angiosperms

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Pine Cones Are Like Hangars for Pine Tree Seeds

Over the past year I’ve written about the making of pine tar and the drinking of pine needle tea. But why stop there? Pines are a fascinating group of plants, worthy of myriad more posts, and so my exploration into the genus continues with pine cones and the seeds they bear.

Pines are conifers and, more broadly, gymnosperms. They are distinct from angiosperms (i.e. flowering plants), with the most obvious distinction being that they don’t make flowers. Since they are flowerless, they are also fruitless, as fruits are seed-bearing structures formed from the ovary or ovaries of flowering plants. Pines do make seeds though, and, as in angiosperms, pollen is transported from a “male” organ to a “female” organ in order for seeds to form. Rather than being housed in a fruit, the seeds are essentially left out in the open, which is why the term “naked seeds” is frequently used in reference to gymnosperms.

seed cone of Scots pine (Pinus sylvestris ‘Glauca Nana’)

In the case of pines and other conifers, the seeds may be naked, but they’re not necessarily homeless. They have the protection of cones, which is where the female reproductive organs are located. Male, pollen cones are separate structures and are smaller and less persistent than the cones that house the seeds. A cone, also known as a strobilus, is a modified branch. A series of scales grow in a spiral formation along the length of the branch, giving the cone its shape. On the inside of these scales is where the seeds form, two per scale. First they are egg cells, and then, after pollination and a period of maturation, they become seeds. The scales protect them throughout the process and then release them when the time is right.

With more than 120 species in the genus Pinus, there is great diversity in the size, shape, and appearance of pine cones. While at first glance they don’t appear all that different from one another, the cones of each species have unique characteristics that can help one identify the pine they fell from without ever having to see the tree. Pine cones are also distinct from the cones of other conifers. For one, pine cones take at least two or, in some cases, three years to reach maturity, whereas the cones of other conifers develop viable seeds in a single year. Pine cones are also known to remain on the tree for several years even after the seeds are mature – in some species up to 10 years or more – and they don’t always part with their seeds easily. Lodgepole pines (Pinus contorta) require high temperatures to melt the resin that holds their scales closed, the cones of jack pine (P. banksiana) generally only open in the presence of fire, and the seeds of whitebark pine (P. albicaulis) are extracted with the aid of birds (like Clark’s nutcracker) and other animals.

immature seed cone of lodgepole pine (Pinus contorta)

Every pine cone is special in its own right, but some stand out in particular. The largest and heaviest pine cones are found on Coulter pine (P. coulteri), measuring up to 15 inches long and weighing as much as 11 pounds with scales that come to a sharp point. It’s understandable why the falling cones of this species are frequently referred to as widowmakers. Longer cones, but perhaps less dangerous, are found on sugar pine (P. lambertiana). The tallest trees in the genus, the cones of sugar pine consistently reach 10 to 20 inches long and sometimes longer.

Pine tree seeds are a food source for numerous animals, including humans. Most are so small they aren’t worth bothering with, however, several species have seeds that are quite large and worth harvesting. Most commercially grown pine nuts come from stone pine (P. pinea) and Korean pine (P. koraiensis). In North America, a wild source for pine nuts is found in the pinyon pines, which have a long history of being harvested and eaten by humans.

immature seed cone of ponderosa pine (Pinus ponderosa)

The seeds of many pines come equipped with little wings called samaras, which aid them in their dispersal. Upon maturity, pine cone scales open and release the seeds. Like little airplanes leaving the hangar, the seeds take flight. Wind dispersal is not an effective means of dispersal for all pines though. A study published in Oikos found that seeds weighing more than 90 milligrams are not dispersed as well by wind as lighter seeds are. When it comes to long distance dispersal, heavier seeds are more dependent on animals like birds and rodents, and some pines rely exclusively on their services. The author of the study, Craig Benkman, notes that “bird-dispersed pines have proportionately thinner seed coats than wind-dispersed pines,” which he points out in reference to Japanese stone pine (P. pumila) and limber pine (P. flexilis), whose seeds weigh around 90 milligrams yet rely mostly on birds for dispersal. Benkman suspects that the seeds of these two species “would probably weigh over 100 milligrams if they had seed coats of comparable thickness as wind-dispersed seeds.”

Whitebark pine, as mentioned above, holds tightly to its seeds. Hungry animals must pry them out, which they do. Pine seeds are highly nutritious and supplement the diets of a wide range of wildlife. Some of the animals that eat the seeds also cache them for later. Clark’s nutcrackers are particularly diligent hoarders, harvesting thousands more seeds than they can possibly consume and depositing them in small numbers in locations suitable for sprouting.

