Monday, March 31, 2014

The Shepherd's Purse - A Tiny Harbinger of Spring

Winter in my part of Texas is usually a non-event.  We might get a couple of days below freezing here and there, and sometimes some ice or snow, but after a day or two we warm right back up.

Not this year.  It has been much colder than we are used to and seems to have lasted forever, so I was delighted to find this little, often-overlooked wildflower growing in my backyard.  It is one if the first flowers I come across each spring.  Capsella bursa-pastoris - the Shephard's Purse - so called because its triangular seed pods supposedly resemble a type of purse shepherds used to carry.  This is one time that Google failed me because I couldn't find a single image of a shepherd with a purse.

Shepherd's Purse has very tiny flowers, so it can easily be overlooked.  What catches your attention are the triangular-shaped seed pods.  It is taller than most of the weeds in my backyard right now so it is easy to find.  (Sadly, none of my grass has decided to begin growing yet so the weeds are having a field day.)

The individual flowers are so small that one of them looks like it could be pinned to President Lincoln's lapel on a penny.

By the way, I cut several stems of Shepherd's Purse, stuck them in water and took them to the lab with me.  They look great so if you like very tiny cut flowers, you are in luck.

This image shows the flowers at the top of the stem of Shepherd's Purse.  Plants grow from the tips so the higher up you go on the plant, the younger the structures.  Notice the unopened flower buds in the center.  The opened flowers show several stages of seed development leading to the seed pods on the outside of the photo.

Here is a close-up of the flower buds at the extreme tip of the stem.  Interestingly, the petals breaking through the sepals are purple.  By the time they open, as you will see in the images below, the flower petals are white.  
The flower at the top of this image is younger than the one at the bottom.  On the bottom flower you can already see the swelling. flattened seed pod.

This is a close up of a single flower.  The stigma, part of the female reproductive structure, is in the middle.  On the upper margin of the stigma you can see individual pollen grains.  The four structures surrounding the stigma are the stamen which produce the pollen grains.

A word about flower structure.  The role of a flower is to have sex.  Of course, plants have a hard time going to single bars and getting involved in on-line dating since they are stuck in the ground, so they have to depend upon other organisms or physical forces to spread their pollen around.  To insure that their flowers will get pollinated, flowers tend to make a lot more pollen than they might need to get the job done.

Complete flowers have all of the commonly recognized flower parts - petals, sepals, stamen (the male parts), and pistils or carpels (the female parts).  Not all flowers have all of these parts, however, and are called incomplete flowers.  Lots of trees and grasses have flowers without petals. Petals, of course, attract pollinators like insects.  Insects are very, very good at going from flower to flower taking pollen with them.  The insect has no idea that it is spreading pollen - it is just looking for nectar (sugars) and pollen (protein) to eat.  Plants provide nectar and pollen to attract insects and other pollinators, but not because they love insects.  They do it to have their pollen reliably transferred.  This is not to suggest that plants are planning or thinking about attracting pollinators.  It is a simple case of natural selection - plants that can most effectively attract pollinators are the most successful reproducers, increasing their numbers in the gene pool.

If petals attract pollinators and plants like grasses and trees make flowers  that don't have petals, how do they insure pollination?  They make TONS of pollen which is spread by wind.  If, like me, you have seasonal allergies you are very much aware of grass and tree pollen this time of year.

In this image you can see a close-up of the developing fruit or seed pod.  The yellow circle in the center is the stigma and it is covered with pollen.

Once a flower is pollinated, it no longer has use for the stamen, petals, and sepals, so it cuts off their food supply and they fall off.  In the image above you can see the triangular seed pod and a single petal and stamen.

What I have been calling a seed pod is in reality a fruit.  By definition, a fruit is a structure that contains the seeds.  The fruit is the developed ovary of the plant.

This seed pod has been opened so you can see the seeds inside

A close-up of a single developing seed.  You can clearly see the cells that make up the seed.

Another dissected seed pod revealing the seeds inside.

A single flower.  [44x]

The beautiful thing about scanning electron microscopy is the remarkable depth of field it allows. Instead of having to focus on only a single structure in an image, a much larger area of the specimen can be in focus.  (Of course, we do sacrifice color, but color is overrated at times.)

In this image you can see the pistil in the middle of the image with its stigma on top and the ovary below.  The stamen with their pollen encrusted anthers and supporting filaments surround the pistil.  You can also see the bumpy cells that make up the petals in the background. 
Stigma to the left, stamen in the center with pollen, and petal to the right.
Here is a stigma that is still attached to the seed pod.  The little mouth-like structures on the stigma and on the seed pod are stomata for gas exchange.

Seeds inside of the seed pod.

Single seed inside of a dissected seed pod.  The little boxes are cells.

Here are a couple of pollen grains from the Shepherd's Purse.  
What is the difference between a weed and a wildflower?  It is in the eye of the beholder.  Plants that you don't want growing where you don't want them are generally called weeds.  But before you reach down and yank that weed out of your yard, take a second an really look at it.  You may have discovered a wildflower.

All of the images in this blog are covered by a Creative Commons License.  You may pretty much use and modify them anyway you like as long as you credit Eastfield College, Mesquite, TX and don't sell them.

Murry Gans
Microscopy Lab Coordinator
Eastfield College