Category Archives: Plants

Physics on the Farm: Brassica in a Whole New Light

At the farm, we gently wash the vegetables in preparation for the distribution. It’s a meditative process: gently we lay the earth bedecked root crops in the first tub of water. Swish, swish! Swish, swish! One can imagine radish tops as the tail of some exotic koi. One by one, each vegetable in turn, passes through a couple of changes of cool water, so that they’re free of clods and are radiant when you pick them up.

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One afternoon, while washing the collard greens, John noticed that the leaves took on a silvery sheen when submerged. Green above water, silver below. What was going on? The answer is a combination of botany and physics.

Collard Green leaves (as well as the leaves of other Brassica) are covered with a waxy cuticle, a waxy layer that the plant secretes to deter pests from munching its leaves. The waxy cuticle makes the leaf slightly waterproof and that means air bubbles adhere to the surface when the leaf is plunged under water. (Fire ants take advantage of a similar development in their exoskeletons when they make waterproof rafts of themselves to cross rivers or survive floods … but that’s another story!)

But why would a miniscule layer of air look all silvery? This is where the physics comes in.
Light bends when it travels from one medium to another medium of a different density. In the case of our submerged collard green, from the water into the air bubble on the leaf’s surface. When passing from a more dense (water) to a less dense (air) medium, it is possible for the light to get “trapped” in the bubble and not be refracted back out again. This happens if the angle at which the light enters the less dense medium is greater than 48.6 degrees. At that angle, the light entering the air bubble is reflected off the boundary between the air and the water and does not refract – bend or have it’s speed changed enough to pass back through the boundary. This results in what is called ‘total internal reflection’, and we see a silvery surface. Neat, huh?

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For a more detailed explanation of the physics involved see: http://www.physicsclassroom.com/class/refrn/u14l3b.cfm

For more on the fascinating fire ant rafts see: http://www.uvm.edu/~cmplxsys/newsevents/pdfs/2011/ant.pdf

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The Mystery of the Missing Kernel

Ever rip open an ear of corn and find gaps where plump kernels should be? Sometimes a whole row will be missing, sometimes a kernel here or there.  Have you ever wondered why that happens?  What makes one kernel develop and another not?  It’s really quite remarkable.  To understand what’s happening, we first have to realize that corn (Zea Mays) is a flowering plant.  By flowering plant we don’t necessarily mean a plant with large showy blossoms or one that richly perfumes the air.  Flowering plants are plants which produce a seed that is protected by a fruit. In the corn plant each corn kernel develops from the female part or ovule of the plant; each kernel is actually an individual fruit with a seed inside.  If the ovule isn’t fertilized by pollen, the fruit won’t develop and voila, gaps amongst the rows of kernels in an ear of corn.

But, as you will have observed, a corn plant doesn’t seem to make it easy for the pollen to reach the ovule given that the male and female parts of the plant are in separate flowers, the tassel (male) and the ear (female), and the cob is so tightly wrapped by leaves.  So how can the pollen get to the kernels?   This is where corn silk comes into play.

Reproductive parts of a corn plant

The tassel on a corn plant is the stamen which contains the anthers, the part of a flowering plant that produces pollen, the male component of reproduction in plants. The silk on an ear of corn is the stigma and style, the means of collecting pollen and providing a pathway to the ovule, the female component of the plant.  Pollen shaken from the tassel by the wind falls on the silk.  It is at this point that the mystery deepens.  In order to form a kernel, how does the pollen get down the silk, under the leaves, and through the ovule wall?  It burrows. Or more precisely, the gamete burrows.  Within a grain of pollen are three nuclei: one whose job is to fertilize the ovum, one whose job is to help produce the endosperm (the kernel, the starchy food for the seed) and a third, whose sole job is to create a tube for the other two to travel down the interior of the silk into the ovule.

A pollen grain forges a path.

Modified from: http://www.agry.purdue.edu/ext/corn/news/timeless/silks.html

Timing is essential for a full ear of corn to occur. Pollen can be released only after the tassel is dry enough, normally mid-morning after the morning dew has been burned off. If the weather is too wet or too dry, the anthers will not open to release the pollen.  Pollen is very light and distributed easily by the wind, which is why it is important to plant corn in a block of rows rather than a single row to increase the likelihood of pollination.  Fortunately pollen doesn’t travel far (from 20 to 50 feet from the parent plant) and silks are covered with fine, sticky hairs that trap the pollen grains.  A pollen grain, once released, can only successfully fertilize an ovule for between 18 and 24 hours.  Fortunately, a single tassel can produce up to 25 million pollen grains and more than one grain of pollen will fall on any given silk. Plus, pollen gametes are speedy!  Pollen tube growth begins within minutes of the pollen grain’s contact with the silk.  A pollen tube can grow the length of a silk (up to 12 inches!) and fertilize the ovule in 12 to 24 hours.

So quite a few things have to go right to grow a single kernel of corn: temperature and moisture levels, silk development timed with pollen release, and pollen viability. And then, of course, there are corn borers and smut to control.  Getting an ear of corn is a bit more complex than one might have supposed!

For more fascinating information about silk growth and the timing of pollination, read: http://www.agry.purdue.edu/ext/corn/news/timeless/silks.html

And for an overview of pollination: http://ohioline.osu.edu/agf-fact/0128.html

 


Braconid Wasps versus Tomato Hornworms

          Ah, tomatoes.  We’ve harvested the first of the season.  How plump, how juicy, how tasty!  With such bounty in the offing, we look down the long days of summer with delight.  But there’s a kink in our path, a stumbling block, a veritable bug in the program, you might say.  Tomato hornworms. Neon green and gaudily stripped and dotted, these voracious destroyers can grow to an enormous size and devour an entire tomato plant in a day or two if not stopped.  What to do?  We go on hornworm hunts.  Dawn and dusk are best when they aren’t hiding under the greenery away from the blazing sun, but hornworms, for all their great size, can be elusive, and the hunt time-consuming.  Fortunately, we have allies.