Even large seeds that naturally fall from their cones have a chance to be dispersed further. As the seeds become concentrated at the base of the tree, ground-foraging rodents gather them up and cache them in another location, which Benkman refers to as secondary seed dispersal.

Particularly in pine species with wind dispersed seeds, what the weather is like helps determine when the hangar door will open to release the flying seeds. When it is wet and rainy, the scales of pine cones close up. The seeds wouldn’t get very far in the rain anyway, so why bother? When warm, dry conditions return, the scales open back up and the seeds are free to fly again. You can even watch this in action in the comfort of your own home by following the instructions layed out in this “seasonal science project.”

immature seed cones of limber pine (Pinus flexilis)

mature seed cones of limber pine (Pinus flexilis)

Further Reading:

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Photos of pine cones were taken at Idaho Botanical Garden in Boise, Idaho

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Seed Oddities: Apomixis and Polyembryony

Plants have uncanny ways of reproducing themselves that are unparalleled by most other living things. Offshoots of themselves can be made by sending out modified stems above or beneath the ground which develop roots and shoots (new plants) at various points along the way. Various other underground stem and root structures can also give rise to new plants. Small sections of root, stem, or leaf can, under the right conditions, push out new plantlets in a fashion that seems otherworldly. (Picture chopping off a bit of your finger and growing a whole new you from it.)

These are some of the ways in which plants reproduce asexually, and it’s kind of freaky if you think about it. Plants can clone themselves. But one major disadvantage of reproducing this way is that clonal offspring are genetically identical to the parent plant, which truncates any advantage that might be gained by genetic mixing between two separate plants. For one, it means that a plant population composed of all clones is at risk of being wiped out if something in the environment comes along (such as a disease or change in climate) and none of the plants in the population have adapted any sort of resistance to it.

New plants forming along the lateral stems of Ranunculus flammula – via wikimedia commons

That’s where seeds come in. Seeds are produced sexually, when the gametes of one plant fuse with the gametes of another. Genetic recombination occurs, and a genetically unique individual is born, housed within a seed. Unless, of course, that seed is produced asexually. Then the seed is a clone, and we’re back to where we started.

Apomixis is the process by which seeds are produced asexually. In flowering plants, this means that viable seeds are formed even when flowers haven’t been pollinated. In some cases, pollination stimulates apomixis or is required to produce endosperm; but either way, the result is the same: an embryo containing an exact copy of the genes of its single parent plant.

To understand this process, it’s important to familiarize yourself with the basic anatomy of an ovule, the part of a plant where embryos are formed and which ultimately becomes a seed. In gymnosperms, ovules sit inside cones; in angiosperms, they are surrounded by an ovary. The wall of the ovule is called an integument. A small opening at the top of the ovule, known as a micropyle, is where the pollen tube enters. Diploid cells of the nucellus compose the interior of the ovule, and within the nucellus resides the megasporocyte, which is where meiosis occurs and egg cells are produced. In sexual reproduction, a germ cell introduced through the pollen tube fuses with the egg cell to form a zygote and eventually an embryo. In the case of apomixis, the fusion of germ cells isn’t necessary for an embryo to form.

ovule anatomy via wikimedia commons

There are three main types of apomixis: diplospory, apospory, and adventitious embryony. In diplospory, the megasporocyte skips meiosis and produces diploid cells instead of haploid cells (germ cells). These unreduced cells go on to form an embryo inside of the embryo sac, just like an egg cell would if it had been fertilized with a pollen cell. Additional unreduced cells go on to form endosperm, and the ovule then matures into a seed. This type of apomixis is common in dandelions (Taraxacum officinale). As much as bees love visiting dandelion flowers, their pollination services are not required for the production of viable seeds. Yet another reason you are stuck with dandelions in your yard whether you like it or not.

In apospory, an embryo develops inside of an embryo sac that has been formed from cells in the nucellus. Embryo development within the megasporocyte is bypassed; however, pollination is usually necessary for endosperm to form. Plant species in the grass family commonly produce seeds using this type of apomixis.

Adventitous embryony is also known as sporophytic apomixis because an embryo is formed outside of an embryo sac. Cells from either the integument or the nucellus produce an embryo vegetatively. In this case, a sexually produced embryo can form along with several vegetatively produced embryos. Extra embryos die off and a single, surviving embryo is left inside the mature seed. But not always. Two or more embryos occasionally survive, including the sexually produced one. The mature seed then consists of multiple embryos. This phenomenon is called polyembryony and is common in citrus and mangoes. When it comes to plant breeding, polyembryony is incredibly useful because the asexually derived seedlings are exact copies of their parent, which means the desirable traits of a specific cultivar are retained.

