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,

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.



Big Picture Science Website Technical Difficulties

May 22, 2011

The website is back online and seems to be functionally normally.

The leaves that didn’t fall in fall

March 14, 2011

Most deciduous trees live up their name and drop their leaves in autumn, but you may have found a curious exception. Some trees or shrubs have leaves that turn brown, but do not fall off. Members of the oak family often exhibit this phenomenon. The leaves that didn’t fall in fall are called marcescent leaves. The term comes from a Latin root that means “withering.”

This Gambel's oak hadn't dropped its leaves in November.

I observed marcescent leaves on the Gambel’s oaks (Quercus gambelii) this past fall. Nearby members of the same species had shed most of their leaves. What survival advantage does this give the plant? Whenever a genetic trait is commonly present, it probably confers some survival advantage. For the oak shrubs along the foothills of the Colorado Rockies, there is a strong threat to survival in the form of mule deer. Cute little bambi grows up to be death to plants on four hooves.

The marcescent leaves on the oaks could help protect them from the onslaught of the large herbivores. In winter, deer are browsers. They eat twigs, buds, and I have even seen them eat the foliage of conifer branches that have blown off the trees or been broken off by heavy snow. The marcescent leaves are thought to deter browsing, probably because they are not tasty and have little food value. The growing points, the meristems, hide behind these dry leaves, protected against drying by their bud scales. In the spring, the old leaves will fall as the new buds start to grow.

This oak, near the first one, had lost most of its leaves by the same time.

However, a study in Denmark showed that the deer there avoided beech and hornbeam branches with marcescent leaves, but not oaks. The dead leaves on the beech and hornbeam branches had a high lignin content. Lignin is a very complex substance that is hard to digest. Perhaps Colorado oaks have less palatable leaves, or the leaves could give a different benefit.

He's cute, but he is murder on plants. Twigs on trees and shrubs are among his favorite foods.

Another theory is that the leaves help hold more wind-blown snow, and therefore bring more moisture to the shrub. In this semi-arid climate, every bit of moisture is important. While this may be a part of why Colorado scrub oaks keep their leaves, it doesn’t really fit the picture for taller oak trees.

Some oak species, notably pin oaks (Quercus palustris) keep their petioles alive through the winter and shed their leaves in spring. In other cases, the petiole appears to be brown and dead. Another factor could be sudden fall freezes that kill the petiole before it has completed the abscission process. There are many steps to abscission, the “cutting away” of the leaves. They involve enzymes, and these protein molecules, as well as cell structures, may be damaged by sudden cold.

It is likely that many factors play into the puzzle of the leaves that didn’t fall in fall. They will, however, fall in spring.

Global choices – desirable features for classroom globes

September 29, 2010

Maybe it was the fall Sun angle coming into my greenhouse that got me thinking about globes. A while back I decided to replace my old globe. After all, it still has the USSR on it. It is a Cram’s Imperial World Globe, and it has two features that I want in a globe, the analemma and the ecliptic.

The best features for a globe depend on what you want to show with it. It would be hard to find a globe that has everything. A classroom probably needs at least two globes, one for the Earth in space and the physical features of the Earth, and another that shows the countries, longitude, latitude, and time zones clearly. Of course, beginners need a simple globe, like the ones found in many Montessori early childhood classrooms that show the oceans and continents without labels. Elementary and older children will likely enjoy the complex globes with more in-depth geography information or with the ecliptic and analemma.

The analemma and the ecliptic are important for studies of the Earth in space and the relationship between the Earth and Sun. Before I get into what these two markings are, I found a couple of other interesting globes in my search.

Replogle Globes has a “blank” political globe that can be labeled with a dry erase marker. The act of labeling it would certainly help children consolidate their geographic information. This globe is called the Geographer. Replogle also has a relief globe, called the Atlantis, that has ocean features indented, along with raised elevations.

Back to markings for study of Earth in space, let’s start with the simpler one, the ecliptic. The tilt of the Earth’s axis means that the Sun’s rays strike the Earth at different places as the Earth rotates. If there was no tilt, the Sun would always shine directly at the equator. As it is, that only happens twice a year, at the equinox times.

