Cones, Fruits, and Seeds

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Of Fruits and Seeds

Samuel Butler wrote that “a hen is only an egg’s way of making another egg.” We might think of a plant as a seed’s way of making another seed. For seed-bearing plants, the seed serves several functions: dispersal, dormancy, genetic diversity, and to provide nutrients to the seedling once it germinates. Conifers produce their seeds on cones (though sometimes the cones can be heavily modified and act like the fruits of flowering plants), while flowering plants produce seeds within a fruit. In this section you’ll find lots of information on the anatomy of seeds, the functions of seeds, dispersal mechanisms, and the diversity of seed/fruit/cone morphology. Before reading, it might be helpful to browse the galleries below to get a sense of the diversity of seeds, cones, and fruits.

Gymnosperm Cones
(Click image for gallery of examples)

Angiosperm Fruits
(Click image for gallery of examples)

Where seeds come from

Showing simplified diagram of alternation of generations in a fern

We might be more precise than Samuel Butler was and say that a GAMETOPHYTE, which produces GAMETES which fuse to produce seeds, is a SPOROPHYTE’s way of making another sporophyte. If botany jargon’s not your thing and you’re already weak at the knees, feel free to accept it on authority that a seed is very important and skip ahead to seed function. But if you want to dive into the nitty gritty of seed anatomy and how a seed forms from the development of the fertilized egg (produced by the megagametophyte) in gymnosperms and angiosperms then read on (for more on pollination and fertilization, check the page on flowers).

Plants have a complex life history, with an ALTERNATION OF GENERATIONS, alternating between a DIPLOID sporophyte and a HAPLOID gametophyte. These two stages have very different appearances (similar to insects, like a monarch butterfly, that exhibit complete metamorphosis). Most of the plants we’re familiar with (e.g. paper birches, basil, saguaro cacti) are sporophytes. Sporophytes are plants (the “-phytes” part) that, not surprisingly, produce SPORES by meiosis. Meiosis is reduction division and results in producing 4 haploid spores.

Most of the non-seed plants (e.g. mosses, horsetails, and virtually all ferns) are HOMOSPOROUS, i.e. producing a single type of spore that develops into a bisexual gametophyte. In homosporous plants, spores serve as dispersal units, serving a similar function to seeds, though they’re a single cell in their dispersal stage and largely reliant on water for dispersal. In mosses (see the life history below), the gametophyte is the dominant stage and produce the sporophyte as part of and on top of the gametophyte. The spore takes root in the soil and grows into another gametophyte.

Detailed life cycle of moss, showing alternation between diploid sporophyte and haploid gametophyte stage

Contrast with all seed plants, which are HETEROSPOROUS (hetero = different), and produce MICROSPORES and MEGASPORES. In biology, the male of a species is the individual which produces the smaller gamete, and in heterosporous plants, the microspores develop into the male gametophyte as it produces the smaller, sperm cells. We know male gametophytes as the pollen grains in seed plants. The larger megaspore develops into the female gametophyte (a structure that develops inside of the ovule). In heterosporous, the sporophyte is the dominant stage and they produce the gametophytes as part of a structure on the sporophyte. Gametophytes fuse and develop into seeds, the agent of dispersal. We see the reverse of the life cycle with homosporous plants and the dominant stage is the sporophyte, which gives disperses the next generation through seeds, not spores.

While heterospory has evolved multiple times in plants, it is most prominent in SPERMATOPHYTES, which includes the GYMNOSPERMS (conifers, cycads, ginkgos, and gnetophytes) and ANGIOSPERMS (flowering plants). This clade represents a significant advancement over the more primitive vascular, spore-bearing plants (e.g. ferns, horsetails, mosses, liverworts). While the spores of these plants are effective dispersal units in some circumstances, they are just a single cell and rely primarily on water for their dispersal. Seeds serve a similar function to spores but offered increased protection from drought and could be dispersed by wind, water, and animals. Seeds first evolved in the ancestors gymnosperms, which translates to “naked seeds,” referring to absence of ovary walls which leave unfertilized ovules unenclosed (the seed eventually develops a seed coat). Gymnosperms likely evolved, radiated, and succeeded in response to the desertification associated with Pangea’s interior, where spores provided inadequate protection from the elements. In gymnosperms, seeds form directly on the surface of leaves, which are often heavily modified to form the bracts of CONES (as in pines, spruces, hemlocks, etc), bright red fleshy ARILS of the “fruits” of yew, or fleshy scales of juniper GALBULI (PDF).

The “naked” ovule of a typical gymnosperm as compared to the “encased” ovule of an angiosperm.

In angiosperms (“encased seed”) everything is a bit more evolved and therefore complicated! Seeds still form on modified leaves (called CARPELS), but in contrast to gymnosperms, the OVULES are enclosed within an ovary wall, the “angio” part. During fertilization (well, DOUBLE FERTILIZATION as two sperm enter the carpel), one sperm cell fuses with the egg cell to become the zygote, while the other fuses with the two polar nuclei to become ENDOSPERM, and the integument of the ovule becomes the seed coat. Ugh, right? So there it is, we now finally have ourselves a seed.

