Onion outside in

Jun 12, 2018 • 4:53 AM UTC

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Microorganisms

TCIS Outreach

We are a group of students, volunteers and staff working with TIFR Hyderabad's Science Education and Outreach program: http://www.tifrh.res.in/~outreach/

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We started with an onion from the outside. First there’s a dried up layer or ‘tunic’ that we don’t eat.

Tunic of onion

Onion tunic under foldscope The cells in the tunic must be dead. Even the cell walls are disintegrated, quite unlike the dead xylem cells that we saw in matchsticks . Later we found, in the wonderful Walter Dioni’s website, that we might also see crystals of calcium oxalate and other chemicals in the tunic of the onion . That is for other times.
Next below it was a semi-dried layer which too we often discard.

Semi-dried layer of onion

Semi-dried layer under foldscope In this layer the cell walls still remain somewhat visible.
Now come the nice fleshy layers of onion, or ‘scales’. You can remove the skin, or ‘epidermis’, from the outer or the inner side of each scale. The textbook tells us to take the inner epidermis but we tried both sides, about 3 samples of each. The outer epidermis of all the scales was pinkish in color and did not stain so easily. But it did have a nice surprise for us!

Inner and outer epidermis Here is the outer epidermis of one scale:

Outer epidermis of onion scale
Here is the inner epidermis:

Inner epidermis of onion scale

Inner epidermis stained with methylene blue

Inner epidermis with dark field The dark field image is taken with a finger in front of the camera lens held in sunlight. In it the nuclei are seen clearly, better than in the stained slide.
We found that the cells of the inner epidermis were fairly uniform in shape and size. In the outer epidermis the cells were of different shapes and sizes. We were further surprised to find stomata in the outer epidermis! Again, Walter Dioni told us , the scales are actually modified onion leaves. If the scale grew out into a leaf then its outer epidermis would become the lower epidermis of the leaf. That’s why the outer epidermis of each onion scale has stomata and the inner one (usually) doesn’t.
Onion cells are larger than most plant cells: the average size of onion skin cells is about 400 microns . In comparison, most other plant cells are between 10-100 microns in size .
Onion is Allium cepa . We also looked at garlic, or Allium sativum . There is just one epidermis surrounding the garlic clove, which is actually a bulb. This is what it looked like.