Category Archives: Essays

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

 


It’s hot! But no drought … not quite, not yet.

Not us! Not yet.

“Crunchy” is not an adjective that one usually wants to apply to one’s lawn.  But that’s precisely what the grass is this blazing, thirsty July … crunchy.   And yet, according to the weather service, we’re not in a drought.  Well, that all depends on how you define a drought, now doesn’t it?  What’s a drought?

Hydrologic drought is when the groundwater aquifers, reservoirs, and stream flow are below normal. The massive snows of this past winter recharged the aquifers and while the stream flow for the Christiana River is running between the 24th and 74th percentiles (See http://md.water.usgs.gov/surfacewater/streamflow/christina.html ) this is still normal for this time of year on average.  In addition, the large, established trees that rely on subsurface water tables seem in good shape, their leaves full and plentiful. (See http://www.drought.unl.edu/vegdri/VegDRI_Main.htm)  So, accordingly, there’s no drought by these measures, close maybe, but not yet.

A meteorological drought is defined as “a period of abnormally dry weather sufficient to cause a serious hydrological balance.” (Huschke, R.E., ed., 1959, Glossary of meteorology: Boston, American Meteorological Society, 638 p.)  This can be variously defined as an “absolute drought”, a “partial drought”, or a “dry spell”.  An absolute drought is a period of at least 15 consecutive days with less than 0.01 inches of rain or more on any given day. A partial drought is a period of at least 29 consecutive days, the mean daily rainfall of which does not exceed 0.01 inches. A dry spell is a period of at least 15 consecutive days with less than 0.04 inches or more on any given day. If we check the monthly rainfall for June and July, we’ll discover that, meteorologically speaking, we’re not only not in a drought, we’re not even in a dry spell!

But from a farmer’s (or lawn owner’s!)  perspective, it’s quite another thing.  A more recent delineation of the different types of drought includes “agricultural drought”.    Agricultural drought isA climatic excursion involving a shortage of precipitation sufficient to adversely affect crop production or range production.” (Rosenberg, N.J., ed., 1979, Drought in the Great Plains–Research on impacts and strategies: Proceedings of the Workshop on Research in Great Plains Drought Management Strategies, University of Nebraska, Lincoln, March 26-28: Littleton, Colorado, Water Resources Publications, 225 p.) Agricultural drought occurs when there isn’t enough soil moisture to meet the needs of a particular crop at a particular time. Agricultural drought happens after meteorological drought but before hydrological drought. (From: http://www.drought.unl.edu/whatis/define.htm )

Yep. Our crops and your grass have definitely been adversely affected.  Sounds like a drought to me!

For the farmer, the difficulties are threefold:  too much warmth too soon, too little or inconsistent, intermittent rain, and too much sun. Warmer than normal temperatures with abundant rain early in the growing season cause plants to put forth lots of top foliage before they are ready to flower.  If the rain shuts off later in the season as the blossoms are becoming fruits, then the plant struggles to maintain the greenery it initially grew to the detriment of the fruits we look forward to eating.  Warmer than normal temperatures encourage abundant growth, but too much sun, and the tender leaves burn resulting in the loss of entire crops of tender leafy green crops: lettuces, spinaches, etc. and the damaging of newly germinated seedlings.  In fact, sometimes the seeds themselves bake in the too warm soil and never germinate at all.  Rain could to some degree help, but without it, there isn’t much we can do for the seeds already planted. That’s one reason why we do multiple plantings over time of the various crops and have plenty of additional seed on hand.  It’s not a cure-all, but it does mitigate the effects of the drought IF it doesn’t last too long!

So what do we do?  Well, last year we could  water, and did water, sometimes three times a day. The plants were thirsty, and we were glad to do it. Early in the morning, in the evening, and sometimes even in the dark!  With watering cans in hand we walked along the rows, becoming cloudy, indistinct forms after sunset.  There’s something very fairy tale like about watering by moonlight while the deer roam nearby in the hush of the night.

But this year, this crunchy July, we’re giving up our water cans for drip irrigation.  Hooray!  More on that in our next newsletter.

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