What’s with the hair?

Hairy leafed Ficus platypoda

What’s with the hair?

Why do some figs have hairy leaves and what purpose does the hair serve?

It’s a common belief in horticultural circles that plants in arid zones have hair to help prevent the loss of moisture. A recent paper about Ficus brachypoda, seems to confirm this. It mentions that brachypoda’s leaves become more hairy as the environment becomes more arid. Problem solved!

Hairless Ficus brachypoda
Typical glossy leaf Ficus brachypoda, near Uluru.

 

I’m not so sure! The collected specimens of Ficus brachypoda and wild plants that I have seen don’t seem to reflect this. Plants from semi arid areas like Alice Springs and Uluru, have glossy leaves with no visible hair. While the Ficus brachypoda that do have hairy leaves, grow in the tropics. A reverse of what you would expect.

It seems that other hairy leafed figs are also not growing in arid areas. Ficus rubiginosa, in a temperate zone, Ficus platypoda and cerasicarpa in the tropics and Ficus destruens in the wet tropics. All regions that have plenty of rainfall for at least half the year.

So if the hair isn’t there to retain moisture then why is it there?

We have body hair to help our sweat evaporate and in the process lose body heat. I’ve been wondering if the same is true for hairy leaf fig trees.

Growing in the tropics of Western Australia, Ficus platypoda is possibly the hairiest of all the Australian Figs. During the summer these plants are exposed to flooding rains and temperatures sometimes exceed 45 degrees celsius. It would seem that the biggest issue for these plants may be over heating, not a lack of water.

If the hair did prevent the loss of water it could inturn cause the plant to heat up and possible cook. Put some leaves in a plastic bag with some water, leave it in the sun and you will see how easily a plant can cook. With plenty of water available during the hot summer months it seems plausible that the hair increases evaporation and releases excess heat.

When a glossy leaf is cut from a Ficus brachypoda, it can stay looking fresh for days, yet if a hairy leaf is cut from a Ficus platypoda, it will start to wilt within hours. Ficus platypoda’s hairy leaves seem to lose water quicker than brachypoda’s glossy leaves.

Instead of the leaf hairs being a sign of a dry arid environment they may also show the plant grows in a hot, moist environment, where water can be readily used to keep the plant cool.