My Favorite Harbinger of Fall

The Late Blooming Aster

(Symphotricum patens - I think)

I love warm weather, but Texas summers are often pretty brutal. I am always delighted when the break in the weather occurs in the fall and our temperatures drop from highs in the 100's to highs in the 80's - often over night. 

When this little aster shows up in my yard and I know that fall is near.  Now that the temperatures have finally changed for the better, these asters are everywhere. 

With flower heads only about 5/8ths of an inch across, these little guys are often over looked, but if you take the time to really look at them, you are in for a treat.  The petals are white to lavender.

 Dissecting Scope Image

This flower head is in a small jar filled with water to keep it turgid. 

Dissecting Scope Image

This is not a simple flower, but a composite flower - made up of several individual flowers.  The flowers in the middle are called disk flowers and have no petals.  The flowers on the outside are called ray flowers and have only a single petal.  If you count the petals you will know how many ray flowers there are.

Dissecting Scope Image

A close up of the disk of this composite flower.  Notice on the image above that 8 of the disk flowers have yet to open.  The little dots in the image are pollen grains.

Disk Flowers - SEM [20x] 

Even though this is only a 20x magnification, the SEM provides excellent depth of field which allows almost the entire flower disk to be in focus.  The flowers in the middle are not open. The flowers farther out have opened and the anthers are protruding.  These will also open up and a stigma will emerge from the center.



 Outer covering of anthers (pollen obvious) - SEM  [120x]

 Anthers - SEM [25x]

On the bottom left of the image you can see a stigma from a ray flower.

 Dissecting Scope Image

Pollen on anther

Composite flowers can be difficult to figure out, so I took the flower head apart to look at the disk and ray flowers.

 Dissecting Scope Image

 Dissecting Scope Image

This is a ray flower with its single petal.  Seed dispersal in this aster is just like a dandelion - once the seed forms in the ovule it will be dispersed by wind.  The pappi in the image will become the parachute that carries the seed.

Dissecting Scope Image

A close up of the stigma of a ray flower.  I am very pleased with this image - you can clearly see the pollen grains on the stigma

But what about those stamen?  They really don't look like the stamen on simple flowers.  In the images below I have used a needle to dissect a disk flower to find the stamen.

 Dissecting Scope Image

Even on this low magnification image you can see that the anthers on the inside of the flower.

 Dissecting Scope Image

An excellent image showing pollen lined up on an anther.

 Dissecting Scope Image

With the disk flower fully open, the stamen are finally revealed.  The anthers are covered in pollen and supported by filaments.

 Tip of unopened stamen - SEM [55x]

The pollen is essentially oozing out of the stamen

Close up of stamen with pollen - SEM [185x]

 Anther with lots of pollen - SEM [62x]

 Filaments that support anthers, Pappi - SEM [71x]

The final two images are of pollen.

 Pollen - SEM [750x]

 Pollen - SEM [1,000x]

Pollen about 25 microns in length - small!

It was a lot of fun to study the composition of this aster using the tools available in the Scanning Electron Microscope Lab here at Eastfield College.  It is amazing what you discover when you are able to see the world in such tiny detail.

The SEM Lab at Eastfield College is open to students and faculty everywhere.  We have two scanning electron microscopes.  Our smaller scope is so simple to use that I can have you capturing images within 15 minutes.  The larger scope provides more flexibility and control, and I am here to help you use it. 

If you are interested in seeing or using our scopes, please contact the lab.

Murry Gans

Scanning Electron Microscope Lab Coordinator

Eastfield College

Mesquite, TX

mgans@dcccd.edu