Anatomy of a seed

And what a seed. It’s got everything it needs to carry a plant’s genes into the next generation:

  1. An embryo to hold the genes and programming for development of the next-gen sporophyte. It’s always more complicated, and the embryo consists of embryonic cells that have begun to differentiate into the roots (RADICLE), shoot (HYPOCOTYL), and leaves (PLUMULE). Seeds also contain COTYLEDONS, or seed leaves, that typically look very different from the true leaves and are often used in place of endosperm to store nutrients to fuel the seedlings early growth
  2. A seed coat to protect the embryo from the elements and allow for a period of dormancy
  3. Endosperm to nourish the developing seedling after germination

Anatomy of a fruit

Anatomy of a drupe (peach), type of fruit with a single seed surrounded stony endocarp

Angiosperms are so awesome because not only do they get seeds, like gymnosperms, but they also got a cool package to put that seed into, the fruit. Gymnosperms are mostly dispersed by wind, and the seed coat is modified into a wing. You’ll remember the angio part of angiosperms, the ovary wall that encloses the ovule. After fertilization, the ovary wall really kicks into high gear and develops into the PERICARP. The pericarp is like a costume a seed wears to the ball and comes in all sorts of shapes, sizes, colors, and textures. And the primary purpose of all this modification is to aid a plant in the dispersal of its seeds. And with so many ways to disperse your seeds, no wonder there are so many costumes. A plant might modify its pericarp into a big bright red sweet fruit as a delicious offering to animals who eat the fruit and poop out the seeds, a wing that carries the seed on the wind, a floatation device that rafts the seed to a distant island, or a catapult that launches the seed out into the air.

Example labeled fruit anatomy

Functions of a Seed

Plants evolved from aquatic algae in environments along the edge of the ocean. In these habitats they still had access to lots and lots of water. As plants strayed farther from the coasts and wet environments, they faced the ecological problem of drought. While spores worked in wetter areas, seeds were an efficient and effective method of weathering sustained periods of drought. Using plant organs already present, the early gymnosperms twisted, pulled, expanded, engorged, and tweaked their leaves in all different ways to develop the seed, which could store energy, and a tough seed coat to protect from the environment. Angiosperms, which evolved much later, made further modifications to their leaves, evolving big showy flowers and bright shiny fruits, and in doing so, relied more on animals than wind to spread their seeds. But in the end, regardless of all that variation in how seeds and seed containers were made and looked, the functions of seeds remain the same:

  1. Dormancy: Seeds can lie dormant during difficult times of year (extreme cold, drought); seed longevity, the length of time that a seed can remain viable, measured in years, allows a seed to wait for canopy and soil conditions to be just right (black cherry seeds, for example, waiting for a fire remain viable for up to 100 years). Longevity depends on the durability of the seed coat and the viability of the embryo.
  2. Dispersal: Plants can’t walk so they let their seeds do the walking for them. More on this under dispersal syndromes
  3. Diversity: Like you and me, seeds are the product of sexual reproduction, resulting from the fusion of gametes, and are genetically different from their parents
  4. Nourish the embryo: Spores, if you remember, are just a single cell. There’s not much space to pack in nutrients to sustain the early stages of growth in a developing sporophyte. Seeds rely on energy stored in either cotyledons or endosperm to fuel early growth of the seedling. Of course the amount of nourishment provided to the seed depends on the reproductive strategy of the species. Some invest a lot (e.g. hickories), others barely anything (paper birch, balsam poplar)

Dispersal syndromes

Dispersal-ochory Syndromes
If you’re going to fill an ecological niche, there are only so many ways to do so; it’s how you wind up with flying squirrels and sugar gliders looking almost identical even though flying squirrels and whales are closer on the evolution tree than they are to sugar gliders, which by they way have more in common with kangaroos than squirrels, or whales for that matter. The pseudo-doc Darwin IV: Alien Planet takes this idea of CONVERGENT EVOLUTION to describe how an alien ecosystem might look incredibly familiar to our eyes (and ears and noses and taste buds). We could look at the morphology of aliens and interpret how they exist within their native ecosystem by extrapolating out from “syndromes” or suites of adaptations that organisms having for dealing with specific ecological problems. This is essentially how we interpret fossils to understand the behavior of dinosaurs and other extinct species.

