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

 


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