Physics Photo of the Week

Nature's Thermometer - Rhododendron Leaves

 

Rhododendron leaves are a good temperature indicator, especially when the temperature is between 20 deg F and 32 deg F.  The leaves shrivel up and practically point downwards.  The plants look like they are shriveling up to die!

But no, they are merely responding to the significantly below freezing ambient temperature which was about 20 deg F (-7 deg C).  The next day, Jan 8, 2022, the temperature was above freezing.  The second picture shows my dog investigating the warmer rhododendron leaves that have fully "recovered"


I do not know why or how the leaves respond to temperature like this.  Since it is a physical phenomenon about the temperature that water freezes, it is perhaps a mechanism in which the water in the leaves' cells freezes and the cell walls can stretch to accommodate the the larger volume of the water.  Some plants are destroyed by freezing temperatures - presumably because the expansion of the freezing water ruptures the cell walls as happens to tomato plants.  Most temperate plants do not change shape and are not harmed by freezing as in daffodils which will soon bloom in sunny areas of the south.  I would love to receive a response from a botanist.

Physics Photo of the Week is published weekly during the academic year on Fridays when Warren Wilson College is in session. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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