We can treat the forest in our backyard as an alien planet, and look for patterns in morphology of fruits and seeds and connect these to dispersal mechanisms. These repeated patterns in morphology associated with various dispersal mechanisms are called DISPERSAL SYNDROMES and are repeated across ecosystems throughout the world. You can work forward and backwards, start with the fruit morphology and determine the dispersal agent or look at a dispersal agent and determine what the seed or fruit should look like. And you can do this as easily in your backyard or while following Alfred Russel Wallace’s footsteps to Malaysia. The types of dispersal that you’ll encounter in our trees are:

  • Anemochory: Dispersal by wind
  • Hydrochory: Dispersal by water
  • Zoochory: Dispersal by animals
  • Barochory: Dispersal by gravity (basically any round fruit that can roll away from the trunk)

Other mechanisms not found in Vermont trees:

  • Autochory: Self-assisted dispersal (e.g. ballistics in jewelweed, filaree spiraling into the ground)

Anemochory (dispersed by wind)

There’s a trade off for wind dispersed seeds, the same that bird’s must face: being light at the sacrifice of not having much stored energy (it’s why no birds hibernate). Most seeds in this category have very little stored nutrients to supply the growth of seedling development before photosynthesis kicks in. There are two types of aerial seeds, winged and fluffed.

Winged Seeds
Virtually all the conifers have winged seeds (except junipers and yew).

  1. Paired samaras: Maples
  2. Single samaras
    1. One wing
      1. Asymmetric: Pines, firs, spruces, larches, hemlock, larches
        1. Toothed wing: Musclewood
      2. Symmetric: Ashes
    2. Two wings: White cedar, birches, elms
  3. Inflated bag in small clusters: Hophornbeam
  4. Round seeds dangling from a curved bract: Basswood

Fluffed Seeds
While winged seeds are rather common in our trees and have a lot of variety in form, fluffy seeds are confined to a single family, Salicaceae (cottonwoods, aspens, poplars, and willows). These seeds are viable only for a few weeks and when they fall it seems like its snowing.

Hydrochory (dispersed by water)

Coming soon...

Zoochory (dispersed by animals)

Animals are important in the ecology of seeds in a couple of important ways. First, they bring seeds from the parent tree to new a location. They can either transport it already in the digestive tract (as with berries, see below) or to be cached for later usage (as with nuts and other "non-perishables"). Transporting the seed is good in that the offspring is less likely to compete with the parent tree but bad in that the new location may not be a favorable one for the seed. But there's a way of betting on getting dropped off in a place that suits your niche.

Second, some species need their seeds to pass through the gut of an animal before it can germinate. This process, called SCARIFICATION, dramatically increases FRUGIVORE (fancy name for an animal that eats fruit) it digests the fleshy part of the pericarp and begins breaking down the TESTA, or seed coat. Seed coats keep a seed in dormancy by preventing the embryo for accessing the essentials for germination: water, oxygen, and light (temperature too, but seed coat doesn't do much to buffer the embryo from this). The seed coat can be broken down by UV light (as in radishes) or frost/thaw cycles (as in sugar maples) or abrasion or acidification (or some combo of these). Once the embryo gains access to water, oxygen, and light it begins the rapid process of growth. Once the animal expels the waste, include the largely undigested seeds, they're deposited in a nice bed of fertilizer.

For nuts or other species that don't have a sweet, tasty fruit surrounding the seed, an animal that ingests the seed kills its chances of making it to the next generation. What to do? These species are similar to K-selected species, species that put a lot of parental investment into offspring. Nut bearing trees put a lot of energy into supporting the early growth of their young seedlings (1800 calories in a pound of acorns). Most of that energy is stored in carbs, but with ample protein and about a third fat by weight. And that's not for the squirrels. Trees that produce fruits on the other hand put most of their energy into producing large sweet, watery enticements surrounding inedible (and often toxic) seeds. Almost no fat or protein in fruit. So an energy dense nut needs protection. Some have hard shells (walnuts), other produce high concentrations of tannins (acorns). But a squirrel is a tenacious beast. And so an added measure of insurance to guarantee some make it to the next generation is through masting. Mast trees produce copious quantities of nuts in one year followed by 1 or more seasons of want. This pattern avoids a squirrel (or other seed predator) population from syncing up with seed production and just destroying the harvest each year.

Birds
Birds are tiny and don't have teeth, and so the fruits aimed at birds are also tiny and don't require much chewing. They also can be high up on a tree and out on tiny branches where a bird can perch. These fruits are often sweet and abundant in each year. Like nectar in flowers, the sweetness is an offering to the disperser: you take my seeds elsewhere and I'll give you this sugary snack in return. And that sweetness takes sunlight to produce. So these fruity fruits (apples, cherries, buckthorn fruits, grapes, etc.) tend to grow in full sunlight. They need to make sure they get deposited in full sunlight and have 2 methods of this. One is by relying on birds, which often perch on edges or fence rows while they digest and then poop out the seeds. The other is by having seeds that are viable for a longer period of time. This way if they're deposited in the forest they can patiently build up a seed bank and wait until an opening in the canopy arises and then spring quickly into action.

Mammals
Mammals are pretty good dispersers, but don't go quite as far

Plesitocene Megafauna
It's likely that most of our really large seeded or large fruit-bearing trees (e.g. Kentucky coffee tree, Osage orange) had co-evolved with the now extinct Pleistocene megafauna, like giant ground sloths, giant beavers, and mastodons. Their ridiculously large seeds are too large to pass through the guts of most our mammals

Resources

  • Trees: Their Natural History by Peter Thomas