Friday fabulous Flowers: at the stage of seed dispersal

Our Magnolia virginiana, sweet bay, flowered well for pollen dispersal, now the flowers are at the stage of seed dispersal, a gentle reminder of what a fruit really is.  The fruits are an aggregate cone-like fruit that is basically green and protective, so not very attractive, but then the individual fruitlets dehisce rather like little milkweed pods exposing bright red-orange seeds that sometimes dangle on a thread. The actual fruit continues to dry out and turn brown. The seed coats are actually dark brown but covered by a fleshy red-orange aril that contains quite a bit of lipid, just what birds need as they get ready for a southerly migration, which is exactly what these seeds are advertising. 
From our kitchen table it was obvious a flock of birds was after this food source, and it took awhile for us to figure out who was there.  Turned out to be a mixed flock of Swainson's thrushes and red-eyed vireos.  This morning all the seeds exposed were gone; there will be more.  Successful dispersal was achieved!

Seed dispersal display

Fruits are flowers at the stage of seed dispersal.  As such many have attractive displays for the purpose of attracting seed dispersers with the promise of a reward, either the edible fruit itself or a fleshy seed coat, or a fleshy tissue surrounding the seed, an aril.  Here's the seed dispersal display of a sweet bay magnolia (Magnolia virginiana).  Magnolias have many pistils in each flower and each makes a fruitlet, usually a tough little dry follicle that opens along one seam to release or reveal one or two aril-covered seed.  The bright orange-red fleshy aril is both the visual attraction and the reward, the seed has a dark-brown tough seed coat. The seeds actually dangle on a thread to attract even more attention from birds who after digesting off the aril regurgitate the seed.  Displays like this can last days, and disappear quickly when a flock of cedar waxwings stop by.

Multilayer interference iridescence or shiny!

Wow!  Those are some shiny fruits!  Quite a few shiny, colorful things actually lack pigments and their apparent color is due to iridescence, the way then bend and reflect light.  Supposedly these fruits (Pollia condenstata) are the shiniest of all such biological materials.  Now of course plants make attractive fruits to lure seed dispersers, in this case most likely birds, and the birds seek such displays to get a nutritive reward and everyone goes away happy.  However these really, really attractive fruits are deceptive providing no reward at all, so any bird that eats these fruits disperses the seeds and considering some effort was involved bascially gets cheated.  This works because such fruits mimic a similar rewarding plant, in this case possibly a species of Psychotria (right) that has bright blue rewarding fruits.  You don't get fruits colored like this in the temperate zone.

Of fish and figs

One of the more interesting aspects for tropical forests are fish that function as seed dispersers. In Costa Rica the machacas (pictured, ~35-50 cm long) have excellent eyesight above the water and a quite a number will race out from under the over hanging tree crowns to snag a chunk of banana or a fig much to the delight of each year's class of students. In the Amazon other members of the same family, the characins (tetras, piranha), have proven to be excellent dispersers, especially in flooded forests, because fruiting trees are an island of resource, a patch of food, so foragers must move from place to place to find the patches. Fruit-eating fish in that ecosystem can move seed up to 5.5 km, dispersal on a par with large birds and mammals. However, over fishing could have an impact on tree reproduction, an excellent example of interconnections not at first evident.

Rainforest Field Trip - Luscious Lips

On many occasions the Phactor has tried to illustrate the problem plants have in dispersal of their pollen and seeds. The basic rule is pretty simple: attracting dispersers requires a display, and the bigger the display the better the dispersal. Another problem is that flowers are often either too small or too ephemeral to make a large, long-lasting display, so very often bracts, modified leaves associated with flowers, are used to produce larger, long-lasting displays for many plants. Some of these displays last long enough to also serve in seed dispersal, and this pair of red bracts belonging to Psychotria poeppigiana in the coffee family, looking quite like a pair of luscious lips, lasts long enough that after attracting pollinators to the rather small white flowers, they attract seed-dispersers to the blue fruits. With only 1 fruit remaining, their job is about done, and the lips aren't looking so kissable.

Attractive display for dispersers of tropical plant

It's been awhile since blogging about the tropics. Those of us who study flowers are used to seeing diverse displays to attract pollinators. This is also true for fruit displays attracting seed dispersers. Now of course both of these are floral displays, but at different times in the flower's functional life, and most people just aren't used to thinking of fruits as flowers at the time of seed dispersal.

What is truly unusual is to have the same display being used for attracting both pollen and seed dispersers because they are almost always at different times and for different organisms. This a member of the Gesner (African violet) family from Costa Rica, Columnea purpurata, and the leaves are arranged to make a flat array along a stem forming a canopy over its flowers and fruits. So this is photographed from below as this branch extends out over your head (it grows as an epiphytic shrub). Both the flowers' persistent calyx and the subtending bract are bright orange and hairy, and the entire cluster of flowers, at both dispersal stages, produces a single display. At the right hand end of the cluster you can just see a yellow corolla tube and at the left hand end an orange berry is visible (arrows). The reason one display works for both dispersal stages is that birds are the dispersers of both pollen (hummingbirds) and seed (diverse frugivores), and such a large, bright display is needed in the shady understory.

Ludicrous fruit display

The Phactor noticed this ludicrously colorful fruit display this morning on the tree outside his building. What a wonderful presentation this fruit display makes! Looks like something from a Georgia O'Keefe painting. Do you recognize this common genus in fruit?

This fruit is from the bigleafed magnolia (Magnolia macrophylla). The pendent, cone-like fruit is a lurid pink and consists of fruitlets, one from each pistil, in the form of a one or two-seeded follicle (like a milkweed pod). When the fruitlet splits open the seeds dangle forth on a thread and are covered by a fleshy pink-orange aril, a reward for the bird dispersers. The actual seed is black and has a hard shiny seed coat. This time of year a flock of grosbeaks or cedar waxwings will find this tree and an hour later every single seed will be gone.

Unusual tomato plants

OK, quick tomato growers quiz. What's wrong with these tomato plants? Nothing actually; they are normal enough tomato seeds sprouting normal enough tomato plants. What makes them unusual is that they are sprouting from inside an intact tomato fruit, and you don't see this happen very often.

Like many plants, seeds inside tomatoes are inhibited from germinating. Fleshy fruits function by attracting and rewarding animal seed dispersers, and they broadly fall into two categories: fruits with many small seed and fruits with few, large-seeds. The former get eaten with the fruit, pass through the GI tract unscathed, and get deposited in a nitrogen rich environment ready to grow. Some seeds actually require passage through the GI tract to readily germinate. The seeds are numerous enough that while a few get destroyed by chewing, the majority get swallowed intact. And the digestion process removes the inhibiting tissue from around the seed, and may even render the seed coat more permeable.

When seeds get larger, animal dispersers like ourselves generally pick them out and discard them rather than swallowing them. Something about the size of a watermelon seed is about the break point on swallowing versus picking out. Fruits with big seeds (grapefruit) or pits (inner part of fruit with enclosed seed) like peaches and mangos are no brainers. You can follow fruit eating monkeys through a rainforest and see, based on what they drop and defecate (don't follow too closely), that their choices are very similar to yours.

For some reason, probably having something to do with the way it was stored, the environmental conditions overroad the inhibitors and kicked these tomato seeds into premature germination, so here they are growing their way out of the fruit.

HT to a curious botanical colleague.