are also when dispersed and it's important to note that we have multiple layers here so we have inside a seed young developing new sporophyte generation that we're gonna call the embryo and it's diploid on the outside of that we have a protective layer which helps the seeds survive harsh conditions that are called the seed coat and then internally we have the remains of the megagametophyte tissue so this is all haploid tissue and that contrasts sharply with what we're going to see in angiosperms so that megagametophyte tissue is nutritive for the developing embryo and is haploid now last but not least these seeds are usually enclosed by some structures that help them disperse through the wind eventually landing and growing through lots of mitoses eventually into the next sporophyte generation. Although the gymnosperm lifecycle on the surface looks rather complicated it's important to note that it still follows the same basic alternation of generations.

 

The life cycle that we've learned about sporophytes give rise to spores grow into gametophyte s' gametophyte smake gametes which fuse to make a zygote that grows into the next sporophyte generation that same pattern is repeated in this life cycle you should also recall that seed plants are all heterosporous. So they have different sizes of spores that eventually develop into specialized Kamino fights for the male.

pine cones Gymnosperm.