It takes two flowers to make a squash

June 8, 2012

A summer squash plant with both pistillate and staminate flowers. This is a yellow squash, as you can see from the ovaries of the pistillate flowers.

Squashes, melons, pumpkins, cucumbers, and gourds all belong to the squash family, Cucurbitaceae. There is a common pattern of the flowers that children enjoy finding, and that often escapes adults. I’ve pointed it out to many long-time gardeners, who hadn’t noticed it before.

This is a young pistillate flower of a patty pan squash. The ovary is green now, but it will turn white as this squash matures.

Most members of this family are monoecious, which means each plant has flowers with only stamens along with other flowers which are only pistillate. These are commonly called male and female flowers. They are easy to tell apart if you look beneath the corolla. The ovary is inferior (located beneath the other flower parts) or, to put it another way, the other flower parts are epigenous (they sit on top the ovary).

 If you want to find the pistillate (aka female) flowers, just look for a tiny ovary – a baby squash, cucumber, etc. – on the stem under the corolla. You can find the little ovaries well before the flowers open, so it is easy to see which flowers will produce the desired fruit. The mature ovary of a flowering plant is a fruit, so to a botanist, squashes, cucumbers, and melons are all fruits.

The staminate flowers of the squash family have a plain stem beneath the corolla.

The staminate flowers have a plain stem beneath their corolla. Inside the filaments and anthers of their stamens are joined together into a knob-like structure that resembles a pistil. Inside the pistillate flower’s corolla, you can see the three-carpellate structure of the pistil. There are three stigma lobes that have two branches each. The fruit shows the three carpels as well. Look at a cross section of a squash or fruit of other family members to see this.

This is a staminate squash flower that has been split along the corolla and opened to show the fused anthers and filaments of the stamens.

The stigmas of the pistillate flower have several lobes. This flower had bloomed, and its corolla was removed to show the stigmas.

The next question that comes up is often “Why doesn’t my squash plant produce more squashes?” Sometimes the temperature is to blame. It affects the sex of squash flowers in ways that aren’t always obvious. When I lived in the mountains of Colorado, I found that although zucchini plants would grow, they seldom produced fruits. The plants would form female flowers, but seldom have staminate ones, so pollination didn’t happen. The cold soil temperatures were to blame. With other members of this family, cold temperatures cause only staminate flowers to form. You can read more about this on the website of the Ontario Ministry of Agriculture, http://www.omafra.gov.on.ca/english/crops/facts/00-031.htm

Conversely, temperatures above 95 degrees F can also cause flowers to drop instead of developing. There could be a number of factors operating in this case, including moisture stress.

Although squashes and begonias don’t commonly come to mind as relatives, if you look at the flowers of a begonia, you can see the same pattern – monoecious plants with inferior ovaries. The begonia family and the squash family both belong to the squash order, Cucurbitales.

In this view of begonia flowers, the staminate flower is on the top. It has a plain stem. The pistillate flower below has a green, winged ovary.

A front view of begonia flowers. The pistillate flower is on the left. The staminate flower has four tepals; the pistillate has five.

 


What color is your Doug fir?

June 13, 2010

When you read the title of this post, did you say to yourself “Well, it’s green, of course, that is if it is alive.” Doug firs (Douglas fir, Pseudotsuga menziesii) have some parts that aren’t green, however, and those parts are a visual confirmation of an important property of life – genetic variation in a population.

If you look at the branch tips of a Douglas fir early in the spring, you may notice its cones. This is provided that it is making cones that year. These trees don’t make cones every year. Each year some will form a few cones, but in a cone year, almost all the trees in an area grow cones at the same time. I’m not sure how they manage this trick, but it is a good one for a wind-pollinated species.

The bright pink structures are the ovulate cones of this Doug fir. The pollen cones are the smaller ones below.

Like other conifers, Doug firs have pollen cones and ovulate (aka seed) cones. Both pollen and seed cones grow on the same tree – the species is monoecious. I’ve noticed that the Doug firs where I live have different colors of cones. Some of my trees have deep rose ovulate cones, while others have lemon yellow ones. The pollen cones on the tree match the color of the ovulate cones, but they aren’t quite as intensely colored.

This Doug fir has yellow cones. The ovulate cone is on the upper right.

 

As the ovulate cones develop, their characteristic three-pointed bracts protrude from between the cone scales. Most Doug firs around here have straight bracts, but some have curly bracts. The straight or curly feature remains after the cones have dried and fallen from the tree.

Why should Doug firs have different colors of cones and different types of bracts? I certainly can’t tell you the exact advantage, but I do know that variations in every population are important. They are that species’ library of solutions to life’s problems. Life tinkers – tries this and that. Some traits may work better for some situations, while others may be an advantage in different conditions.

Developing Doug fir cone with curly bracts.

Most Doug firs have cones with straight bracts, like this one. The photos of this one and the curly bract one were taken on the same day.

