Green Scene

No, these aren’t cigars — they’re rhododendron leaves

Posted

Years ago, I remember a film discussing how difficult it is to conceptualize the need for various measurements. Today, as I write this column, I am home waiting for a major snowstorm, and temperature is on everyone’s mind. 

But how is temperature measured? Hot and cold are relative terms, and the state of water may not be an intuitive benchmark.

It is unclear whether the ancients had any way of accurately measuring temperature, although there is evidence they were grappling with the issue. The development of the thermometer as an instrument that could define temperature in units of equal size began in the Renaissance. 

Before there were thermometers, there were thermoscopes, incompletely sealed devices that demonstrated changes in water volume due to temperature changes. The etymology of the thermometer is from the Greek thermos meaning “hot” and metron from “measure.” In 1654, Ferdinand II, Grand Duke of Tuscany, invented the first sealed thermometer using alcohol as the liquid.

Robert Hooke, 17th century scientist with interests ranging from biology to architecture, was the inventor of the compound microscope, opening the doors of microscopic life to scientists. However, he also was the first person to suggest the freezing point of water should be a starting point for measuring temperature. 

In 1714, Daniel Gabriel Fahrenheit refined the alcohol-based thermometer of Danish astronomer Olaus Roemer by using mercury as the enclosed liquid. Mercury was the liquid of choice because it had a linear and identical response to increments of heat.

  In 1724, Fahrenheit wrote about his eponymous measuring system, establishing 32 degrees as the freezing temperature of water, and 212 degrees as its boiling point. A mixture of water, ice and the salt ammonium chloride served as the baseline of Fahrenheit’s new system at 0 degrees.

I, however, have a crude botanical measure of extreme cold. When we moved into our present house, there was a rhododendron outside the kitchen window. Our first winter, I noticed that the rhododendron did something peculiar when the temperature was severely cold. The broad, oval, leathery leaves would hang straight down and curl up cigar-like. As soon as the temperature warmed up, they would unfurl and lift up, no worse for the wear. 

This way I could easily decide how to dress for the outdoors.

Years later, while walking through the Lady’s Border at the New York Botanical Garden, I spotted a small tree that appeared rhododendron-like, but somehow daintier. Finally spotting the plant label, I learned that I was looking at a Daphniphyllum macropodum from the order Saxifragales and family daphniphyllaceae. It’s unrelated to rhododendrons, which belong to the order Ericales and the family ericcaceae. 

At the time, it was quite difficult to find one for sale locally. Finally, I bought one and planted it in the front yard, and that first winter, it went through the same routine of leaf hanging and leaf curling that I already experienced with my rhododendrons. 

Last winter, I finally decided to establish the temperature parameters of the Daphniphyllum (alas, my rhododendrons had died). Keeping records, it seems to me that the cutoff temperature is 26 degrees. Warmer, the leaves appear normal; colder they curl up, hang down, and look thoroughly miserable.

This observation has lead to a certain amount of frustration. Except for me, the only person interested in this question is Erik Nilsen, a scientist at Virginia Tech in Blacksburg whose research focused only on rhododendron leaf movements. Nilsen informed me that Seok Gon Park has investigated leaf movements in Daphniphyllum.

There are about 1,000 rhododendron species, and they are widely distributed, inhabiting tropical zones (cold hardiness to 50 degrees Fahrenheit), arctic, alpine, and temperate zones (cold hardiness to minus-58 degrees. 

The genus name comes from the Greek rhodon, meaning “rose,” and dendron for “tree.” Rhododendrons originated about 65 million years ago in the circumboreal area, the temperate to arctic portions of North America and Eurasia.

Nilsen has divided the phenomenon into two separate categories — leaf-drooping and leaf-curling. Nilsen has shown that leaf drooping is a function of light intensity, leaf turgor potential (the flow of water into the cell that governs pressure and tissue stiffness) and temperature. 

Plants, surprisingly, can be damaged by too much exposure to intense light. Therefore, the leaves protect themselves by lowering themselves, thereby controlling light exposure on the leaf blade.

Leaf curling has nothing to do with turgor, which is demonstrated by minimal leaf curling under conditions of total turgor loss. Despite curling under experimental conditions of total darkness, Nilsen believes that curling is a mechanism to protect photosynthetic machinery from too much light in the wintertime when there is little shade in the forest.  It seems that leaf curling, however, is “positively correlated with the relative cold hardiness of the species.” 

I prefer to think that even the leaves are shivering!

Have a thought or comment for Sura Jeselsohn? Email her at greenscenesura@gmail.com.

Daniel Gabriel Fahrenheit, Olaus Roemer, Robert Hooke, Sura Jeselsohn,

Comments