Tag 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