Tag Archives: darwin

Living flypaper

On the edge of a mountain bog in Maine, a thumbnail-sized plant grows amid the mosses:

sundew

This is sundew, Drosera, a carnivorous plant, ready to ambush.

Darwin devoted twelve chapters of his 1875 book, Insectivorous Plants, to the anatomy, behavior, and physiology of a European species of Drosera. He writes:

During the summer of 1860, I was surprised by finding how large a number of insects were caught by the leaves of the common sun-dew (Drosera rotundifolia) on a heath in Sussex. I had heard that insects were thus caught, but knew nothing further on the subject… I gathered by chance a dozen plants, bearing fifty-six fully expanded leaves, … it was soon evident that Drosera was excellently adapted for the special purpose of catching insects, so that the subject seemed well worthy of investigation.

The results have proved highly remarkable; the more important ones being—firstly, the extraordinary sensitiveness of the glands to slight pressure and to minute doses of certain nitrogenous fluids, as shown by the movements of the so-called hairs or tentacles; secondly, the power possessed by the leaves of rendering soluble or digesting nitrogenous substances, and of afterwards absorbing them; thirdly, the changes which take place within the cells of the tentacles, when the glands are excited in various ways.

We now know that sundews are forced into a carnivorous mode of existence by the poor soils of the bogs in which they live. They are starved of nitrogen and, not being able to find any through their roots, resort to feasting on flying nitrogenous sources, aka insects. (If extra nitrogen is added to their roots, they back off from carnivory.) The “dew” on the plants’ leaves is sweet and sticky; the droplets lure and trap passing sugar-seekers. The plants’ movable hairs and leaves then draw their victims into the center of the rosette of leaves where glands digest then absorb the meal.

Insects also serve as pollinators of the sundew’s flowers. You’ll note that the flower stalks holding opening buds in the pictures above are very tall. Natural selection evidently says: don’t eat your pollinator for lunch.

My camera could not capture the full beauty of the sundew’s leaves. The following photo by “I, Petr Dlouhý” (generously shared under a Creative Commons license) gives a glimpse. The last thing a gnat sees before The End:

Drosera_rotundifolia_leaf

De rerum…corpus individuum? On the nature of the periodic table.

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Through the caprices of classroom scheduling, I find myself teaching a writing class in a seminar room usually devoted to the study of the grammar of electrons and the narratives of covalent bonds. My colleagues in the Chemistry Department have posted a periodic table on the wall at the front of the classroom. This arrangement is typical. You’ve likely sat in such a room. Science classrooms, especially lecture halls, often have the Table lifted in front of and above the pews, in the same location as the stained glass window or crucifix above a church’s high altar.

There is a message here that extends beyond the practical utility of having a convenient chart of the elements at hand. Behold, congregation: This is the nature of our Universe. Order exists beyond the confusion of form. And indeed, this is the nature of things (thank you, Lucretius), if we restrict our gaze (as we must when we teach any class) to one particular scale of our world (about 0.3 nanometers, atom size). But the Table’s lofty and lonely position — it is seldom accompanied by any other words or symbols — is also misleading. The world is ranged in rows and columns only in one very particular way. For a biologist or a subatomic physicist, other images are more appropriate. Years ago, when I taught Introductory Biology to a large lecture-hall of first-year students, I would lower the projection screen to veil the Table, replacing it with images of ecological communities or individual living creatures. My purpose was not to deny the power or utility of the atomistic view, still less to claim that the study of chemistry is not relevant to biology, but to reclaim visual space within our science classrooms for the messy beauty of life: unpredictable, diverse, braided.

From Darwin, at the close of The Origin (I quote from the first edition):

It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us…whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Out of physical order, an unfolding process, one that cannot be contained within ruled lines.

In every classroom from whose walls the Table gazes, let us place an equally-sized artwork of the local community of life, humans included. A twin for what Whitman called “the figures…ranged in columns before me.” An entangled bank interpreted through art and science, the “endless forms most beautiful and most wonderful” of the human mind. That, or a classroom window.

Orchid seeds

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My ankle brushed against the dried flower stalk of a cranefly orchid and puff! a cloud of sandy dust billowed across the surrounding leaf litter. I got down on the ground for a closer look: the orchid’s fruit capsules were mature and starting to split apart.

Each capsule is roughly the size of a pinto bean. Inside are thousands of seeds. To the naked eye the massed seeds look like piles of very fine sawdust; with a squint we can make out the individual seeds. A camera lens and digital zoom lets us see a little closer.

Cranefly orchid leaf with its distinctive purple underside. The leaf appears in fall then dies in the spring.

Cranefly orchid leaf with its distinctive purple underside. The leaf appears in autumn then dies back in the spring.

