If you follow any agronomists on social media, or perhaps are a corn producer yourself, you have likely seen the annual flood of images with corn ears broken in half. Of course, the experience wouldn't be complete without the photographer bragging about the impressive number of kernels around the ear. But - have you noticed that the number is always even?
A recent post on my Twitter account (https://twitter.com/rachel_veenstra/status/1420051713361338368) generated a lot of interest in this question of why corn ears have an even number of kernels around. This blog post is dedicated to answering that question and some new ones that came up in the discussion!
Why does corn always have an even number of kernels around the ear? ... because corn doesn't like to be odd. (joke credits to Joel Schneekloth of Akron, Colorado)
We are currently at R3 (milk stage) in our plots across the state of Kansas for our corn tillering research project. That said, focus for our research has really shifted to ears, their development, and of course, the number of kernels they have!
Setting the Stage
To answer this question, we have to go back in time - let's say around V5 (fifth leaf - five fully expanded, collared leaves).
At this moment in the corn plant's life, important things are happening. The tassel is developing, which means the growing point will be elongating soon. As the tassel is developing, the ears are also initiated and differentiating. At least in my own experience, we don't usually hear much about this early ear development process, only "tassel initiation" and eventually kernel rows as a yield component.
What does "differentiating" mean?
Differentiate means to change, to grow, or to become something different. In this case of reproductive differentiation in corn, I am talking specifically about the development of reproductive organs (ex: the structure of the tassel and ear) and sex determination.
This image shows a tassel in early differentiation stages - the presence of branches means the sex determination process has begun!
Corn is a special crop, because although the plants are monoecious (have both male and female organs on the same individual - hermaphrodites), these organs are separated. We know this "spatial separation" as the ear (female - found part way up the plant) and tassel (male - found at the top of the plant).
The plants DO NOT start out with this distinct sex separation though! When the flowers first develop in these different parts of the plant, BOTH male and female flower components are present - these are called perfect flowers.
The Answer.
In early stages of ear development, small branches grow from the center of the developing cob and ridges form along the ear as these branches arrange themselves in rows. Imagine a cog - such as one from a clock or your machine of choice. If a developing ear in early stages were to be sliced cross-wise, each ridge would be equivalent to a tooth on the cog. These ridges are actually a type of leaf-like structure also part of the flower development in other grass crops.
Each of these ridges will differentiate into two rows of spikelets running down the length of the ear. Because spikelets always develop in pairs, the potential number of rows will always be even, barring any abnormal environmental conditions, pollination failure, or special development cases. (see end of post for some "odd" examples)
This paired development is obvious when looking at an ear of corn based on the kernel alignment. Twin rows will have kernels directly opposite each other, while non-paired rows will be arranged alternately. In this image, the rows in the center are paired.
FUN FACT: If an even number of ridges develops on an ear (such as 8 or 10 - resulting in 16 or 20 rows around), the ear should have very straight rows. BUT, if the number of ridges is odd (such as 7 or 9 - resulting in 14 or 18 rows) the ear is likely to have a twisted appearance. Next time you pick up an ear, check if the number of kernel rows is divisible by 4. If it isn't, are your rows twisted?
Deeper Into the Physiology.
Both tassels and ears are made up of many spikelets (individual floral units) containing multiple florets (individual flowers). Spikelets develop in pairs (from the ridges), and corn has two flowers per spikelet.
In each flower of corn, three stamens (male organs) and one pistil (female organ) are present. In ears, only one of the flowers per spikelet (the upper floret) develops and the stamens are aborted.
Each pistil contains one ovary, which is attached to a single silk (called the "style") and will (assuming pollination & fertilization are successful) become a kernel.
FUN FACT: In some corn varieties, such as Country Gentleman sweet corn, both florets are fully functional. That is, TWO kernels develop from a single spikelet, which results in really "messy", odd-looking ears without organized rows. Check it out! https://www.rareseeds.com/store/vegetables/corn/country-gentleman-sweet-corn
As mentioned, the male or female parts are selectively aborted as the flowers (florets) develop. In the ear florets, all stamens (male parts) are aborted, leaving only the pistil (female). The opposite process happens in tassels. (This process doesn't always work correctly - more on "tassel ears" in this video: https://youtu.be/Yf78JqKp7b0) The abortion process is accomplished with chemical signals transmitted through the plant as hormones.
An Analogy to Remember.
To put this all another way, let's compare a corn ear to a basketball tournament with lots of games (spikelets) - each happening two at a time (thus the even number of rows).
Each game has two teams (florets).
Each team has 4 players - 3 men (stamens) and 1 woman (pistil).
All of the men (stamens) foul out and are eliminated from the game (floral component abortion).
In each game, there is only one winner (the upper floret survives).
The winner of each game, the woman in the higher-ranked team, receives a prize (in our case, the pistil in the upper floret becomes a kernel).
... this "one winner" mentality is not the case for teams that don't follow the rules (looking at you, Country Gentleman sweet corn) - and both women are winners (both florets survive to become kernels).
Examples of situations with an uneven number of kernels around the ear:
Pollination failure or other environmental problems causing kernel abortion.
Corn varieties without orderly rows - each ridge still develops into paired spikelets, which now have two functional flowers each, but breaking the ear in half may result in an odd count because the rows are not aligned!
Interested in learning more about corn reproductive physiology? I've listed the resources I used down below, and am always happy to help you find answers if they're elusive!
Isn't corn cool?!
References.
Bonnett, O. T. (1948). Ear and tassel development in maize. Annals of the Missouri Botanical Garden, 35(4), 269-287. https://doi.org/10.2307/2394693.
Hsu, S. Y. (1976). Microspore development in maize: the maturation of upper and lower flowers among an assortment of genotypes. Retrospective Thesis and Dissertations. 5747. Iowa State University. https://lib.dr.iastate.edu/rtd/5747.
Nielsen, R. L. (2007). Ear Size Determination in Corn. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/EarSize.html.