Sinningia Fruits

[This is only distantly relevant to the current topic, but has it ever occurred to you that there are three eight-letter words in English which contain exactly the same letters, are intimately related in subject matter, but are completely unrelated to one other linguistically?  It is a remarkable example of evolutionary convergence.  See elsewhere for the three words. The subject matter in which they are, ahem, intimately related is reproduction.  Go for it.]

I was going to say that sinningias have typical gesneriad fruits: dehiscent dry capsules.  But then I remembered that about a fifth of all gesneriads are cyrtandras, which have berries.  Some new-world gesneriads (like columneas) have berries too.  Streptocarpus and boeas have dry fruits which spiral as they mature, so that they can unwind to release their seeds.  Nematanthus have berry-like fruits which split open to reveal the seeds.

In view of all that, it seems unwise to claim that there is any such thing as a typical gesneriad fruit.

Nonetheless, there are three things which all gesneriad fruits have in common: two carpels, one locule, and parietal placentation.

Wait wait!  I can explain.

Carpels

Carpels (see the etymology) are the structural subunits of a fruit.  Each nubbin (technical name: drupe) on a blackberry is a carpel.  Each of the five chambers of an okra pod is a carpel.

Carpels are like the ancestral origins of the parts of a fruit.  Imagine a leaf, with seeds along the leaf margin.  This leaf corresponds to a carpel.  In most flowering plants, the leaf edges meet and coalesce, to form an ovary, with the ovules (future seeds) on the inside.  This creates one or more interior chambers (the locules) within the protection of which the seeds mature.

Locules

A bell pepper is a good example of a fruit with a single locule but more than one carpel.  The internal "ribs" where the seeds are attached are called placentas, and mark the edges of the original carpels. In evolutionary terms, the placentas are the descendants of the leaf edges which merged to form the ovary.

One can visualize a fruit that has fewer locules than carpels as one where the walls separating the carpels have "undeveloped" away, leaving fewer internal chambers.  Usually, when there are fewer locules than carpels, there is only one locule, and this is the case in the Gesneriaceae.  Fewer flower parts are considered a sign of great sophistication in the plant kingdom, but beware!

Placentas

In human reproduction, the placenta (see the etymology) is fetal tissue.  However, in plants, the placentas are maternal tissue.  Only the embryo and endosperm are progeny tissue (I'm pretty sure the seed coat is maternal tissue, but I should check that someday).  Through the placentas, the parent supplies nutrition to the progeny.  Or to put it another way, the progeny steal nutrition from the parent.

[This is a fascinating subject.  The mother mammal or mother plant has many chances to reproduce; therefore it does not wish to risk everything on this current opportunity.  The embryo (or seed), on the other hand, has only once chance: this one.  So it is not inclined to a give-and-take attitude.  The parent/seed (or mother/embryo) interaction is a somewhat civilized war: progeny want everything the mother can afford, while the mother organism will impart what it can, given the odds.  The intriguing part is that the mother and the progeny share at least half of their genes (in reality, more than 99%), so that the abilities that the progeny are using against the mother organism are almost certainly abilities that they inherited from the mother -- and which the mother used against its own progenitor!  Think about that the next time you celebrate your mother's birthday.]

This picture of a fruit of Sinningia warmingii (the fruit is actually upside down) shows the two edges of one of the two placentas in a sinningia seed pod.  Try to visualize them as one edge each of the two leaves (carpels) which went into making up the ovary, which became the fruit.  The edges curled around and in, so that the point of junction of the leaves was a little way back from the leaf edge.  When the fruit dehisces (splits open to release the seeds), it does so down the middle of each carpel "leaf", so that each half of the split-open fruit contains one half of each of the two carpels.  In the picture, the boundary between the two carpels runs right between the two placenta edges.

Most sinningia fruits split open along the midrib of the carpel (defined above).  The placentas, which mark the junction between the two carpels, are therefore in the middle of the split fruit wall, as shown in the picture above.

Sinningia gerdtiana has a different mode of opening: it splits along the boundary between the placentas.  This can be seen in a picture.

The picture to the left, from Dale Martens, shows yet another way in which a sinningia fruit can open.  In this fruit of Sinningia speciosa, the capsule opens by splitting along the junction with the calyx, so that the fruit wall pops off like a cap.

Here is another picture from Dale Martens, showing the fruit with the cap removed.

This is not a typical sinningia fruit, because the calyx has seven lobes, instead of the usual five.  This is a result of breeding for large flowers in S. speciosa.  The four sets of placentas are due to the same enlargement.

In this case, the placentas (and thus the seeds) appear to be anchored to the base of the ovary instead of to its walls.