Tuesday, November 29, 2016

The Life of a Tree: 3. Growth

Most animals are integrated in another important way: their growth.  You (as a representative animal) grew from the cell that resulted from conception in a highly organized dance in which cells actually moved from place to place, divided (at different rates in different places and times), and differentiated into all the many tissues and organs that make up the adult human.  (Just how all of this is organized and regulated is cutting-edge science today.)  Then, once you had reached adult size, your cells stopped dividing.  Just stopped.  --with the exception, of course, of those needed to replace skin, blood cells and the like, and those triggered to divide in wound healing, and so on.  

Plants do this is a very different and simpler way.  Plant cells are encased in a cell wall that strengthens them, provides overall structure to the plant, and effectively fixes the individual cells in place--and is one reason plants don't move.  Instead of growing everywhere, land plants grow at shoot and root tips, at regions called meristems.  Cells in a meristems divide, then those that end up on the side opposite the tip differentiate into vascular tissue, fibrous tissue, cortex "filler," leaf primordia, etc., while those that are nearer the tip continue to divide, gradually leaving the differentiated stem or root cells behind as the shoot elongates.  

 Shoot (Coleus?) and root (onion) meristems.  Tissue slices have 
been stained to show individual cells and their nuclei more clearly. 

One consequence of apical growth that surprises people is that the tree branch you have to duck under this year will never be any higher: its height was established when that branch was only a twig.  (Those lower branches tend to die over time, though, often leaving the lower trunk bare.)   

Unlike animals, most plants have no "adult size": as long as they live, trees and shrubs continue to grow.  Because each year's shoot growth begins with last year's buds, to cease to grow is death.

Besides growing at tips, woody plants (trees, shrubs and vines) also grow around their circumference.  A layer of tissue under the bark, a lateral meristem called vascular cambium, adds cells inward to form water-carrying xylem tissue, and outward to form sugar-carrying phloem tissue.  (When you look at a piece of wood, you are looking at xylem, and the tiny holes sometimes visible in end grain are the cut ends of water-carrying tubes called xylem elements.)  While xylem is long-lasting, typically carrying water for many years before finally clogging up and becoming dark-colored "heartwood", phloem is only active for a short period, eventually becoming a second kind of lateral meristem: bark cambium.

This is a good place to talk about the chemistry of animal and plant strength.  We mobile animals are a peculiar mixture of delicacy and strength: our individual cells are floppy, insubstantial little blobs of Jello, but together they secrete proteins that form immensely strong fibers (mostly collagen) that make our bodies tough and strong and yet flexible.  The walls of plant cells are made of a very different fiber called cellulose, which is a polymer of sugar molecules instead of a protein.  

 Loose cells are easily scraped off the inside of your cheek with a dull toothpick.  
These are stained to show the nuclei.  Notice how floppy they are!

Cells in the leaf tissue above have been soaked in a salt solution so they have wilted.
The living tissue of each cell (complete with green, disc-shaped, sugar-generating chloroplasts)
have collapsed, leaving the box-like cell walls intact.  Think of severely wilted lettuce.

 A cross-section through a stem shows thinner-walled cortex cells with thicker-walled cells.  Producing linen begins with beating stems of flax to separate these strong vascular fibers
from the rest and spinning them into thread. 

The little cellulose boxes in which plant cells live make them individually tough even as they trap the cells in place.  (In fact, green plant cells are hydraulic structures: their rigidity is due to internal water pressure in exactly the same way a football's rigidity is due to air pressure.)*  Animals must continually bathe their cells in a mild salt solution that prevents them from either shriveling up (too much salt) or exploding (too little).  This vulnerability is the reason athletes must watch their electrolyte (salt) balance.  Though plant cells may wilt with too little water (or too much salt) , these cells are immune from damage by fresh water because their cell walls are strong enough to prevent their bursting.  Together, these cell walls form the fiber of countless natural products, from the cotton in our clothes, to the rope that formed the rigging of tall ships.  In a form stiffened by other molecules (lignin prominent among them), this fiber becomes the wood that builds our homes.  Wood, sometimes disparaged in comparison with modern materials, remains stronger for its weight than any other substance. 

*I remember the first time I handled a deflated football: the pigskin is soft and flexible (like a plant cell wall).  Inside it is a delicate but air-tight rubber bladder (like a cell membrane).  The pigskin prevents the inflated bladder bursting just the way a cell wall prevents the plant cell membrane from bursting.  

Saturday, November 26, 2016

The Life of a Tree: 2. How do Plants Have Sex?

--Continuation of an essay comparing plants to animals.  (Part 1 is below.)

