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.

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.

A pine cone tale

April 7, 2010

A major goal of Montessori botany studies is to help children learn to observe and understand plant structures. There are a lot of things going on in the plant world that take a sharp eye and careful observation to find. The life cycle of pines is one of them. It is important for the teacher/guide to show children inconspicuous plant structures such as pine cones throughout the year and explain to them what is happening.

Most people are familiar with conifer cones, although they tend to call all of them “pine cones.” Few have followed the development of a cone through the year – or two years in the case of pines – that it takes for a cone to mature. I have been photographing the development of pine cones and here’s a look at their life cycle.

Pines have two kinds of cones on the same tree, pollen cones and seed cones. The latter are formally called ovulate cones. The trees don’t usually form cones every year. In cone years, the cycle begins as the new shoots elongate in the spring. The seed cones form at the end of the new growth. They look like tiny pink-to-purple bristles.

These are young seed (ovulate) cones on the new shoot of a ponderosa pine.

The pollen cones cluster at the base of the new shoots, beneath the terminal bud. Most of the pollen cones form on the lower branches of the tree, away from the seed cones, but sometimes they form on the same shoot as the seed cones. The wind usually won’t take pollen from the base of the tree to its upper branches, so the arrangement of seed and pollen cones encourages cross-pollination. 

These two cones formed in the previous spring. The one on the right died during the winter. The left one is starting to grow.


By early July, the living seed cone has quadrupled in size. Its scales are noticeably green.

Conifers use wind pollination, which requires a lot of pollen to work, and in cone years the trees produce an amazing amount of pollen. Pollen cones tremendously outnumber seed cones. After they release their pollen, most of the spent pollen cones drop off the tree. You can sometimes find dried ones in the branches later in the summer, however.

It takes careful observation to find the budding ovulate cones, even though they can be colorful. They hide among the new needles and are most easily seen from above, the bird’s eye view we don’t usually have. It doesn’t help that the ovulate cones usually form on the higher branches. You may need to pull a branch down so that the children can see the tiny new cones. The little cones of pines don’t grow much during their first year. In the fall, they have become browner and drier looking, but are nearly the same size as they were in the spring.

In the second spring the pine seed cones rapidly enlarge. A shoot I photographed had a pair of seed cones, but one of them had died. It provides a size scale to show how much the live cone has grown. Fertilization is a slow process in pines. It takes about 15 months for the egg cells to form and the pollen tube to grow and deliver the sperm to the eggs. The scales of the seed cones are green until late fall. By that time the seeds are mature. The cone dries and the scales spread apart, releasing the winged seeds. The dried cone may remain on the tree for months or years, until a strong wind brings it down.

In the fall, the seed cone has dried. Its brown scales spread apart and the seeds are released.

In case you need help finding your local pines, look for a conifer tree with needles in bundles of two to five. Other conifers, such as firs or spruces bear their needles singly. Their cones mature in one year, but they can be even harder to see because the seed cones form in the tops of the tree.

Take a look around this spring and see if you can locate some young cones to show your children and to follow through the cone life cycle.

The winged seeds of the pines blend in with the soil and rocks very well.

Blooming trees that are hard to see

March 21, 2010

Happy equinox and happy tree watching!! Some trees in your area may have already bloomed, and others may be still to come. Some trees put on such a lovely show that no one can miss their blooms. For others, it takes a sharp eye and sometimes a hand lens to see the blooms. I’d like to show you the details of a few of them. For this post, I’ve chosen ash and elm trees.

Elms, genus Ulma, have their own family, Ulmaceae, which is part of the Rosales order in the rosid branch of flowering plants. They are very early bloomers, and as a result they may have their blooms frozen. If they are able to form fruits, these grow before the leaves. The trees have an early display of “spring green” fruits, which then turn brown and blow away as the new leaves are emerging.

The staminate and pistillate flowers of Siberian elms open while the weather is still freezing.

What’s the advantage in blooming so early and risking a late frost? These trees are wind-pollinated. The leaves would block the wind from the branches where the flowers form. Instead the trees take the risk and produce large numbers of fruits (“elm seeds”) as a trade-off.

The flowers of elms are tiny clusters of staminate and pistillate flowers that have tiny tepals, not showy petals. The stamens have dark anthers and the pistillate flower has two tiny, furry stigmas, usually light-colored. The ovaries start to enlarge soon after the trees bloom, and you can see the enlarging green disk that will become the fruits.

The samaras of elms grow quickly and mature before the leaves open. This is a Siberian elm.

The fruit is known botanically as a samara, which literally means “elm seed” in Latin. A samara is a winged fruit that is wind-dispersed. Elm fruits have a single seed surrounded by a membranous ring.

The flower clusters of the American elm are more open. Each flower has a long stalk.


The developing ovaries of the American elm are covered with short hairs.

Ash trees, genus Fraxinus, belong to the olive family, Oleaceae, which is part of the Lamiales order in the asterid branch of flowering plants. They bloom later than elms, but still quite early. Ash trees are dioecious, which means that they have their staminate and pistillate flowers on separate trees. The staminate or male trees are often planted as street trees. They have the advantage of not producing fruits, so not requiring a lot of clean-up. Their distinct disadvantage is that they produce abundant allergy-triggering pollen.

The staminate inflorescences bear two stamens per flower, like all the olive family. They start as tight, globular clusters. The more open inflorescences of pistillate flowers show the tiny green ovaries that rapidly enlarge. The fruits are another samara.

Early on, the immature stamens form tight clusters as they emerge from their buds.

Later the stamens mature as the cluster opens. The pollen flies as the anthers split open.

This pistillate ash tree has the old, dried fruits (samaras) from the previous year, along with the green inflorescence of this spring.


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.