No organism can predict the future, so the best survival strategy is to have lots of genetic diversity. This is the reason for cross pollination and sexual reproduction. The species that shuffles its genes and deals them out in all combinations has the best change of continuing on, whatever the environmental conditions.

Certainly there are some species that don’t seem to change, at least on the surface, but they just change more slowly. I prefer the term “slow-evolving species” to “living fossil” because the latter gives the false impression that the species doesn’t have variations and doesn’t change. Today’s ginkgoes are not the same as the Triassic ones, even if the leaves look the same.

When you are out in the field with your children, point out the variations that you can see. There may be an albino among a stand of blue flowers or some that have different shades of color. These are outward manifestations of genetic diversity in the population. Many more traits that we can’t see have variations in a natural population, and that’s a good thing for long term survival.  Hurray for being different – diversity is an important characteristic of life.

This rose-colored sugarbowl or leather flower (Clematis hirsutissima) is very unusual.

This purple is the usual color for sugarbowls. It was growing near the pink one.


Pollen cones

April 13, 2010

I was so concerned with getting the photos of the seed cones (ovulate cones) into my last post that I forgot to show the pollen cones of the ponderosa pine. Here are a couple of views.

The young pollen cones of the ponderosa pine.

 

One pink seed cone and many pollen cones ready to release their pollen.


What makes influenza so changable?

May 5, 2009
An influenza virion (single particle of virus)
An influenza virion (single particle of virus)

With all the publicity over the new swine flu strain, people who teach kids biologymay be getting questions from their students that are hard to answer. I have found the information in children’s literature to be limited. It doesn’t do a good job of explaining the influenza virus and its ability to change so rapidly.

First, let’s look at what is in an influenza virion – a single particle of the virus. In the center of the virion, there is a coil of RNA complexed with protein. This coil is wrapped in a viral envelope, which is a membrane the virus modifies and takes from its former host cell. There are viral proteins on the outside of the membrane. They look like little knobs sticking out from the virion surface in the photomicrograph.

This illustration, which is a public domain image from the Centers for Disease Control, was made by spreading the virus on a transparent film and adding a stain that blocks electrons. The stain filters down into the low spots and makes them look dark in the photomicrograph, which was made with an electron microscope. Virions of almost all viruses are too small to see with a light microscope. Influenza virions are about 1/20 to 1/10 of a micrometer in diameter. Their host cells have diameters that are thousands of times larger.

The RNA carries the instructions for how to take over a host cell and turn it into a virus factory. Viruses can have RNA instead of the DNA that cells use to hold information. That’s not what makes influenza unusual. Most viruses have their RNA or DNA (they have one or the other, not both, like cells have) in a linear strand or a closed circle. Influenza has its RNA in eight separate pieces.

When influenza replicates, its RNA-protein complex enters a host cell and uses the cell’s resources and machinery to make many copies of the molecules that make up its virion. As the cell fills with viral RNA and proteins, the eight pieces of RNA that make a complete set of viral genes form a complex with protein. Then this nucleic acid-protein complex moves to the cell membrane, to areas where viral proteins have been inserted into the membrane. It buds out of the cell and gets its envelope in the process.

Now if only one virus has infected the cell, then the virions will have the RNA from that virus alone. It is possible, however, for two different virions to infect the same cell. When that happens, the resulting virions can hold an assortment of RNA segments from either virus. The RNA segments are packaged randomly, so they can have many new combinations of genes. Instantly new strains of influenza arise that may have different proteins on the outside. If a person has not been vaccinated against those proteins or has not been infected by a strain of influenza that has those proteins, then the new virion may be able to spread very well in its host. That’s good for the virus, but bad for the person.

Another complication with influenza is that there are strains that infect birds and mammals such as pigs, as well as people. This new influenza virus was called swine flu because some of its genes appear to have jumped from pig influenza virus to a strain that infects people. This may have happened if a sick person and sick pigs were in close company. Note that pork meat cannot carry influenza virus, and eating pork certainly can’t infect people with influenza.

Now back to the little knobs on the outside of the influenza virus – they are called hemagglutinin and neuraminidase. When a person has antibodies to the neuraminidase, they keep the virion from entering a host cell. This means that the antibodies protect the person from the disease. Strains of influenza are named for the type of their hemagglutinin and neuraminidase. The new swine flu strain is called H1N1, because its hemagglutinin and neuraminidase both type 1. Human influenza viruses usually have H1, H2, or H3, and N1 or N2. There are many other types of these outer proteins in the virions that infect other species (and occasionally jump to humans). The H1N1 designation doesn’t tell everything about the virus. Within this type, there are mild and severe virus strains. After all, there are several more genes in the virions.

The big question is “What can we do to protect ourselves from the flu?” One thing that is seldom mentioned is to get more sun exposure or take a vitamin D supplement. There is increasing evidence that high vitamin D levels are important for good immune function and that influenza infections increase as people’s vitamin D levels drop in winter. With summer on its way, it will be easier to get some sun – just don’t get sunburned. Enjoying the outdoors and nature while getting sun exposure ought to be a fun, easy, healthy thing to do for yourself.