Wisconsin Fast Plants Network

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Counting Hairs

In a population of Fast Plants, there are a number of traits that students might observe. One trait in particular, hairiness, provides an excellent opportunity for investigations into modeling evolution with artificial selection. The hairy phenotype is highly variable—a population can have plants that range from nearly hairless to quite fuzzy.Yet, the phenotype is not highly sensitive to environmental fluctuation—hairiness will be variable regardless of high/low fertilizer or temperature. The high variability and resistance to environmental variation make hairiness an ideal trait for an artificial selection experiment.

With only a passing glance, students might ask what is so interesting about hairs? But the answer is everything! Plant hairs are not yet fully understood, but scientists hypothesize that hairs play a role in defense against predators, aid in the absorption of water and nutrients, and aid in the reflection of sunlight.


It may seem like a daunting task to count every hair on a plant, but fear not! Observations suggest that hairiness on one part of a plant is correlated to hairiness on other parts of a plant (i.e. a hairy plant is hairy everywhere and a bald plant is bald everywhere). It may be most efficient to count the hairs in one place on each plant, for example along the petiole or the margin of the first true leaf.


There are multiple methods that you can take to make counting hairs simple and enjoyable. No matter which method you choose, make sure that your work area is well lit (use a bright desk lamp, if available). It may be easiest to view the hairs by back-lighting and holding the plant in front of a dark contrasting background. If available, a hand lens can be used to magnify the hairs. (See https://charge.wisc.edu/discovery_lens/ for Fast Plants’ Discovery Lens Kits)

1) Count on The Plant – If you don’t wish to trim leaves off your plants, hairs can be counted by carefully viewing the first true leaf. Make sure to rotate the plant and view it from all directions to avoid miscounting hairs.

2) Remove the First True Leaf – This method is the most simple as it allows the student to easily move and rotate the leaf to take an accurate count of the hairs from all viewpoints.

3) Photograph the Leaf – Students can photograph the leaf (attached to the plant or removed) and can view the photo to count the hairs. Don’t forget to include a ruler in your photo for scale! If your school allows it, you can even have the students use their cell phones to take pictures, then counting the hairs can be homework.


Please also check out the blogpost "In Support of Trichomes" at the following link http://fastplants.ning.com/profiles/blogs/in-support-of-trichome-selection for further information.

If you choose to photograph the leaves, you can then use image-processing software to take your investigation into hairiness even further. ImageJ (free for both Windows and Mac) is a piece of software that allows taking measurements from photos. One application of this could be to measure hair density of a leaf margin or petiole. Instructions for using ImageJ to measure leaf density are as follows.

1) Install ImageJ (free) http://imagej.nih.gov/ij/download.html
2) Open the image that you wish to measure
3) Set the scale
       a) Use the line tool to draw a line on the ruler in your image (circled in red in the image below)
       b) The line should represent a known distance (i.e. a specific number of inches or centimeters, based on the ruler in your image)
       c) In the Analyze menu, click Measure
       d) In the Analyze menu, click Set Scale (see image below)       e) Input your measured values (reported from step C and the known                         distance from B) and click OK (see image above)
4) Use the freeform line tool (circled in red in the image below) to draw a line around the object to be measured            
      a) In the Analyze menu, click Measure
      b) Record the reported value (in the image below, ImageJ is reporting the pictured leaf to be ~15cm in perimeter)



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