Yes, insects are animals.
(If it eats, then it's likely an animal.  Most also move.  None make their own food.)
Here are two damselflies mating; sexual reproduction is nearly universal 
among life forms more complicated than bacteria. 

Plants reproduce sexually in fundamentally the same way most animals do: they produce sperm and eggs with half the usual chromosome number, then sperm from one individual meets egg from another, and they fuse to form a new individual with genes from both parents.  But there are differences.  

First, all plants "alternate generations": they form individuals with half chromosome numbers (called haploid) as well as full chromosome numbers (diploid).  This is most striking in mosses and liverworts, where these individuals may live independent lives.  It would be as if your eggs or sperm went off on their own, grew multicellular bodies, and lived independent lives before producing sex cells that finally resulted in a baby!  In "higher" plants such as conifers and flowering plants, the haploid individuals are very small and incapable of living long without dependence on the more familiar adult, so that only botanists notice them.  (A pollen grain is actually such a haploid individual: it is made up of three cells wrapped in a tough, water-resistant shell that enables it to survive until it reaches another flower.)

Alternation of generations in the life cycle of ferns.  The diploid "sporophyte" is the familiar fern
we see.  The haploid "gametophyte" lives underground and resembles (I'm told) a wad of chewing gum: after fertilization (fusion of egg & sperm) the gametophyte grows upward from it.

Second, transportation is an issue.  For most animals, sperm get to eggs either by swimming through the water in which both parents live or, in the case of land animals, are brought into close proximity by mating.  Land plants face the same challenging waterless environment as land animals, but with the handicap of being unable to move.  Land plants of moister places, such as mosses and liverworts and ferns, employ a swimming sperm strategy just as some marine animals do.  Other land plants overcome the challenge of immobility by producing pollen to carry the sperm from male to female.  Those that produce pollen include conifers and flowering plants.  Most such plants have evolved one of two solutions to transporting their pollen: allowing dusty pollen grains to waft on the wind, or making a business arrangement with an animal that can do the transport.  

Wind pollination works pretty well where individual plants are in close proximity, such a field of grass.  Many trees also are wind pollinated.  (Many people are unaware that grass and most trees are flowering plants: since they do not need to attract animals, their flowers are usually small and not brightly-colored.)  These are the plants that might make you allergies act up. 

 Pollen grains can be quite beautiful.  The shapes are specific enough  that palynologists studying pollen grains in old lake sediments can name the plants that grew nearby thousands of years ago.
(Spiky projections help some stick to insects, etc.)

For those plants that engage the services of animals to transport their pollen, insects--and especially bees--are the most common.  (So many crop plants are pollinated by honey bees, for example, that the decline in honey bee populations caused by Colony Collapse Disorder actually threatens the American food supply.)  Other animals that have partnered with flowering plants include wasps, moths and butterflies, bats, and birds.  The coevolution of plants and pollinators has led to business partnerships in which the plant offers a reward (often nectar) to lure the animal, then dusts it with pollen, and allows it to go off to visit other flowers.  Sometimes these arrangements have become so specific that a single species of plant depends on a single species of animal, which in turn has become wholly dependent on that plant.; this is highly-efficient for both species, but also very risky in the long run, should one or the other go extinct.  Orchids are famous for pulling a rather dirty trick: growing flowers that imitate particular insects, luring the males for mating--then sending  the poor, disappointed creatures to go elsewhere with their pollen load--likely to another orchid to repeat the attemp! 

Third and arguably strangest for us, as animals, to consider is the matter of gender.  Although gender is changeable in some animals (slipper shells come to mind), we mostly think of our fellow animals as either male or female.  But gender varies in plants.  Most flowering plants are simultaneously male and female: you can find dusty, male pollen-producing stamens and also sticky, female, pollen-receiving stigmas on the same plant, and most often even in the same flower.  Rarer is the case of plants that produce only male or female flowers.  Around here, the "gendered" trees include ashes, quaking aspen, and red maple and ash-leaved maple.  Though most flowering plants produce both eggs (which remain inside the flower) and sperm (in pollen),  they do not readily pollinate themselves: chemical recognition seems to make self-pollination rare in most species.  This is probably because in-breeding is as bad in the long run for plants as it is for animals.  

Stamens are the male parts; the anther makes and releases the pollen.  The pistil (carpel) is the female part; pollen lands on the stigma, grows a tube down the style into the ovary and to the egg inside the ovule, and the sperm move down the tube to fertilize the egg.  (The names of many parts were borrowed from animal anatomy, but only roughly correspond.)