Now, in late September, the tilt of the axis has begun pointing the Southern Hemisphere at the Sun, and the point at which the rays shine directly on the Earth is moving south, toward the Tropic of Capricorn. At December solstice, the Sun’s rays will shine directly on this tropic. In the Southern Hemisphere, the sun will be more overhead. In the Northern Hemisphere, the Sun will appear to move south and hang low in the sky. As the year progresses from December solstice to June solstice, the Sun will appear to move to the north and will be more directly overhead in the Northern Hemisphere in June, as its rays strike directly on the Tropic of Cancer.  

The ecliptic marking on a globe shows where the Sun’s rays are hitting directly on the Earth for any day of the year. On September 29th, this point is just south of the equator, as we have just passed September equinox. Traditionally the ecliptic is placed on the globe so that it crosses the equator at the Prime Meridian (zero degrees longitude) and the International Date Line (180 degrees longitude). The ecliptic marks the apparent path of the Sun and its plane is the plane in which the Earth orbits. When the Moon is near this imaginary line, eclipses happen, hence the name “ecliptic.”

This is the analemma on my old Cram Imperial globe. The diagonal line near the bottom is part of the ecliptic. It shows where the Sun shines directly overhead in the month of November.

If the orbit of the Earth was circular instead of elliptical, then the ecliptic marking would be all we need on a globe to show the relationship of the Earth to the Sun. As it is, Earth’s elliptical orbit makes the analemma necessary. The analemma is the odd figure 8-like object that is usually printed in the Pacific Ocean. That’s where there is enough “blank” space to put it. Its pattern results from two things, the elliptical orbit of the Earth plus the tilt of the Earth’s axis. If you had a nail or rod in a board, and if you marked where the shadow of the nail head lay on each day at noon, the dots would make a shape like the analemma. The name “analemma” comes from a Greek term that essentially means “something that sticks up” and refers to a sundial.

Kepler’s laws of planetary motion say that the planets have elliptical orbits, and the Sun is at one focus of the ellipse. Furthermore, if you drew a line from the planet to the Sun, the line would mark equal areas over equal time periods. What this means is that the Earth moves faster when it is in the end of the ellipse nearer the Sun and slower when it is farther away. You can see this in an almanac – there are five more days from the March to the September equinox than from the September to March one.

As the Earth’s orbit speeds and slows, our clocks run at a constant rate, which makes them get out of sync with the Sun. The Sun sometimes reaches its zenith before the clock says it is noon, and sometimes the Sun doesn’t get directly overhead until afternoon. The clock and the Sun agree only 4 times a year, at the solstices and around mid-April and early September. You can read the difference between the clock and the Sun on the analemma.

Knowledge of the relationship of the Earth to the Sun gives us a greater appreciation of our wonderfully complex planet. Globes with the ecliptic and analemma help children learn more about this.

It looks like I will be keeping my old globe for a long time. In my quest for a new globe, I spoke with a cartographer with Cram globe makers (now a subsidiary of Herff Jones Education) who told me that the ecliptic was removed from globes when they were converted to digital images, over 10 years ago. He also said that many Cram globes still have the analemma. If you are looking for a new globe, I suggest that you see it before you buy or else be able to return it if it doesn’t have the features you want.

A Rare Plant Treat

July 6, 2010

I was in Seattle recently and enjoyed a visit to the greenhouses at the University of Washington. I was delighted to find a collection of Welwitschia plants there, one of which was forming cones. This bizarre plant of the Namibian Desert belongs to the gnetophyte lineage of seed plants. It is one of three wildly different branches of this clade.

Welwitschia plants in the UW greenhouse have tall pots to accommodate their long tap roots.

The gnetophytes are one of five extant lineages of seed plants. The others are the cycads, the ginkgo, conifers, and the angiosperms, aka flowering plants. The angiosperms greatly outnumber the others, but that has become the case in the Cenozoic Era. In the Mesozoic Era, there were many other types of gnetophytes, but presently the three lineages are the genus Ephedra, genus Gnetum, and genus Welwitschia.

