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The leaves that didn’t fall in fall

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

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 29^th, 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

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?

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

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 www.bigpicturescience.biz.)

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.

Botany marches on – Part 1, basal angiosperms and monocots

Warning! I’m going to get into some detailed botany here, so if you are not deeply into the flowering plants and their classification and you do not own my book, A Tour of the Flowering Plants, you may not wish to wade through all this. It is basically for advanced elementary and secondary Montessori botany studies and for anyone else that owns the book. A Tour of the Flowering Plants is still quite useful to show the appearance of angiosperm families, whether or not you are concerned about the latest classification.

Last fall, the Angiosperm Phylogeny Group published a third report concerning the classification of the orders and families of flowering plants. The report, published in the Botanical Journal of the Linnean Society, is called APG III. There is a summary of APG III on Wikipedia. http://en.wikipedia.org/wiki/APG_III_system My book, A Tour of the Flowering Plants, is based on the 2003 report from this group, which is called APG II. The book has some further advances that were published on the Angiosperm Phylogeny Website of Peter Stevens. http://www.mobot.org/MOBOT/Research/APweb/welcome.html

I’ll go over the APG III changes, beginning with this post. If you own my book, you can decide if you want to add notes to it. You may contact me via my website (www.bigpicturescience.biz) if you would like a list of the pages and changes for bringing A Tour of the Flowering Plants in line with APG III.

Will the changes keep coming over the years as we acquire more and more DNA data? I think that future changes will be modest and will not affect the general structure of the angiosperm tree. There aren’t that many unplaced groups left. There will likely be little surprises, like one I give below for Nymphaeales.   

For now, let’s start with the first branches of the angiosperms, grouped as the basal angiosperms in A Tour of the Flowering Plants. The Nymphaeales got another family, Hydatellaceae, which was formerly placed in the grasses. This shows how much DNA studies can reveal, and what studies of morphology may not be able to distinguish. This family is tiny, both in size and numbers of species. The only reason I mention it is the idea that when plants adapt to living in water (or any other extreme environment), they often change form so much that they don’t resemble even their closer relatives. You can see the plant here: http://www.ubcbotanicalgarden.org/potd/2007/03/hydatellaceae_1.php

The order Chloranthales was previously unplaced – no one was sure what its closest relatives are. Now it is considered to be a sister group to the magnoliids. The magnoliids themselves have not changed in APG III. This branch of the flowering plants includes the laurel and black pepper families, as well as the magnolias. Although their seeds have two cotyledons, they are not closely related to the eudicots. The eudicots are the traditional dicots minus the magnoliids and the basal angiosperm lineages.

In A Tour of the Flowering Plants, I used terms for branches of monocots that have since disappeared. You don’t have to worry about whether to call the Liliales and Asparagales “lilioid monocots” or “petaloid monocots.” Just call them monocots and go on. The only lineage of monocots that gets a special name now is the commelinids. “Lilioid” and “petaloid” should be understood as informal terms that refer to plants that were traditionally lumped in the lily family. Most of them have large, showy tepals.

The major change in the monocot is the grouping of several small families as subfamilies under the enlarged families Amaryllidaceae, Asparagaceae, and Xanthorrhoeaceae. This is all within the order Asparagales. Here’s the breakdown:

The enlarged Amaryllidaceae has the agapanthus subfamily (Agapanthoideae, equivalent to the former Agapanthaceae), the onion subfamily (Alliodeae, equivalent to the former Alliaceae), as well as the amaryllis subfamily (Amaryllidoideae, equivalent to the former Amaryllidaceae). The members of this enlarged family have their flowers in umbels that are enclosed by two bracts when the flowers are in bud.

The enlarged Asparagaceae is really big. It has the Brodiaea subfamily (Brodiaeoideae), the scilla subfamily (Scilloideae, which includes the former hyacinth family, as the tribe Hyacintheae), the agave subfamily (Agavoideae, which includes the former Agavaceae), the Nolina subfamily (Nolinoideae, equivalent to the former Ruscaceae), as well as the asparagus subfamily (Asparagoideae). Yet another subfamily holds several Australasian species such as the cabbage tree, Cordyline.  

Asparagaceae members have flowers in racemes or in umbels that have three or more bracts at their base. The umbels, if present, do not have the pair of enclosing bracts seen in Amaryllidaceae.

The enlarged Xanthorrhoeaceae (the grass tree family) includes the daylily subfamily (Hemerocallidoideae) and the asphodel subfamily (Asphodeloideae), as well as the grass-tree subfamily. I did not include the grass tree family in A Tour of the Flowering Plants because it is native to Australia and not commonly used in North American landscaping.

The cattail family, Typhaceae, got a second genus, Sparganium, the bur-reeds. It’s not hard to see these two aquatics as relatives.

I’ll address the APG III changes to the eudicots in another post.

Pollen cones

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

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

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.