The plan of individual flowers varies widely, also.  Flowers which are perfect (both male and female) and complete (having all the typical parts such as sepals and petals) are common.  But all possible variations are found.  Flowers of grasses, for example, have no petals (having no need to attract pollinators), while many (lilies and tulips, for example) have sepals that have become so like the petals that they are collectively known as tepals.  The Norway maple, a common city tree, can have--growing in a single bunch--perfect flowers, male flowers and female flowers!

Tuesday, November 22, 2016

The Life of a Tree: 1. Integration--and the lack of it

Humans are very much animals.  We think like animals, and we have the most regard for the other animals that are our closer relatives.  Plants are so different from us that they're hard to understand.  That very foreignness may be one reason I like them. 

Let's contrast them. 

Yes, insects like these milkweed tussock moth caterpillars are animals.
(Ask yourself: does it eat? then it's likely an animal.  Most also move.)

Most animals move.  Animals have sophisticated senses: chemical sensors (the nose), light and image sensors (eyes), vibration sensors (ears), etc.  Animals--at least, those much more complicated than jellies, sponges, flatworms, and clams--have nervous systems that provide very sophisticated and centralized coordination and control.  They circulate blood in a loop-like circulatory system, they have nerves with a brain that receives and processes information and responds back.  A sensory stimulus to any part of the body might result in a response by any other part or by the entire body.

Ponkapoag Pond, Blue Hills Reservation, August 2016.

Plants have none of this.  They do not generally move, and need not seek food since they make their own using light energy; indeed, photosynthesis is one of the characteristic traits of the plant kingdom.  They have two more-or-less independent one-way transport systems: one (called xylem) transports water and nutrients from the roots toward the stems and leaves where they are important to photosynthesis; another (called phloem) transports the food (sugars and the like) made in photosynthesis to tissues that cannot make their own, such as the stem and roots. 

Plants respond to stimuli, also.  They can sense gravity: roots grow reliably down, shoots upward.  Plants are sensitive to light, and in particular use the length of darkness to help regulate seasonal changes.  Temperature is a stimulus that, for example, triggers the beginning of spring growth.  Plants are sensitive to various chemicals, and can (famously) respond to the chemicals produced by nearby plants under insect attack: they will increase their own production of defensive toxic chemicals.  But there is no nervous system and very little centralization; responses are decentralized and local. 

A tree I remember on my college campus had grown up and enveloped a streetlight; as fall lengthened into winter, all the leaves turned and fell except those in a sphere around the light: these remained green as they became increasingly tattered and finally died in the winter without turning--all because those leaves did not experience the long darkness that signals fall to a tree.

Trees seem to behave in coordinated ways in sending roots out into more moist or fertile soils, or increasing root or shoot growth in a way that balances the two.  But these are very simple and mechanistic responses: roots in richer soil have more resources and so will grow faster, while a cut tree stump will often sprout rapidly because it has all the resources of a disproportionately large root system supplying water and nutrients.  The result only looks purposeful

Plant vascular tissue.  Xylem--a dead tissue that resembles household plumbing
--transports water and minerals from roots to leaves, drawn upward by evaporation of water
from leaf cell surfaces.  Phloem, made of living cells, transports sugar by bulk flow
from where it is most concentrated (in summer, the leaves where it is created) to where it is
least concentrated (stems and roots that need food but cannot make it as green leaves can).

Sunday, November 20, 2016

The beginning of the end of fall

Only a week after the black oaks in the neighborhood were at there most brilliant, they are fading to brown.  Oaks like these are "tardily deciduous," meaning the dead leaves hang--sometimes all winter--before falling as late as spring budbreak.  Red oak leaves are brown and dead, white oaks are mostly bare.  The first sugar maples helped kick off the color fest weeks ago, but they're individualists, and one or two still have a few yellow leaves.  Red maples paralleled the sugar, and are losing their last leaves.  River birches are sparse and yellow.  Among native trees, basswood is still yellowing, while green leaves still cling to some paper birches.  

Some lesser folk such as nightshade remain green to the bitter end.  Multiflora rose, too, is green as long as there is any light and warmth at all left in the year. 

Witchhazel is alone among the woody plants of my acquaintance in being in flower right now, though the flowers are looking a bit the worse for wear.  The seeds are forcefully ejected from the pods not at the end of flowering, but the following fall, just as the flower buds are about to open.  The witchhazel on the corner had a good fall last year, and sent many children of into the world in the second half of October, as this year's flower buds swelled and burst.  

Mostly oaks along my street.  11/5/16

Black oaks (Quercus velutina) on 11/11.

Last and brighest of the neighborhood black oaks.  11/13

November 19th: black oak scarlet fades to brown.

