Evolution of Plants; part 4, The Spermatophytes.

The Spermatophytes means plants that produce seeds as opposed to spores. There are three groups the Cycads, the Gymnosperms and the Angiosperms.   Gymnosperms are the conifers, Angiosperms are the flowering plants but first we need to consider the Cycads.

These are perhaps the least well known of the three, but you can sometimes purchase them in garden centres. They look like rather robust ferns often with very dark leaves. They usually cost quite a lot and are very slow growing. You also quite often see them in botanical gardens where they can grow to some size. Botanists like them because they are very much the link plant between the Pteridophytes and the Spermatophytes.

Given time they will produce what looks like a massive cone like structure in the centre of the plant, in fact two types of cones, male cones which are longer and thinner.

 

 

 

 

 

 

 

 

 

Then female cones which are rotund and can be quite enormous. Also you only get male cones on one plant and female cones on another.

 

 

Now the big break through with this group is that the male microspores, we could now call it pollen is transferred to the female macrospores (ovules)by wind or even by insects, commonly beetles and not bees but none the less it is very similar to flowering plants. Also the female macrospores are not shed and so remain in their cone. So no more random dispersal and hope that the little antherozoid can swim and locate the female oocyte somewhere maybe? Also no more reliance on a layer water for the antherozoids   to swim in. This really is a major step forward for land plants.

The only remnant of the old system is still the involvement of a swimming antherozoid, however not in a layer of water. When the microspore/pollen grain gets blown to the female cone, or transferred there by some random insect, there is mucilage on the entrance area and as this dries out it reduces in volume and sort of sucks the microspore in and towards the entrance to an archegonium in which the female oocyte is located. Whilst this is happening, inside the microspore a small amount of cell division takes place producing what is strictly a gametophyte….remember them from the days of the mosses and liverworts. So this is all that is left of that generation, just a few cells and Two of these cells becomes  antherozoids.  They no longer have much resemblance to the sperm like antherozoids of the mosses and ferns but they do have little swimming tails in fact lots of tiny one arranged in a sort of spiral arrangement and the antherozoid is sort of top shaped. No matter it does not have far to swim, less than a millimeter and all in a nice mucilaginous medium, so it can hardly fail and most important not  dependent on rain or dew or splashes of water. So these pants can grow anywhere on land as long as there is soil water and light and nutrients.

Once the oocyte is fertilised it is of course then an oosphere and this develops into a diploid seed. This seed is very similar to the seeds of flowering plants with two seed leaves (cotyledons) a shoot and a root. It is not however contained in a seed pod or nut or such like, it is described as naked and simply has an outer seed coat.

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