Blog written by Caleb Hinojos.
(Posted by David Gladden)
Hey everyone and welcome back to the blog. I hope that the holidays are treating you well. Over the past couple of weeks, I have visited the garden a few times and have been impressed with the work that has been done to clean the place up. Thank you to the volunteers that do the physical labor. While looking around the garden for interesting shots I looked at our signage. Unfortunately, the one pictured below had fallen over. As I staked it back into the ground I was reminded of the mission of the garden. To make a habitable area for pollinators. However, we talk so much about the pollinators that we seem to neglect the pollen which they help to spread around. I then went home and checked our past blogs to see if we had one that actually covered what pollen is. Turns out that we don’t. I think we should change that.
 | This is a photo of some pollen from the rock rose pavonia (Pavonia lasiopetala). From this perspective it looks like little more than balls of Cheeto dust or miniature caviar. However, these small particles are essential so much more than what you might think. Let’s begin by taking a look at what actually lies within the small granules. |
This diagram gives a basic overview of the main components of a pollen grain. We can see that there are 6. The exine is the outer covering of the granule. It helps to protect the cell from desiccation, temperature fluctuations, and a variety of other external pressures which can degrade the cargo. The next layer is the intine. This is helps give shape and structural support to the tube cell, and also develops the pollen tube for fertilization. The cytoplasm contains the organelles of the tube cell while the tube cell nucleus has the DNA. The generative cell is quite interesting. As you can see it lives within the larger tube cell, but its purpose is to divide into two sperm and travel through the pollen tube into the ovary.
In the next photo we have a great example of a flower that makes pollen and relies on insects to distribute it. This is termed entomophilous. Let’s dissect the scene a bit. First notice the speckled arms coming out of the central stalk. These are called filaments while the structure at the end of them is called the anther. This is where the pollen is made. These two structures together make up the male stamen. At the top of the stalk is a tri-spoke structure. The arms are called the style while the bulb at the end is called the stigma. This is where the pollen will be deposited and begin making the pollen tube so it can reach the ovary. Take notice of where the honey and carpenter bees are located. If you zoom in on the photo you can see that the carpenter bee has its entire back covered with pollen. This is good news for the passionflower. As the bee crawls all over the central stalk there’s a good chance that it will brush up against the stigma and voila. The pollen has been delivered.
Although it may seem problematic for the flower to have to rely on another species for fertilization it actually makes a lot of sense regarding their structures. Unlike wind pollinated plants, anemophilous, entomophilous plants usually have their reproductive organs more protected. This means that a broadcasting of gametes into the air reliant on wind for transport have a much lower possibility of actually reaching a stamen. However, by offering a reward from these structures and utilizing display colors to attract insects greatly increases their chances of fertilization.
 | In our garden one of these insects which has caught my attention this year are those in the genus Megachile. These are the leafcutter, resin, and mortar bees. They commonly live in short burrows underground but can also utilize hollow stems, and woody plants as a nest. Unlike many other species of bees these do not have “baskets” on their legs but will carry pollen on their underside. Although it may be difficult to tell if you look at the photos you can see how fuzzy they are on the underside. Once they have collected a certain amount of pollen, they will make their way back to their nest and enter a chamber. She will then make what is referred to as a bee loaf by adding her saliva to the pollen and kneading it. Next, she will proceed to lay a single egg on the loaf which will be all the food the larva needs to mature. |
Once the egg is laid, she will go and find a leaf, cut a circular section of it from the edge, and use it as a seal for that specific chamber. She will have multiple chambers per nest and the males are laid at the upper chambers. When the spring comes the males will emerge, wait for the females to reproduce, and then die quickly after while the females start the process all over again. The next photo shows a simple diagram of the chambers and evidence to look for.
Up until this point I have covered the basic properties of pollen by discussing it as a food source for certain invertebrates, and as a source of fertilization for the plant. However, one of the more interesting aspects of pollen that you may not immediately think about is how important pollen can be with regard to environmental forensics and environmental planning. One of the best examples that I know of is that in Joshua Tree National Park. In the distant past the range of the Western Joshua Tree, Yucca brevifolia, extended much further north in California than it currently does. Evidence of this was found in a pack rat midden where pollen from nearly 10,000 years ago was preserved. This was possible due to the exine of the pollen, the urine of the rat, and the fact that the midden itself was in a sheltered area. After it was found the researchers were presented with a task. Should we as the dominant species on this planet put resources into expanding the tree back into its historical range? Ultimately, due to climate change it was determined that the resources needed to expand the tree back to its historical range would not be worth the cost. What makes this so interesting to me is the fact that they were able to reach this question due to pollen granules. Each plant makes pollen that has a specific exine layer. When these are scanned using an electron microscope, we receive an image like the one below. Although this is a niche case, it really demonstrates how humans are able to unravel stories and determine futures based on nearly microscopic multicellular units.
Up to this point I have covered why pollen is important to a plant, why it is important to certain invertebrates, as well as how important these granules can be to Homo sapiens in niche fields. I would be remiss however if I didn’t mention what is probably near the top of the list things that pop in your head when you think of pollen. Allergies. Although there are some of us who are fortunate to not have a reaction to the little invaders, the majority of people seem to be affected in some way. Why does this happen though? Well, as pollen enters your body it may be recognized as an intruder. If this is the case your immune system will utilize antibodies which are carried around by mast cells. When they come into contact with the invader they will bind to it, and subsequently release histamines. For those who don’t know, histamine acts to help remove unwanted molecules such as pollen by forcing us to have some type of reaction. This can be runny noses, itchy skin, and even induce vomiting. Any way for our body to expel the unwanted guest is acceptable to histamine. This is why many of our allergy medications are antihistamines.
If you have made it this far into my writing, I appreciate that. I hope that you have learned something new, and have a shifted perspective on what role pollen plays in a pollinator garden as well as in our daily lives. Also, if this piece caught your attention, then please consider reaching out and putting in work for the garden. We always need/want volunteers who are excited about helping out. I hope that the year of 2024 went well for you, and if it didn’t then I hope you are remaining optimistic for the fresh start that 2025 will provide. Cheers until next year.
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