Cranefly orchid capsule, split open and shedding seeds.

Thousands of seeds in one capsule.

A tiny puff of air is all they need to take flight.

These seeds owe their existence to pollination by noctuid moths. The moths suffer the indignity of carrying orchid pollen on their eyes. The cranefly flower has a slight twist and the direction of this twist determines whether the left or the right eye of the moth receives the pollen.

The wind-blown seeds’ future depends on where they land. Successful growth requires (or is greatly helped by) the presence of decomposing wood, so this orchid is one of thousands of species in these forests that depend on old logs and fallen branches.

Regular readers of Ramble will be interested to know that this orchid’s only close relatives live in east Asia. It joins many other plant species in reminding us of the ancient connections between the forest of the southeastern US and those of eastern Asia.

The abundance of orchid seeds has impressed botanists for centuries. Here is Charles Darwin calculating that one plant could in a couple of generations of unchecked seed production “clothe with one uniform green carpet the entire surface of the land throughout the globe.”

“[seeds] are produced by orchids in vast profusion. Not that such profusion is anything to boast of; for the production of an almost infinite number of seeds or eggs, is undoubtedly a sign of lowness of organisation, … a poverty of contrivance, or a want of some fitting protection against other dangers. I was curious to estimate the number of seeds produced by some few Orchids; so I took a ripe capsule of Cephalanthera grandiflora, and arranged the seeds on a long ruled line as equably as I could in a narrow hillock; and then counted the seeds in an accurately measured length of one-tenth of an inch. In this way the contents of the capsule were estimated at 6020 seeds, and very few of these were bad; the four capsules borne by the same plant would have therefore contained 24,080 seeds. Estimating in the same manner the smaller seeds of Orchis maculata, I found the number nearly the same, viz., 6200; and, as I have often seen above thirty capsules on the same plant, the total amount would be 186,300. As this Orchid is perennial, and cannot in most places be increasing in number, one seed alone of this large number yields a mature plant once in every few years.

To give an idea what the above figures really mean, I will briefly show the possible rate of increase of O. maculata: an acre of land would hold 174,240 plants, each having a space of six inches square, and this would be just sufficient for their growth; so that, making the fair allowance of 400 bad seeds in each capsule, an acre would be thickly clothed by the progeny of a single plant. At the same rate of increase, the grandchildren would cover a space slightly exceeding the island of Anglesea; and the great grand-children of a single plant would nearly (in the ratio of 47 to 50) clothe with one uniform green carpet the entire surface of the land throughout the globe. But the number of seeds produced by one of our common British orchids is as nothing compared to that of some of the exotic kinds … What checks the unlimited multiplication of the Orchideæ throughout the world is not known.”

(p. 277-279 in Darwin, C. R. 1877. The various contrivances by which orchids are fertilised by insects. London: John Murray. 2d edition, quote from the Darwin-Online archive.)

Serviceberry: preaching the gospel across the Appalachian Mountains

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Serviceberry roared into flower this week, going from bud to bloom in a few hours, or so it seemed. From a distance these small trees present puffs of bright white in the dusky woods. Seen close, the flowers are indecently large and gaudy: this is no subtle, ground-hugging anemone flower. The serviceberry is famous all over eastern North America for announcing spring with a vigorous fanfare, usually at the head of the line, weeks before other trees and shrubs shake off their winter torpor.

The species goes by many names, surely more names than any other understory tree. Here are the names that I’m aware of, no doubt I missed a few: serviceberry, sarvis, sarvisberry, shadbush, shadblow, juneberry, sugarplum, wild plum, Indian plum.

Etymology is clear for some of these names, but not for others. “Shad” refers to the shad fish (genus Alosa) of the northeast whose return from the oceans to their upriver spawning grounds coincides with the bloom of the “shadbush.” “Plums” refer to the fruits which are consumed with gusto by birds and small mammals and, depending on the variety of tree, by humans.

“Sarvis” is a mountain dialect variant of “service,” but the origins of these terms is not clear. Some claim a corruption of Sorbus, the scientific name of a related plant, the mountain ash. Others associate the name with church services: the return of traveling preachers as winter roads became passable or the use of the blooms as decorations for Sunday worship. I learned of another interpretation from Jay Leutz’s book Stand Up That Mountain. In the higher parts of the Appalachians, where winter freezes are deep and long, the tree blooms as soon as the ground thaws. This time marks the first opportunity to dig graves and hold funeral services for those who have died in winter and whose bodies await burial. On the Cumberland Plateau, at least in the years I’ve lived here, the ground is fit for burying many weeks ahead of serviceberry blooms. Different meanings are heard in different landscapes; our understanding of language molds itself to place.