In sustainable agriculture, we use the most natural methods of pest control that we can.  One of these is to encourage natural predators to take up residence in our fields so that they can eradicate those pests that they find tasty and we’d rather be gone.  A good example are ladybugs whose favorite food are the aphids that suck the juices from plant stems. We are now fortunate that braconid wasps have been making their appearance among the tomatoes.

There are three kinds of parasitism in the natural world:  predators, parasites, and parasitoids.  We’re familiar with predators: foxes, hawks, ladybugs. Usually larger than their prey (on the farming, not the Africa veldt scale!), they eat many individuals over the course of their lives.  We’ve also heard about parasites which live in (or on) a single host their entire life, occasionally debilitating, but rarely killing it.  And then there are the parasitoids.  These are the ‘predators’ that seem most alien to us. A parasitoid spends only a portion of its life in or on a host, using the host for food, and in the process, killing it.  Even the definition can give one the shivers!

Braconid Wasp drawing from Pacific Horticulture

Braconid wasps, small black wasps with transparent wings that are rarely over a 1/2 inch long, are parasitoid.  The adult wasp lays her eggs just under the skin of the tomato hornworm, and while the hornworm is munching along on the tomato leaves, the wasp larvae are eating the worm alive from the inside out!  The larvae once ready to become wasps, burrow out from under the hornworms skin and spin cocoons where they pupate until ready to emerge as full-grown wasps.  Usually, only then, does the tomato hornworm expire.  It’s a long, and to human sensibilities, a gruesome demise, but for the braconids and hornworms, it’s the way nature works.

One of the drawbacks of relying on parasitoids to protect crops is that it is a long-term solution.  It may take a year or two for braconid wasp colonies to have grown in sufficient numbers to adequately control the hornworms, and until then our crops are in danger.  So while we will leave a braconid-infested hornworm alone to suffer its fate, we still pick off and feed the others to the chickens.  In sustainable agriculture, we use a mix of approaches; in the end, we and the wasps will win.

For more on braconid wasps, see: http://www.pacifichorticulture.org/garden-allies/69/2/

 http://aggie-horticulture.tamu.edu/galveston/beneficials/beneficial-04_braconid_wasp_on_hornworm.htm

3rd Year Experiment: Trellises

We do a lot of “experimenting” on the farm. What will be more efficient?  What increases the health and productivity of our crops?  What have others tried?  What’s worked or not, and why?  We’re always trying something new (or old, as the case may be with sustainable farming.)

There has been an addition to the landscape at the farm – trellises.  Many plants benefit from trellising: peas, beans, tomatoes, cucumbers, melons, and peppers to name a few.  Any plant with a vine like stalk can be trellised. Trellising is also good for humans; it takes up less room for those with smaller yards and an urge to garden, and it makes picking the fruits a little easier on the knees and back!

Bamboo Trellis

This is our third foray in trellising.  Our first experiment was based on one that we read about in a book by Shepherd Ogden.  He recommended using lengths of bamboo leaned against and tied to each other, to make a sort of elongated tee-pee, with twine tied into a net.  We chose to do this because we had a free supply of bamboo from a friend’s backyard.  We grew beans on these. They were picturesque and worked well until about two-thirds of the way through the season.  The trellises started to collapse because they were rotting through in places.  While wondering why that might happen, we noticed that Mr. Ogden resides in Maine, where the growing season is cooler, dryer, and much shorter – and we live in the middle of a swamp.  As with so many things about farming, moisture is key!

T post and Twine

We used a second variety of trellising for tomatoes last year.  This approach was one that Thera had used on a farm on which she had interned.  In this version, t-posts were spaced every few plants, and twine was tied horizontally between the posts on both sides of the plant to support it by sandwiching it between the strands of twine.  That worked, but required frequent attention: the addition of more twine, the tightening of old twine, and the weaving of errant tomato vines between the strands.

We’re trying this year’s design, which we hope will be an improvement, because it should last for many years, and be much easier to maintain. Each trellis is composed of two 4′ pieces of rebar, two 10′ pieces of ½” EMT electrical conduit, bent into shape, a 5′ piece of ½” EMT electrical conduit, and two set-screw couplings (to join the 5′ piece to the two 10′ pieces), and twine.

As with all new approaches, we sometimes have to learn as we go. To make the trellis frames, we used a conduit bender, a “cut-off saw,” and a metal grinder.  We thought it would be an easy matter to fit the conduit over the rebar, but were frustrated in this when we discovered rebar often has a burr at the point at which it was cut.  About half the pieces of rebar couldn’t be used, and the other half had to have the conduit forced over them.  To respond to this, we borrowed a grinder, to grind the burr off one end of each piece of rebar, and now the conduit fits neatly over just about every piece of rebar.

Two stem, Vertical Twine Trellis

Once the plants are sufficiently mature, a piece of twine will be looped over the plant, wound around the main stem, and tied to the conduit above.  Each plant will be trimmed back to one or two main growing stems which will climb up this single line of twine.  It’s very exciting to see what will result!

For more on how to trellis various types of plants, see: http://www.gardenguides.com/79086-vegetables-can-grown-trellis.html

From E, I , E, I …OH!   the Omnia Humanitas Farm Newsletter

vol 3 issue 2  / June 2011