Gymnosperms and Fleshy "Fruits"

Fleshy red aril surrounding the seeds of  Taxus baccata.

Fleshy red aril surrounding the seeds of Taxus baccata.

Many of us were taught in school that one of the key distinguishing features between gymnosperms and angiosperms is the production of fruit. Fruit, by definition, is a structure formed from the ovary of a flowering plant. Gymnosperms, on the other hand, do not enclose their ovules in ovaries. Instead, their unfertilized ovules are exposed (to one degree or another) to the environment. The word “gymnosperm” reflects this as it is Greek for “naked seed.” However, as is the case with all things biological, there are exceptions to nearly every rule. There are gymnosperms on this planet that produce structures that function quite similar to fruits.

Cross section of a  Ginkgo  ovule with red arrow showing the integument.  Photo copyright Bruce Kirchoff, Licensed under CC-BY

Cross section of a Ginkgo ovule with red arrow showing the integument.

Photo copyright Bruce Kirchoff, Licensed under CC-BY

The key to understanding this evolutionary convergence lies in understanding the benefits of fruits in the first place. Fruits are all about packing seeds into structures that appeal to the palates of various types of animals who then eat said fruits. Once consumed, the animals digest the fruity bits and will often deposit the seeds elsewhere in their feces. Propagule dispersal is key to the success of plants as it allows them to not only to complete their reproductive cycle but also conquer new territory in the process. With a basic introduction out of the way, let’s get back to gymnosperms.

“Fruits” of  Cephalotaxus fortunei  (Cephalotaxaceae)

“Fruits” of Cephalotaxus fortunei (Cephalotaxaceae)

There are 4 major gymnosperm lineages on this planet - the Ginkgo, cycads, gnetophytes, and conifers. Each one of these groups contains members that produce fleshy structures around their seeds. However, their “fruits” do not all develop in the same way. The most remarkable thing to me is that, from a developmental standpoint, each lineage has evolved its own pathway for “fruit” production.

Ginkgo  “fruits” are full of butyric acid and smell like rotting butter or vomit.

Ginkgo “fruits” are full of butyric acid and smell like rotting butter or vomit.

For instance, consider ginkgos and cycads. Both of these groups can trace their evolutionary history back to the early Permian, some 270 - 280 million years ago, long before flowering plants came onto the scene. Both surround their developing seed with a layer of protective tissue called the integument. As the seed develops, the integument swells and becomes quite fleshy. In the case of Ginkgo, the integument is rich in a compound called butyric acid, which give them their characteristic rotten butter smell. No one can say for sure who this nasty odor originally evolved to attract but it likely has something to do with seed dispersal. Modern day carnivores seem to be especially fond of Ginkgo “fruits,” which would suggest that some bygone carnivore may have been the main seed disperser for these trees.

“Fruits” contained within the female cone of a cycad ( Lepidozamia peroffskyana ).

“Fruits” contained within the female cone of a cycad (Lepidozamia peroffskyana).

The Gnetophytes are represented by three extant lineages (Gnetaceae, Welwitschiaceae, and Ephedraceae), but only two of them - Gnetaceae and Ephedraceae - produce fruit-like structures. As if the overall appearance of the various Gnetum species didn’t make you question your assumptions of what a gymnosperm should look like, its seeds certainly will. They are downright berry-like!

Berry-like seeds of  Gnetum gnemon .

Berry-like seeds of Gnetum gnemon.

The formation of the fruit-like structure surrounding each seed can be traced back to tiny bracts at the base of the ovule. After fertilization, these bracts grow up and around the seed and swell to become red and fleshy. As you can imagine, Gnetum “fruits” are a real hit with animals. In the case of some Ephedra, the “fruit” is also derived from much larger bracts that surround the ovule. These bracts are more leaf-like at the start than those of their Gnetum cousins but their development and function is much the same.

Red, fleshy bracts of  Ephedra distachya .

Red, fleshy bracts of Ephedra distachya.

Whereas we usually think of woody cones when we think of conifers, there are many species within this lineage that also have converged on fleshy structures surrounding their seeds. Probably the most famous and widely recognized example of this can be seen in the yews (Taxus spp.). Ovules are presented singly and each is subtended by a small stalk called a peduncle. Once fertilized, a group of cells on the peduncle begin to grow and differentiate. They gradually swell and engulf the seed, forming a bright red, fleshy structure called an “aril.” Arils are magnificent seed dispersal devices as birds absolutely relish them. The seed within is quite toxic so it usually escapes the process unharmed and with any luck is deposited far away from the parent plant.

The berry-like cones of  Juniperus communis .

The berry-like cones of Juniperus communis.

Another great example of fleshy conifer “fruits” can be seen in the junipers (Juniperus spp.). Unlike the other gymnosperms mentioned here, the junipers do produce cones. However, unlike pine cones, the scales of juniper cones do not open to release the seeds inside. Instead, they swell shut and each scale becomes quite fleshy. Juniper cones aren’t red like we have seen in other lineages but they certainly garnish the attention of many a small animal looking for food.

I have only begun to scratch the surface of the fruit-like structures in gymnosperms. There is plenty of literary fodder out there for those of you who love to read about developmental biology and evolution. It is a fascinating world to uncover. More importantly, I think the fleshy “fruits” of the various gymnosperm lineages stand as a testament to the power of natural selection as a driving force for evolution on our planet. It is amazing that such distantly related plants have converged on similar seed dispersal mechanisms by so many different means.

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8]

Further Reading: [1] [2] [3] [4] [5] [6] [7]

Sequential Ripening

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There are few things better than hiking on a hot summer day and coming across a big patch of ripe blackberries and/or raspberries. If you're anything like me then you promptly gorge yourself on handfuls of these sweet aggregate fruits. However, the genus Rubus never gives its fruit away all at once. Although this may seem like a pain for us humans, there is good reason for it.