Serviceberry also offers us a curious parallel between nomenclature and biology. The genetics of this genus, Amelanchier, are particularly complicated. Dozens of forms exist. These forms hybridize, creating new variants and causing mental (and gastric) distress for any taxonomists who dream of order in the Natural Order. Some forms switch off sexual reproduction and set up small populations of asexual trees. These “microspecies” sometimes then switch back to sexual reproduction and interbreed again with other forms. In all, the genus is a hyperdiverse genetic tangle in which firm species do not exist.

This seems frustrating, but many biologists see the situation otherwise. Here, for example, is our friend Darwin in the second chapter of The Origin, discussing the serviceberry’s unruly relatives in the Rosaceae family:

I refer to those genera which have sometimes been called “protean” or “polymorphic,” in which the species present an inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which as varieties…

Certainly no clear line of demarcation has as yet been drawn between species and sub-species…

Hence I believe a well-marked variety may be justly called an incipient species…

From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms.

For Darwin, these variable “species” and “varieties” were evidence for continuity in nature. Today’s species is yesterday’s variety. Life evolves; don’t expect tidiness.

We can only speculate what the traveling preachers of the 19th century might have thought of having their arrival heralded by an example of Darwinian mutability. I like to think that at least a few may have reached for the wild-plum wine.

Bee comb

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This week I took advantage of what may be the last warm, sunny days of the season to tidy up the bee hives for winter. I removed unneeded boxes of frames from the tops of the hives and shuffled frames within the boxes to keep as much honey in the hive as possible. Thus prepared, winter hives are less likely to blow over in storms and, more important, all the honey is gathered into one place within the hive. In cold winters, bees huddle in a ball around their honey stores, slowly eating the honey as fuel to keep them warm (the center of the hive is as warm as human body temperature). If honey is thinly dispersed, the balmy bee ball cannot form.

I had forgotten just how beautiful the wax combs of honeybees are. The near-perfect six-sided geometry, repeated hundreds of times is a fabulous piece of natural architecture.

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The wax is secreted from chinks in the abdominal exoskeleton of worker bees. The bees then mold the wax into the six-sided pattern using chewed wax particles. This task falls to middle-aged (2-3 week old bees) worker bees. Younger workers look after the brood; older workers leave the hive and forage.

This weekend marks the 153rd anniversary of the publication of On The Origin of Species. It is therefore fitting to include here Mr. Darwin’s thoughts on the wonders of beeswax.

He must be a dull man who can examine the exquisite structure of a comb, so beautifully adapted to its end, without enthusiastic admiration. We hear from mathematicians that bees have practically solved a recondite problem, and have made their cells of the proper shape to hold the greatest possible amount of honey, with the least possible consumption of precious wax in their construction. It has been remarked that a skilful workman, with fitting tools and measures, would find it very difficult to make cells of wax of the true form, though this is perfectly effected by a crowd of bees working in a dark hive. Grant whatever instincts you please, and it seems at first quite inconceivable how they can make all the necessary angles and planes, or even perceive when they are correctly made. But the difficulty is not nearly so great as it at first appears: all this beautiful work can be shown, I think, to follow from a few very simple instincts. (First edition, Chapter VII, page 224).

He elaborated these thoughts with a series of calculations and experiments, summarized in a recent essay at the Darwin Correspondence Project. As you might expect, Darwin concluded that natural mechanisms could explain the structure of bee comb and that sophisticated combs could have evolved from simple beginnings.

This naturalistic view contrasts with the opinions of Darwin’s contemporaries. After reading Darwin’s passage, I pulled down Langstroth’s Hive and the Honey-bee, an important review of bee biology and bee-keeping published in 1859 (the 4th edition, 1878, is the one that I have on hand; post-Darwinian for sure, although Darwin is not mentioned). Langstroth writes of comb:

To an intelligent and candid mind, the smallest piece of honey-comb is a perfect demonstration that there is a “GREAT FIRST CAUSE.”

These enraptured references to the Divine are peppered throughout his work.

Langstroth was a priest, but depression kept him from many of the usual priestly duties. Instead, he studied insects, especially honey bees. Although his theology seems unsophisticated to modern ears, his entomology was not. His careful studies of bee behavior transformed bee-keeping. In particular, these studies led to him a new design of bee hive, a design that is still the preferred hive for most bee-keepers, especially in North America. Unless you’re eating honey from wild nests, you can almost guarantee that the honey in your kitchen came from a Langstroth hive. I use a modified design: Langstroth in the upper portion (from which come the photos in this post) and open in the lower part (no photos — I never open this part, leaving it for the bees to do as they will).