 Basswood, aka linden (Tilia americana), has lost few leaves, and many are still partly green. 11/19

Nightshade hangs on.  11/19

The first witchhazel (Hamamelis virginiana) fruits burst around October 14th.

By 10/29 leaves are gone, most flowers were open and most pods were empty.

November 19th flowers are fading.

Wednesday, November 9, 2016

Science is the surest way we have of knowing anything.

I am deeply troubled for our nation's future, and place in the world.  I  cannot even count all the reasons Donald Trump should never have been elected: from a narcissistic and overweening confidence in his own superiority, to his utter lack of principles, to his complete disinterest in what is actually true, to the deep satisfaction he finds in objectifying and belittling others.  Many, many others more qualified than I will analyze how we came to this pass.  I, for my part, will stick to something I know a little about.

 One of my greatest fears--even eclipsing my worries about a flighty leader with the world's most powerful nuclear arsenal at his fingertips--is for the fate of life on our planet.  Many of the species we share the earth with are on the ropes, others are already gone forever.  Climate change is shaping up to be the most dire single threat humanity has ever faced, and will be the end of vast numbers of our fellow species.  Yet we will be led by a president who has called climate change a Chinese hoax, and congressmen who believe it is a left-wing scientific conspiracy (as if organizing scientists into such a conspiracy wouldn't be harder than herding cats).  The Titanic of our environment is going down by the bow while politicians and citizens alike rearrange ideological deck chairs.  The idea that scientific theories are little better than opinions seems to grip a large part of the electorate--especially conservatives.  This willful disbelief follows the long-standing attitude of millions of Americans toward the theory of evolution--which, thoroughly supported by a full century of successful experimentation, is one of the most successful and powerful of modern theories.

This willful disbelief represents an immense and nation-wide failure of science education.

It is my experience that most science teachers regard the so-called "scientific method" as a brief unit (occupying less than a dozen pages in chapter 1 of some intro textbooks) as something to get through quickly so they can get into the content that will be assessed on the big standardized test.  Many of these teachers will take the time and effort to do a lab to introduce the concepts of hypotheses, experimentation, data and its analysis, and the logic of scientific conclusions that are based on evidence.  Some of these teachers will have students making up their own experiments.  But even of these, very few will put that lab or "scientific method" into the long process and larger context of arriving at "truth" from such experiments.  Very few will look at science as a human enterprise with a social context, evolved over the course of decades and centuries until it has become at one time paradoxically forever uncertain, and the surest and most powerful way we have of knowing ANYTHING. 

This paradox of uncertainty and sureness calls out for a little explanation.  Over many decades, the enterprise of science has been embarrassed by enough conclusions confidently arrived at, so that any scientist worth his or her salt will be very hesitant to make broad assertions of "truth."  We must always qualify and point out that, well, we might be not have it quite right yet.  Scientific knowledge is forever provisional--every "fact" open to falsification at some future time--so that even if we are on the right track, some tweaking will probably be necessary as more is learned.

Scientists are rather allergic to the words "truth" and "proof."  There is too much confidence and certainty to these words.

That very reluctance to proclaim truth--that we in science are forever a little unsure of ourselves, our discoveries forever open to modification--is part of what makes the knowledge so arrived at much surer than knowledge arrived at any other way.  Scientific knowledge contrasts sharply, for example, with religious knowledge: no scientist would take something as fact merely because it is proclaimed by a charismatic speaker, or found in a holy book.  (This, because it has no experimental support or critical peer review, is perhaps least sure of all knowledge.)  Scientists take nothing on faith.

Science is a human enterprise, and scientists are imperfect, biased, and not by nature any more rational than anyone else.  Long training makes them more aware of these faults than most, and safeguards have gradually evolved that make it difficult for any scientist--no matter how charismatic or famous--to pull the wool over everyone's eyes for long.  

A recent case in point.  A couple of years ago research anchored at the Riken Institute, Kobe, Japan announced a simple but revolutionary way to create stem cells from adult cells, by way of "stimulus-triggered acquisition of pluripotency."  Years and careers have been invested in this pursuit by many research labs, so the announcement was met with great excitement.  As is typical, other labs tried to replicate their results, but without success.  Eventually, an investigation found fraudulent science that resulted in key scientific papers being retracted.  Two researchers were hospitalized for stress, and one, the supervisor of the lead investigator, committed suicide.  