Although there are about 60 species of Ephedra and about 30 of Gnetum, there is only one species of Welwitschia, W. mirabilis. Ephedra species are native to arid environments in North and South America, Africa, and Eurasia. They have greatly reduced leaves and look like a bush of tough, narrow stems. Gnetum is the odd one on environment. It grows in the tropics of Indonesia, the Philippines, and parts of Africa. Most of its members are vines, although a few are trees or shrubs. Their leaves look very much like those of angiosperms.

Ephedra, a relative of Welwitschia, growing in Utah. The leaves of this bush are reduced to tiny scales.

Gnetophytes were once thought to be closely related to flowering plants, but the DNA told a different story. They are now considered to be more closely related to the conifers. Characteristics that all gnetophytes share include opposite leaves and having staminate and ovulate reproductive structures on separate plants, i.e. they are dioecious.

Welwitschia grows only two leaves, not counting its two seed leaves, in its whole life time. The leaves arise from a woody stem that has a sort of upside down cone shape. The plants are estimated to live about 1000 years, judging from their growth rates and the length of the leaves on wild plants. The leaves grow from their bases and often split so that it isn’t easy to see that there are only two.

The stem of Welwitschia is woody and has a roughly inverse cone-shape.

The two leaves of this Welwitschia plant are split into several strips. The top of its woody stem is visible.

One of the plants in the UW greenhouse had formed cones. It is a male plant, and there are stamens showing near the bottom bracts. The branch that bears the cones grows from the top of the woody stem, near the base of the leaves. I had read about their cones, but never seen them “in person” before, so finding them was a real treat. 

These are staminate cones of Welwitschia. Their stalk grows from the top of the woody stem.

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.

Botany marches on – Part 2, the eudicots

April 29, 2010

If you did not see part 1 of this post, please take a look at its first paragraph. This post is more on the fall 2009 publication by the Angiosperm Phylogeny Group, the report called APG III. What I’m giving here is how APG III differs from my book, A Tour of the Flowering Plants. This post is primarily for Montessori botany studies and general learners that own my book. (And if you are interested in acquiring A Tour of the Flowering Plants, see

The changes in the eudicots include the assignment of Geraniales and Myrtales to the malvids (eurosids II) branch of rosids. These two orders were previously found to be within the rosids, but their exact location was not clear. Now they are sister lineages that branch near the base of the malvids.

Caryophyllales is listed before the asterids, while the Saxifragales is listed before the rosids. This is the way I depicted these lineages on the informal tree diagram in my book, A Tour of the Flowering Plants. The former Caryophyllales family, Portulacaceae, has been divided, as it held several not-closely-related genera. The genera Claytonia, Montia, and Lewisia have been moved to the new family, Montiaceae.

Lastly the Dipsacales order (in the campanulids or euasterids II branch of the asterids) has had several former families combined into an expanded Caprifoliaceae. The twin flower family – Linnaeaceae, the valerian family – Valerianaceae, the teasel family – Dipsacaceae, and the bush honeysuckle family – Diervillaceae have all become part of Caprifoliaceae. This leaves Dipsacales with only two families, Adoxaceae and Caprifoliaceae.

There have been a number of adjustments in smaller families that are not common in temperate North America. I view these as tweaking the twigs, not establishing the branches. All-in-all, the bulk of the APG scheme has been reaffirmed in the last several years.

What difference do all these changes make? Unless you are an editor of scientific papers, it isn’t absolutely necessary to learn the minor adjustments to the APG scheme. The main thing that students need is the phylogenetic view of life and of the angiosperms. If they see these plants as descendents of a common ancestor and know the major lineages, that’s good. Some may want to delve deeper and that should be encouraged. Mabberley’s Plant Book: A portable dictionary of plants, their classification, and uses (third edition, 2008) by D. J. Mabberley is a valuable reference for further study and to help sort out the changes.

How does the Angiosperm Phylogeny Group classification affect horticulture and native plant identification? I can more easily answer when it will affect these two – that will be over a period of time. Field manuals aren’t going to be re-written overnight and horticultural labels frequently lag behind academic classification. Nonetheless, I think that phylogenetic classification will eventually take over. It certainly makes more sense to focus on the APG scheme and look to the future if you want to address plant classification.