Parasitic ants, unwise language, and a little glimpse of Darwin

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Several weeks ago I came across a curious highway of ants. They were streaming across the leaf litter in a column about a foot wide. The column started under an oak tree, traversed the leaf litter and hiking trail, then ended abruptly about forty feet away in an otherwise unremarkable patch of fallen leaves. Ants traveling away from the tree were carrying white, ant-sized objects in their jaws. Ants moving in the opposite direction were empty-mouthed. At the destination, a few smaller ants milled about, seemingly at ease among the larger ants that I was watching.

I suspected at the time that I was witnessing a raid by a so-called slave-making ant species. My skills as an ant taxonomist are limited and I turned to my colleagues for help. Thanks to James Trager and Ann Fraser, I’ve confirmed my suspicion and been able to tentatively identify the species in question as Formica subintegra and Formica subsericea (an aside: Ant Blog is a great place to seek answers about ants). The first species, the larger one, was attacking the nest of the other and carrying away eggs and larvae. These captured youngsters will be raised in the “den of thieves” and, when they emerge as adults, the newly pupated ants will have no idea that they do not belong. Because ants take their cues from the chemical milieu in which they grow up, the stolen ants consider themselves full members of the alien colony. This trickery buys the captors a work force to maintain the nest and rear more young. In some ant species, the captors are so dependent on the captured workers that they cannot survive without them, having lost the ability to feed themselves and take care of the brood.

In the biological literature this arrangement has, for many years, been called “slave-making.” This makes me deeply uncomfortable. Using a term — slavery — from a human institution that all (or nearly all) modern human societies have agreed is morally unacceptable seems unwise. Further, the “ant slavery” term implies a biological equivalence that does not exist. There is not a single biological parallel between the details of the situations in humans and ants (ants raid other species, ant societies and nervous systems differ radically from ours, etc). By using a term derived from human society, a term that comes with considerable moral heft, we blind ourselves to the otherness of the ants. So in addition to the moral argument (which is strong enough on its own, I think), there are scientific reasons for not using the term: our preconceptions may cause us to fail to understand ant biology.

In other areas of biology, we’ve thankfully tidied up our terminology a bit. Textbooks on animal behavior were formerly strewn with terms like divorce, rape, and prostitution. These days, textbooks generally leave these loaded terms at the door, although more popular media outlets and some scientists continue the unfortunate practice. For example: Wikipedia (of course), BBC, and The Independent (note how the coverage slips so easily into discussion of what is natural for humans; Hume shudders, as explained (of course) on Wikipedia). My point is not that conflict, coercion and suffering do not occur in nature (of course they do), but that the use of human categories to describe animal behaviors can lead us into trouble. This is especially true when those categories carry with them a strong emotional, intellectual or moral charge.

Back to the ants. I was particularly excited to see this process unfold because it has a place in the history of biological ideas. Darwin was fascinated by these ants and used them in Chapter Eight of On The Origin of Species as an example of how natural selection could mold behavior (or “instinct” as he called it). He writes:

We shall, perhaps, best understand how instincts in a state of nature have become modified by selection by considering a few cases. I will select only three, namely, the instinct which leads the cuckoo to lay her eggs in other birds’ nests; the slave-making instinct of certain ants; and the cell-making power of the hive-bee: these two latter instincts have generally and justly been ranked by naturalists as the most wonderful of all known instincts.

Darwin dug up and manipulated a number of nests in England, experimenting with the ants to better understand the nature of the “slaves” and “masters” as he termed them (Darwin was not shy here or elsewhere in his writing about linguistic cross-over from human behavior). He concludes that:

…natural selection might increase and modify the [parasitic] instinct—always supposing each modification to be of use to the species—until an ant was formed as abjectly dependent on its slaves as is the Formica rufescens.

The complete account is available in the many online copies of The Origin (or in the treasured copy of this volume on your bookshelf).

In the years since Darwin, hundreds of studies have been conducted on the socially parasitic ants, many of which are summarized in a short review by Buschinger. One recent study of particular note is the discovery of retaliation by a genus of ant that is frequently attacked by parasites. The host genus is Temnothorax — tiny ants that nest inside acorns (!) and hollow twigs — and the parasite is Protomognathus americanus. Unlike the larva-robbers that I observed, Protomognathus parasitic ants invade and take over the nest of the host. Temnothorax adults are killed and their young are co-opted to work for the parasite. It appears that these attacks are so common that natural selection has produced a counter-measure: genes in some of the host workers cause them to attack the parasite, killing the developing Protomognathus pupae.

The authors regrettably use the terms “slave rebellion” and “revolt against their oppressors” to describe the behaviors that they describe. Surely a human rebellion against slavery is biologically and morally different than a gene variant causing an ant to use chemical cues to bite a pupa? My grousing about language aside, this is a remarkable study. Darwin would have loved to add this co-evolutionary tale to his chapter on the evolution of animal behavior.

From now on, I’ll be examining acorns more closely.