Thursday, 23 April 2015

Feminine sophistication

Gynoecium is without a doubt the most complex organ in a flower. It is here where all the magic of fertilization happens, where pollen meets the ovule and embryos are formed. It has the responsibility of protecting, nurturing and, ultimately, spreading the seeds, not as gynoecium anymore, but as the fruit – the result of its the development. Of course a structure where most action (and purpose) in a plant’s life takes place must be of great sophistication.

Some species, not happy with having solely an ovary, decided to create a peculiar one. In Punica granatum (Lythraceae), the pomegranate – the gynoecium is a highly complex organ and it’s hard to compete with such unique structure.

Pomegranate flower (right) and developing fruit (left).
Foto: Reji

Carpels are placed in different whorls (usually 2 to 3 whorls of carpels), which culminate on a great mess of carpels, arranged in superposed layers in one single ovary (please, take a look at the image!).

Scheme of pomegranate ovary showing layered whorls of carpels. (Original picture: Sinha & Joshi, 1959)

Generally (syncarpous) ovaries are placed in one single whorl due to space constraints. A multi-whorled ovary implies a great deal of logistics, and I wonder what happened in the course of evolution to have such a complex structure selected, a rarity among angiosperms.

But Punica granatum is also unique with respect to the placentation of the ovules. If you observe the ovary, you can see that basal carpels tend to have axile placentation, whereas upper carpels seem to have parietal placentation. Such differences are related with the development of the ovary, but before despair, take a look at the picture below and allow me to explain what is going on in this ovary.

Schematic representation of the growth of carpels. Placentation is axile in all whorls but in the uppermost whorl the placenta is always on the peripheral side of the locule because of the carpels growth. (Original picture: Sinha & Joshi, 1959)

What happens in P. granatum is that placentation is always axile, including the upper whorl. However, because carpels are on the inner surface of a deep floral cup, the inclination of the carpels of the upper (outer) whorl is different from that in the lower (inner) whorl. Thus, the carpels of the upper whorl are directed obliquely downwards, giving the wrong impression of having a parietal placenta. That they are not parietal can already be seen in the fact that they have septa, as gynoecia with parietal placentation have no septa.

Axile vs. parietal placentation

Now, before finishing this post, allow me to clarify the structure of Punica’s fruit. The mesocarp (the fleshy tissue that we eat in many fruits, such as apples, pears, plums, apricots, etc) is represented in pomegranates as a thin white layer – the tissue wall that separates the carpels ovaries. The fleshy part of this fruit is unusually the testa of seeds (or seed coat), and it is called a sarcotesta – the botanical name for fleshy testa. The sarcotesta is many times described as an aril, but it has nothing to do with the aril found in Taxus fruits, which is an appendage of the fruit – different from the outermost layer of the seed. 

The edible part of most fruits is the ovary's mesocarp; in Punica granatum we eat seeds' sarcotesta instead.
(Foto: Eric Beaume)

The symbolism of the pomegranate is present in many cultures from Western Asia to the Mediterranean basin. They tend to represent fertility and prosperity, which is not surprising regarding the numerous seeds each fruit bears and the incredible complexity around an already complex organ - the ovary.