The durian (Durio zibethinus) with spiny fruits and seeds covered by a fleshy edible aril
How did the first tropical trees look like?
The Call to Speculation
In July 1944, botanist Edred John Henry Corner found a fruit on the forest floor in a patch of virgin forest on Singapore island and was stumped.
It was hard to place the fruit into a botanical family.
This fruit was a capsule which had a large black seed enveloped with red flesh (called an “aril”) and was hanging from the fruit by a persistent stalk.
Such a fruit could possibly belong to a number of plant families like the Mahogany Family (Meliaceae), Soap-Nut Family (Sapindaceae), Rukam Family (Flacourtiaceae), #Bombax Family (Bombacaceae), #Cacao Family (Sterculiaceae) or even some others.
Corner later confirmed the fruit to belong to a species of Sloanea of the Oil Fruit Family (Elaeocarpaceae). But the initial difficulty of identifying the fruit got Corner thinking. He wondered about the origins of the tropical forest. During his time, there was a train of thought that trees where the ancestral form of flowering plants.
If this premise was accepted, how then did the first trees look like?
Corner noticed that such a construction of the fruit with conspicuous black seeds enveloped by an aril was rare among flowering plants. He further noticed that many flowering plant families had at least some genera or some species with such a fruit construction.
Dwelling further into the matter, he used the huge Legume family (Leguminosae, now all under the Fabaceae) as a case study. He showed that with many genera of legumes there were a spectrum of species showing aril development in the fruit on one end, and on the other end, the bulk of the members exhibited dry capsules or winged seeds.
Corner also found this pattern repeated in many other large plant families.
A mature and open Sterculia capsule exemplifies one aspect of Corner’s Durian Theory that primitive fruit types might have black seeds that stood out against a red fruit wall.
As the study of trees was one of Corner’s fervent pursuits, he analyzed the growth forms of a large number of trees and deduced that thick unbranched trunks with large pinnate or compound leaves is the primitive condition, whereas trees which are highly branched and small leaved represent the advanced condition. The former condition he termed “pachycaul” and the latter he termed “leptocaul”.
Again, the pachycaul condition is rarer among the flowering plants but may be found in many plant families. The latter leptocaul condition however, has come to dominate the tropical forests.
The Durian theory
In 1949, Corner proposed the Durian Theory as a theory of the origin of the modern tree. In this comprehensive theory Corner attempted to give a hint as to the appearance of tropical trees along with the co-evolution with its seed dispersers.
He argued that:
(i) Primitive flowering plants had capsule-like fruits which were often spiny.
(ii) These fruits split open exposing bright red or orange fleshy walls.
(iii) The seeds were relatively large (and therefore had a limited ‘shelf life’), black, and covered with edible red, orange or yellow arils (an arillate seed)that were not detached unless by the action of animals. These characteristics are well exemplified by the Durian (Durio zibethinus), a tropical tree from the #Bombax family (Bomabacaceae) and hence the name of the theory.
(iv) The primitive flowering plant had a pachycaul construction – in other words, an unbranched stem with a huge terminal flower.
Palms such as Metroxylon spp. may be close to the idealized version of Corner’s primitive tropical tree. It has compound leaves, a single unbranched stem, and terminal flowers.
Corner then argued that it is from the modification of these primitive characters that we get the modern condition of fruits drying up into rattling pods with dry seeds, or succulent berries that do not split open.
With these innovations, seeds then attained powers of dormancy and so trees can extend their geographical range.
His discussion also includes the hypothetical situation whereby trees departed from the pachycaul condition and became increasingly branched with slender twigs and are faster growing, making them more efficient in dealing with cold, drought or damage.
It is important to understand that Corner was not saying that the durian is a primitive tree but rather that certain features it possessed were primitive (e.g. the spiny fruit with a fleshy aril covering the seed).
In other respects the modern durian as we know certainly isn’t primitive in the way Corner was idealizing the primitive tree, but the presence of the spiny fruit with arillate seeds in the durian may be indicative of how plants are arrested in one line of progress while branching into another.
These features, Corner suggested, can be akin to signposts marking the evolutionary route within a botanical family.
Fallen but not Forgotten
In the scarcity of fossil evidence to paint a picture of what the primitive flowering plant might look like at that time, Corner attempted to build up a hypothetical image of how a primitive tree might have looked like and why modern forms look the way they look by studying modern forms.
In Corner’s time, his theory was daring and controversial and was often criticized on the basis of only one facet of his theory without considering the rest. Otherwise, it was flatly turned down on the basis that the fossil record does not lend support.
At the same time, the Durian Theory also made it’s way into the textbooks of the day, and at least a few generations of botanists were indoctrinated in Durianology.
Corner’s theory has yet to be fully re-assessed with a modern synthesis of evidence. It might be said however that there is no molecular evidence in support of the Durian Theory.
For instance, the currently known ‘oldest flowering plant lineages’ like the tropical New Caledonian Amborella or the fossil Archaefructus fit Corner’s idea of a primitive plant.
The Durian Theory may for now be a largely forgotten piece of work in a long forgotten era in botanical science. But for those of us who come across his work, it is not difficult to admire his powers of astute observation and broad-minded genius, and his compelling logic.
For what the Durian Theory was worth, Corner fired the imagination of many, and inspired generations of botanists in his day. I am an ardent fan myself.
What do you think about the Durian Theory? Leave your comments below.
[# note that now the Bombax family (Bombacaceae); Cacao family (Sterculiaceae) are both united under the Mallow family (Malvaceae).]
SOURCES OF INFO
Corner, E. J. H. (1940). Wayside Trees of Malaya. Vol. I: 770 pp; vol. II: 228 pl. Singapore: Government Printing Office.
Corner, E. J. H. (1949). The Durian Theory or the origin of the modern tree. Annals of Botany, (N.S.) 13: 367-414.
Corner, E. J. H. (1949). The Durian Theory or the origin of the modern tree. Ann. Bot., (N.S.) 13: 367-414.
Corner, E. J. H. (1953). The Durian Theory extended – I. Phytomorphology 3: 465-476.
Corner, E. J. H. (1954). The Durian Theory extended – II. The arillate fruit and the compound leaf. Phytomorphology 4: 152-165.
Corner, E. J. H. (1954). The Durian Theory extended – III. Pachycauly and megaspermy – Conclusion. Phytomorphology 4: 263-274.
Corner, E. J. H. (1966). The Life of Plants. London: Weidenfeld & Nicholson.
LINKS
DurianPalace.com
The place for durian lovers or those who will come to love durians
David Tng 