S. Elwynn Taylor
Missouri Botanical Garden Bulletin,
Vol. LVIII, No. 5, Sept.-Oct. 1970, pp. 14-17

Is the extreme tattering of banana leaves an accidental result of wind damage, or is it a significant adaptive characteristic of the banana family? Most who have visited tropical climates have noticed that the leaves of the banana family (Musaceae) are shredded to the point that it is hard to imagine that no serious damage has been done to the leaf. When one looks closely at the leaves, it appears that very little damage is done to the leaf by tearing. The veins of the banana leaf are arranged such that it seems as if the leaf were designed to be torn. One can speculate that in high winds the tearing of the leaf along the vein lines saves the banana plant from blowing down. This in itself indicates that the tattering may be a significant leaf adaptation, but research at the Garden suggests that the tearing has several other implications.

A small leaf on a hot sunny day will be closer to air temperature than a large leaf. The rate of heat transfer from a leaf to the air is somewhat dependent on the size of the leaf; a small leaf transfers heat to the air more rapidly than a large leaf.

The effect of the size of a leaf on its temperature has been dramatically demonstrated using a Bird of Paradise (Strelitzia nicolai) growing in the Climatron. I tore one side of a leaf into strips 2-4 cm wide (about 1-1.5 inches) closely resembling natural tearing (Figure a).

Fig. a. One leaf of a Bird of Paradise (Strelitzia nicolai) in the Climatron has been torn from the edge to the mid rib forming many leaf strips about one inch wide. The tearing resembles that observed for members of the banana family growing in the tropics. Only one side of the leaf has been torn.

I compared the temperature of the torn side with the side of the leaf that was intact. With the aid of a very special camera that is sensitive to temperature differences, a picture known as a thermogram was taken (Figure b).

Fig. b. Color heat sensing camera images of Bird of Paradise. Red is 47.5°C. Dark blue is 10 to 20 degrees cooler.
Fig. c. A heat sensing camera is used to make a thermogram (temperature picture) of a portion of the leaf. The edges of the leaf are near air temperature but the leaf temperature is considerably warmer than the air near the mid rib. The mid rib itself is disproportionally cool, probably because of a high localized transpiration rate. The divided side of the leaf averages much nearer air temperature than does the untorn side. When the leaf is exposed to high natural heat loads, the divided side will not suffer heat damage as severely as will the untorn leaf. The high temperature in the mid black area is 48°C (118°F); the white line indicates a temperature of 46.5°C (116°F) and the edge of the dark area where the picture disappears is 45°C (113°F).

The mid rib proved to be quite cool; apparently transpiration is greatest near the mid rib and evaporative cooling is intensified there. The leaf is quite hot a short distance on either side of the mid rib. The highest temperature was 48°C (118°F) which is hot enough to kill the leaf. The leaf was more than 3°C (5.4°F) cooler at its edges. Tearing the leaf produced more edges, and hence the torn leaf had an increased proportion of its surface at the "edge temperature." When the environmental conditions are such that a tattered leaf heats to 46°C (115°F), an untorn leaf would die, being 3°C warmer. A leaf temperature of 49°C (120°F) is lethal to most members of the banana family.

Fig. d. The leaf shown in Figure b was heated to the thermal death point (47.5°C) in two locations.

Tearing also has a marked effect on transpiration rate. The torn half of the leaf expended only one-third of the total water loss of the leaf. A torn leaf in this particular environment uses only one-half the water required by a similar untorn leaf!

Recent work in our laboratory points out another characteristic of the torn leaf. The net photosynthesis (productivity) is much higher for a torn leaf than for an intact one, partly because the torn leaf is at a more favorable temperature for photosynthesis. There is also an enhancement of CO2 exchange from the air to the leaf. Carbon dioxide is taken into leaves and utilized during photosynthesis. Just as a small leaf transfers heat to the air more efficiently than does a large leaf, carbon dioxide is transferred from the air to the small leaf more efficiently.

In the Canal Zone the majority of leaves (or leaflets) growing exposed to full sun are less than 10 cm (4 inches) wide. Leaves this small will be close enough to air temperature to prevent heat damage under the most extreme weather conditions occurring there. The banana family, however, is conspicuously out of place amid these small leaves, and the size of the banana leaves makes them very vulnerable to heat damage. It is entirely possible that the tearing so common in this family, rather than a mere result of high winds, is an adaptive characteristic enabling the banana to survive the harsh exposed tropical environment without severe thermal damage, without expending excessive amounts of water, and with enhanced net photosynthesis.