Spermatophytes or seed plants can be split into 3 groups and we dealt with the Cycads in part 4 of this series. So now on to the final chapter and here I am combining the two groups Gymnosperms and Angiosperms, conifers and flowering plants. These are huge groups and represent the final chapters in the evolution of the flowering plants ….so far. But in terms of the sexual development and the whole saga of gametophyte and sporophyte and the concept of alternation of generation then these two groups only show a small advance.
The conifers only show a slight change from the situation I described for the Cycads, more on that in a moment. However it has to be said that in terms of morphological development then they have taken a giant step forward. The tree to the left is a meer 800 years old and I photographed it in British Columbia Canada.
Trees like the Giant Sequoia are the biggest living organism on earth and have been for some considerable time. Individuals are estimated to be 2,500 years old and over 250 ft high with a circumference of 108 ft. Several have names like General Grant which is 267 ft high and General Sherman at 290 ft. I am not sure about the names. These trees seem to me to represent a slow steady placid growth and to name them after a short lived (relatively) military commander who’s life would have been a series of ups and downs, adrenalin and sudden changes does not really stack up for me. Also at this juncture it is perhaps also right to mention the Californian Bristlecone Pines which are the oldest living organisms on earth with some estimated to be 4,600 years old. Having said that, yesterday (Nov 1st 2017) I eat my lunch under an Olive tree in the town of El Rocio in Andalucia, Spain which is reputed to be 1000 years old….amazing.
OK back to the sex, the small change from the cycads is that the pollen no longer produces a swimming antherozoid cell. It simply produces a cell with a nucleus and a cell membrane, the nucleus is of course haploid. This cell then passes down a pollen tube which grows out of the pollen gain and grows down to the archegonium and thus to the oocyte (ovule). Once fertilised this grows into a seed and eventually falls out of the pine cone and mat grow into the next generation of the species, more likely it will get eaten by a squirrel.
Final stage, we have the Angiosperms or flowering plants. Here there is no change in the sexual situation, but there are two big advances. First is that the seed is now produced in side a case or vessel. Angio means flask in greek. So the seed is surrounded and protected by an outer coat called a carpel. This surrounding may become quite complex and apart from protection of the seed it may also aid with the distribution of the seed. This can take the form of hooks nuts or wings but also can be various forms of fruits. Also as this whole site is about woodland plants then eliasomes should be mentioned. These are specialized structures on some seeds that ants find a desirable food source and so carry them off a small distance (though it maybe quite far for an ant)and these are quite common on some woodland species.
The other big advance over the Conifers is the modification of the cone into a flower, and there are two directions in which this has developed. Some plants have developed flowers which use animals, predominantly insects to carry out pollination and as such we have a vast range of structures involving petals and sepals to attract animals and to maybe provide a place to alight. There are also the development of inducements like colour, smell, and food (nectar and also excess pollen).
Other plants have developed to use wind to carry the pollen and this involves producing vast amounts of pollen and having flowers that will catch the wind to help disperse the pollen and other female flowers which will filter the wind to remove the pollen. So a big change from the rather uniform structure of the pine cone.
As I said in the first article in this series, where will it all end? Well no doubt not here, evolution has never stopped and it will produce a new phase in plant development. It is very difficult if not impossible to even speculate as to where this will all lead. The only thing I will say is that in the immediate future it will not just be the environment directing the path of evolution but also man may or will have an influence with genetic engineering. However this may just be a blip on the path way as in terms of the passage of time from the liverworts through the ferns to cycads, conifers and finally flowering plants, mans tenure represents just a tick in the passing of time.