Pollen tubes - this'll make your eyes water!

My interest in pollen was raised when I read an article in October 1999 edition of Balsam Post [1], the newsletter of the Postal Microscopical Society. In his article Chris Thomas explains the role of pollen, how to find and observe it and, most fascinating of all, how to make it germinate and grow pollen tubes. Chris himself has kindly given some more details on the next page.

For a clear diagram of what happens with pollen germination and syngamy when the pollen tube enters an ovule see this link and for details of how to collect and mount pollen grains see the booklet by John White [2].

My efforts at growing pollen tubes started with various pollens on sugar solution. Success didn't come until I tried Chris Thomas's method of spreading the pollen on an onion epidermis laid on a moist microslide. Even this hardly worked with our locally grown onions. However, a good big Spanish onion produced instant success with a number of pollens so that's the medium I use now. Leek epidermis is also successful and has the merit of being curved about only one axis, so easier to spread flat on a slide!

First you need a flower with ripe anthers covered in pollen. A warm dry day is helpful for this.

  Where to get the epidermis? Cut the top and bottom off an onion. With a sharp knife, cut a few millimetres deep from the "North Pole" to the "South Pole" of the onion. Now make a similar cut so that the strip that you can remove is about 15 mm wide in the middle. Remove this section from the rest of the onion. Take a piece of epidermis from the inside of the section - i.e. the concave surface (scratching the edge with a finger nail or knife will get it started and then you can peel it gently with fingers) - and put it with the side that was towards the outside of the onion upwards.

Breathe on the slide to moisten it just before you lay the onion epidermis on it and then gently spread the skin to try to make a flat area about a centimetre square. Don't worry if there are a few creases elsewhere. 

Stroke the chosen ripe anther(s) across this flat area and have a look under the microscope, using a low power objective, to see that you actually have some pollen grains on the onion skin. Label the slide with the name of the flower, the date and time. I use waterproof ink (overhead projector pens) direct onto the slide. Now place the slide in a sandwich box or similar in which you have put a layer of damp tissue and put the lid on to keep the air moist.

Periodically observe the slide and, if there has been any growth, sketch or photograph the shape of some of the tubes, noting the time on your picture. This will permit you to work out the rate of growth of the tubes. In some cases you will be able to see the tip of the tube moving if you observe it under high power. I have found a 40x objective good for this.

Of the pollens tried, the fraction of pollen grains that germinated varied considerably. I started this late in the year, so the variety of plants with pollen was limited. Here is a rough guide to the activity that I found but "your mileage may vary" (mine certainly does!):

 poor ivy field pansy
 fair Penstemmon Nasturtium
good Japanese poppy dead-nettle

.

pollen tubes

Penstemmon (6 hours)

On the next page Chris has described his scoring system and the details to record to make your observations of interest as part of a collaborative project.

References:

1) Chris Thomas; "Balsam Post", October 1999
2) John White; "Pollen, its Collection and Preparation for the Microscope", 2nd edition, published by Northern Biological Supplies Ltd. (2.00)

Comments to the author John Garrett are welcomed.
Email:antenna@btinternet.com.

 Visit John's microscopy pages on his own web site.

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Published in the December 1999 edition of Micscape Magazine.

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