The answer to this ripening strategy lies in the seed dispersers. A multitude of animals feed on the fruit of the genus Rubus but by and large the best seed dispersers are birds. Rubus fruits begin to ripen around the time when many birds are beginning to ramp up their food intake to prep for either migration or the long winter to come. Regardless, birds can travel great distances and thus can spread seeds via their droppings wherever they go.

If Rubus were to ripen their fruit all at once, only a handful of birds would make use of the entire seasons reproductive effort. This means that all the seeds of an individual plant would likely fall to the ground in the general vicinity of the parent. By sequentially ripening their fruit, Rubus ensure that their seeds will not only be available for a few weeks to a couple of months, it also ensures that birds, as well as many other animals, will be involved in the distribution of seeds. It's not just the genus Rubus that does this either. Plenty of other berry producing plants ripen their fruitss sequentially. It is a wonderfully successful strategy to persuade mobile organisms to do exactly what the plants require. 

Photo Credit: Nicholas A. Tonelli (http://bit.ly/1q6Gvja)

Further Reading:
http://bit.ly/29ghcwL

Is it a pine? Is it an apple? It's neither!

Pineapples - the fruit that is neither a pine nor an apple. In reality, pineapples are a type of bromeliad. The genus to which they belong, Ananas, is comprised of something like 7 different species, all of which are native to Central and South America. Considering we rarely encounter these plants outside of a grocery store, it is no wonder then that many are surprised to realize how pineapples grow.

The fruit itself is not the entire plant. It is made up of many fruits that fuse together after flowering. The flowers themselves are quite lovely and originate from the center of the hexagonal units that make up the tough rind. The whole inflorescence arises from the center of a large rosette of leaves. Only when you see the entire plant does the bromeliad affinity become apparent. Like all other bromeliads, pineapples undergo vegetative reproduction as well. Small offshoots called "pups" arise from the base of the plant and the axils of the leaves. These can take root and grow into clones of the parent plant.

In the wild, pineapples require pollination to set seed. This is undesirable in cultivation because pollination means lots of seeds that consumers don't want to contend with. Because of this, pineapples are gassed with ethylene, the simplest of plant hormones. Ethylene causes the fruits to artificially ripen without being pollinated. In this way, no ovules mature into seeds.

The dirty little secret about pineapple farming is that it is done at great environmental cost. The dominant producer of pineapples is Costa Rica. Because of the humid, tropical climate, insects and fungi flourish. In order to ensure that production is maximized, pineapple farmers dump thousands of gallons of pesticides and herbicides onto their crops. These farms are largely void of all other lifeforms save for endless hectares of pineapples. This, however, is not a story unique to pineapple farming. The same could be said for all other forms of monoculture farming.

Photo Credits: Fractalux, H. Zell, and hiyori13 - Wikimedia Commons

Further Reading:

http://www.kew.org/science-conservation/plants-fungi/ananas-comosus-pineapple

http://www.theguardian.com/business/2010/oct/02/truth-about-pineapple-production

Fiery Peppers - Evolution of the Burn

Love them or hate them, one must respect the fiery chili pepper. If you're like me then the addition of these spicy fruits can greatly enhance the culinary experience. For others, spice can be a nightmare. Peppers are so commonplace throughout many cultures of the world that it is easy to overlook them. As a plant fanatic, even the simple act of cooking dinner opens the door to so many interesting questions. What is a pepper? Where do they come from? And why are some so spicy?

Peppers evolved in the Americas. The genus to which they belong, Capsicum, is comprised of somewhere around 27 species. Of these, five have been domesticated. They have no relation whatsoever to black pepper (Piper nigrum). Instead, the chili peppers are relatives of tomatoes, potatoes, and eggplants - family Solanaceae.

The fruit that they produce is actually a type of berry. In the wild, Capsicum fruits are much smaller than the ones we buy at the farmers market or grocery store. Centuries of domestication has created such gaudy monsters. The spicy effect one experiences when biting into a pepper is the result of a chemical called capsaicin. It is mainly produced in the placental tissues and the internal membranes. It is in its highest concentrations in the white pith that surrounds the seeds.

Capsicum chinense

Capsicum chinense

As with any fruit, the main goal is seed dispersal. Why then would the plant arm its fruits with fiery capsaicin? The answer to this riddle lies in their wild relatives. As mentioned, the fruits of wild peppers are much smaller in nature. When ripe, they turn bright shades of reds, yellows, and oranges. Their small size and bright coloration are vivid sign posts for their main seed dispersersal agents - birds.

As it turns out, birds are not sensitive to capsaicin. Mammals and insects are, however, and that is a fact not lost on the plants. Capsaicin is there to deter such critters from feeding on the fruits and wasting hard earned reproductive efforts. As such, the well defended fruits can sit on the plant until they are ripe enough for birds to take them away, spreading seeds via their nutrient rich droppings.

It may be obvious at this point that the mammal-deterring properties of Capsicum have been no use on humans. Many of us enjoy a dash of spice in our meals and some people even see it as a challenge. We have bred peppers that are walking a thin line between spicy and dangerous. All of this has been done to the benefit of the five domesticated species, which today enjoy a nearly global distribution. Take this as some food for thought the next time you are prepping a spicy meal.

Photo Credits: Ryan Bushby, André Karwath, and Eric Hunt - Wikimedia Commons

Further Reading:
http://link.springer.com/article/10.1007%2FBF00994601

http://www.jstor.org/stable/4163197…

http://www.press.uchicago.edu/ucp/journals/journal/ijps.html