It’s that time of year when the fist signs of spring tell us that the seasons are changing and life is awakening. Yet just before I deter my focus to the eager angiosperms – I happen to stumble across yew cones. I was originally attracted to them as they looked like little yellow flowers hanging below the dark strap-shaped leaves of a yew tree. They made me wonder about how conifer pollination worked and this inadvertently brought me back to a phenomenon I have struggled to understand for quite some time. Sexual reproduction in plants.
As with most moments of realisation, they happen at the last minute. So for all this time I have been looking at diagrams on the internet and gaining some understanding, yet not managing to grasp how it applied to a real plant, I took a specimen branch of the yew and had a thought to look for youtube videos when I got back home – and hey presto!
In theory sexual reproduction is simple with plants. There is one reproductive cycle that applies to them all – the ‘alternation of generations’, whereby plants alternate between a gametophyte (n) and sporophyte (2n) stage. However the process varies slightly for bryophytes and vascular plants, with the sporophyte stage becoming dominant for vascular plants and the gametophyte becoming microscopic.
So taking this back to my yew cones, I can start to apply this knowledge to the plant I have observed – which I have struggled to do for so long. Firstly, the cones I have are male. They are very pretty, resembling little yellow flowers and as I tap them a shower of dusty yellow pollen covers my sleeve – here begins the multicellular gametophyte (n) stage.
On my sleeve lay the pollen grains which are the male gametophytes. The gametophytes produce the male gametes or sperm. When the pollen grains are dispersed by wind and come into contact with a compatible female cone, the pollen grain germinates producing a pollen tube that transfers the sperm to the ovule containing the female gametophyte. This marks the end of the gametophyte stage. Short and sweet.
The sperm and ovule then fertilise to form the zygote, which in turn develops into a seed. This is the beginning of the sporophyte (2n) stage of the cycle. In the yew, the seed develops a fleshy red outer coating – the aril – which attracts birds such as hawfinches and great tits to come and eat it. The seed is then dispersed elsewhere along with a small package of fertiliser (bird poo). When the seed finally germinates, the zygote undergoes mitosis to produce a yew sapling that eventually develops into a yew tree.
Once the yew tree has reached a certain age it will start developing cones, either male and female. In the case of the female, the cones contain ovules on each of its scales. Each ovule contains a megasporangium which contains a megasporocyte – a unicellular sex cell. It is here where the megasporocyte undergoes meiosis to produce megaspores (n). This marks the beginning of the gametophyte stage of the cycle.
Meanwhile, in the male cones are the micosporangium. These contain micosporocytes. The microsporocytes undergo meiosis to form the microspores (n). This again, marks the beginning of the gametophyte stage of the cycle. The megaspores and the microspores then both undergo mitosis to become the multicellular archegonium and pollen grains, respectively – similar to the pollen grains which earlier coated my sleeve.
It is now clear to me that when you are looking at a vascular plant, such a yew tree, you are generally looking at the sporophyte stage. The gametophyte stage consists of only a very short period when the male and female cones produce pollen and eggs up until the moment the eggs are fertilised. The same process can be applied to angiosperms and bryophytes with slight alterations – all acting out this alternation of generations – which as all sexual selection is intended for -allows the plant to shed off bad genes and add new ones in! Yet one questions still haunts me…which came first – the yew tree or the yew cone?