A Mock Orange Tree - Philadelphus coronarius

I am an amateur naturalist trying to learn something about everything living in my garden.

Photos 1 and 2, taken a few days ago, shows the Mock Orange (Philadelphus) bush that grows at the back of my garden. It is a large plant (maybe 4m x 4m) and a fabulous site in early summer.

And yes, it smells as gorgeous as it looks! A rich honey/jasmine aroma that wafts across my lawn on summer evenings.

Philadelpus has long been popular with gardeners and plant nurseries stock numerous artificial cultivars. The Mock Orange (genus Philadelphus) and 'true' Orange (genus Citus) are really only distantly related. The genus Philadelphus is part of the large Hydrangeaceae family of plants.  I found a species-key here, and my plant keys out as Philadelphus coronarius.

Seeing a thin dusting of yellow pollen on many of the leaves I was inspired to get out my trusty student microscope. I enjoy fiddling around with microscopes and I'm a little surprised that in many years of doing so I've never before looked at pollen. For any beginnner like me who wants to have a go, there are a few tips it may help to know:

Firstly, something I hadn't previously realised is that plants release their pollen in a dehydrated state (15-35% water content is typical [ref.1]). I guess (but don't know) they do this to keep their weight low and so assist their transportation by wind or insects. Also, desiccation may help prolong the active lifespan of the grain. Dehydrated and hydrated grains can look significantly different (see photo 3).

A second thing it helps to know is that pollen grains often have a waxy surface coating. This can be a nuisance for microscopy as it may cause grains to stick together. It also obscures fine surface features of the grains.

Fortunately, both rehydration of pollen grains and removal of their waxy layer is easily achieved by simply wetting them with a few drops of alcohol.

Once you've looked at your pollen slide you can of course simply throw it away. Microscope users will know however, it is possible to make a collection of semi-permanent slides by encapsulating specimens in glycerine jelly. You can buy glycerine jelly especially designed for pollen. The jelly contains red dye that stains the grains and makes it easier to see fine details on their surfaces.

Anyway, photo 3 shows the results of the above: circular/triangular pollen grains about 12microns across.

Seeking to learn some more about pollen, I came across a nice review paper by Edlund et.al. here [ref.1]. The paper highlights various areas where the science behind pollen is unexplored or only partly understood. Take for example the functioning of the outer coating of pollen grains (the exine). This layer can be extremely ornate. Often it is riddled with cavities containing exotic plant proteins. When a dehydrated pollen grain lands on the 'female' stimga in the centre of a flower of the same species, something about the surface of the grain causes it stick fast, when pollen from a different species doesn't. The science behind this 'selective adhesion' is only partly understood.

Once a grain has stuck, chemicals are exuded by the stimga that rehydrate the pollen in a matter of minutes. Once again, this can be exquisitely selective. Two species of pollen grain can be put, side-by-side, onto a single stigma, and only the pollen grain from the correct species will be rehydrated. How nature manages to pull off this clever stunt is again something of a mystery.

With the grain re-hydrated, it germinates and sprouts a single tube that grows its way down the stimga. (There's a fun article by Chris Thomas here that describes how to observe pollen tubes by sprouting grains on a piece of onion skin).  Eventually the pollen tube contacts with a female egg at the base of the stigma and the pollen grain sends its DNA down the tube to fertilise the egg. (Pollen grains are a mechanism by which DNA is carried between plants. Its wrong to think of them as 'male sperm' however since a pollen grain is mostly comprised of bundles of 'normal' vegetative plant cells.)

Some plants rely on wind to spread their pollen. Others, animals and insects. Something new I learnt was that one plant- Lagerstroemia - is so keen to attract the latter it produces two types of pollen: a sterile, yellow, feeding pollen and a fertile, blue one.

My first microscope observations of pollen were great fun and I learned a lot. It turned out my Mock Orange had an another interesting microscopic feature for me. What it was however, will need to wait for another posting.


Reference
[1] Pollen and Stimga Structure and Function, A.F. Edlund, R. D. Swanson, Preuss, The Plant Cell 16:S84-S97 (2004)