Some would see this as a failure of the scientific enterprise; I see it as an unlovely success.  Granted, when fraud occurs it should not get this far--and it seldom does--but the participation of other scientists innocently repeating the work eventually brought problems to light.  Science corrected itself, as it nearly always does.  All of these built-in safeguards, from ultimate reliance on solid evidence, to participation by colleagues, to peer review, to the scrutiny of the larger scientific community--all these make the resulting "bricks" of knowledge strong and sure enough to be built into wonderful theories--the large, explanatory frameworks that are the height of scientific achievement, and help to organize the modern human understanding of the universe.  Einstein's theory of gravity, for example, organizes our understanding of much of the large-scale structure of the universe; quantum theory organizes the very small; and biological evolution is the central organizing theory for the life sciences; all three of these theories have, I think, passed the century mark. 

Compare this with any other department of knowledge, from religion, to philosophy, to history, and you realize that none even approaches the rigor of science.  Tentative though it seems, no knowledge is more deserving of trust than scientific knowledge. 

Why doesn't every American citizen know this?  Because science teachers do not teach it adequately.  Few science teachers have a hands-on research background, and many are pressed for time.  Coming up with good materials is difficult and time-consuming.  

One solution I discovered a few years ago is an organization called Evolution and the Nature of Science Institutes.  This little gem of a shoe-string organization promotes learning about biological evolution in the larger context of science as a robust, largely self-correcting human enterprise.  ENSI lays great stress on the messy, imperfect, human nature of the scientific enterprise--warts and all--yet shows how robust the resulting knowledge can be when scientific habits of mind, the integrity of researchers, and science's checks and balances its safeguards are taken into account.  ENSI has many ideas and lessons that send students out in journeys of open-ended discovery--and without the "answer key" that no real scientist ever has.  I personally cherish many of these. 

There is more to it, of course.  It is perfectly possible for a student to skate through any number of science lessons, writing the expected answers and passing the tests, without ever really internalizing the way science works.  The old, established childhood understandings can go into hiding, but emerge and reasserts themselves when it's "safe."  Countering this, the "conceptual change model" begins by first unearthing the long-held, sometimes unconscious, theories acquired in the limited experiences of childhood, and then holding them up to the light of day.  New knowledge can then wrestle honestly with old, with a better chance that the former will be truly vanquished.  

Only by teaching, warts and all, the how of science alongside the what, can we do our part to insure an educated electorate who demand educated leaders.  And on this, our very future depends.  

Monday, November 7, 2016

Leaves don't fall--they're pushed!

Red maples on West Elm Street, Brockton on 11/6/16--only a day or so after strong winds.

How else could you explain the piles of leaves now gathering, simultaneously, under so many trees?  It was very windy a day or two ago, yet only now, in stillness, are the leaves falling in large numbers. 

Many assume that in fall leaves die and fall off, while in fact the deciduous "habit" is an active process of gradually dismantling cellular machinery, retrieving some elements for recycling into next year's growth, and then cutting loose the "used up" leaf that remains. 

After the greater part of phosphorus and potassium and especially nitrogen are withdrawn from the leaf to be stored in trunk or roots, a special "breakaway" zone called the abscission zone develops at the base of the petiole (leaf stem).  Part of this zone includes a waterproof "bandaid" coated with fatty suberin that prevents "bleeding" from the wound that will result.  After this, the slightest breeze--or even just the weight of the leaf--will break it loose. 

View through a microscope.  The twig is on the left, the leaf grows out on the right.  Each little chamber you see is a single cell.  You can see how the leaf is being separated from the twig beginning at a sort of notch.

If you look at any twig you will see the leaf scar beneath each bud that marks where a leaf was attached.  Towards the middle of the leaf scar is one or more bundle scars that mark where the abscission zone cut through the vascular tissue that transports water and minerals upward (called xylem), and sugar downward (called phloem).  These scars vary in shape and arrangement from species to species, and are a good aid to identification.  

Saturday, November 5, 2016

The Value of Looking Down

--maybe not all the time--but definitely look down every so often as you walk.

Out walking the dogs on the same route I've walked perhaps a hundred times, a single leaf stopped me in my tracks.  

It was certainly a maple leaf, and was broad like the commonplace Norway maple, but this leaf was toothed more like a red maple (also commonplace).  (If you're curious, you can learn to distinguish these here.)  It is also more strongly-veined than either of these. 

Only one tree fit the bill--sycamore maple (Acer pseudoplatanus)--an introduced tree.  I know it only from the neighborhood where I grew up: it grows on the grounds of the old Aldrich Estate, where I assume it escaped from cultivation.  (The one little sycamore maple I could reach there without trespassing died last winter.)

Where is the tree this leaf came from?  I'm sure it isn't near the street, or I would have been able to find it.  Probably it is in a nearby backyard.  I will keep my eye out for others; maybe I will be able to triangulate.