Depiction of seed with three viable embryos after germination.

Polyembryony can occur in a number of ways, and not always as a result of apomixis. In some species, additional embryos “bud off” from the sexually produced embryo. This is called cleavage polyembryony and is known to happen frequently in the pine family (Pinaceae), as well as other plant families. Another common form of polyembryony in gymnosperms is simple polyembryony, in which several egg cells in a single ovule are fertilized resulting in the development of multiple embryos. This doesn’t always mean there will be multiple seedlings sprouting from a single seed. Most embryos usually fail to mature, and only one prevails. However, sometimes more than one survives, and if you’re lucky, you’ll find a seed with multiple plant babies pushing out from the seed coat.

Up Next: Vivipary!

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Inside of a Seed: Gymnosperms

“Every tree has to stay where it put down roots as a seedling. However, it can reproduce, and in that brief moment when tree embryos are still packed into seeds, they are free. The moment they fall from the tree, the journey can begin.” — The Hidden Life of Trees by Peter Wohlleben

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Seed plants – also known as spermatophytes – make up the largest group of plants on earth. Seed plants consist of five divisions, and among them the angiosperm division (a.k.a. flowering plants) dominates in its number of species. The four remaining divisions are referred to collectively as gymnosperms. This incudes the cycads (Cycadophyta), Ginkgo biloba (the only living species in the division Ginkgophyta), gnetophytes (Gnetophyta), and the conifers (Coniferophyta). Conifers are by far the largest and most widespread gymnosperm division.

Angiosperms and gymnosperms have different evolutionary histories, resulting in their distinct genetic and morphological differences. That being said, an overly-simplistic way of differentiating the two groups is to say that, while both groups produce seeds, angiosperms produce flowers and fruits while gymnosperms produce pollen cones and seed cones. There are always exceptions (Ginkgo biloba, for example, doesn’t produce cones), but for the most part, this is the case.

Pollen cones (top) and seed cones (bottom) of mugo pine (Pinus mugo) via wikimedia commons

Sexual reproduction in gymnosperms follows a familiar pattern. Pollen, which contains the male sex cells, is produced in pollen cones, which are essentially miniature branches with modified leaves called scales that house the male reproductive organs. Mature pollen is shed and carried away by the wind. Lucky pollen grains make their way to the female cones, which are also modified branchlets, but are a bit more complex. Scales sit atop bracts, and on top of the scales are ovules – the female reproductive structures. During fertilization, the bracts open to collect pollen and then close as the seed develops.

When pollen lands on an ovule it forms pollen tubes that help direct the male sex cells to the egg cells inside. The process is similar to pollen tubes extending down the style of a flower. In flowering plants, additional pollen cells combine with cells in the ovule to produce endosperm, a storage tissue that feeds the growing embryo. This doesn’t happen in gymnosperms. Instead, haploid cells within the ovule develop into storage tissue and go on to serve the same role.

The ovule eventually matures into a seed, and the cone opens to release it. The seed sits atop the scale rather than enclosed within a fruit, as it would be in an angiosperm. For this reason gymnosperms are said to have naked seeds. The development of seeds can also be much slower in gymnosperms compared to angiosperms. In some species, seeds don’t reach maturity for as long as two years.

Seed cones and winged seeds of mugo pine (Pinus mugo) via wikimedia commons

Seeds in the genus Pinus are excellent representations of typical gymnosperm seeds. Their basic components are essentially identical to the seeds of angiosperms. The seed coat is also referred to as an integument. It was once the outer covering of the ovule and has developed into the seed covering. A micropyle is sometimes visible on the seed and is the location where the pollen cells entered the ovule. The storage tissue, as mentioned above, is composed of female haploid cells that matured into storage tissue in the ovule. Like angiosperms, the embryo is composed of the radicle (embryonic root), the hypocotyl (embryonic shoot), and cotyledons (embryonic leaves).

Angiosperms can be divided into monocotyledons and dicotyledons according to the number of cotyledons their embryos have (monocots have one, dicots have two). Gymnosperms are considered multi-cotyledonous because, depending on the species, they can have a few to many cotyledons.

Seedling of Swiss pine (Pinus cembra) showing multiple cotyledons via wikimedia commons

For the sake of this introduction to gymnosperm seeds, I have offered a simple overview of the production of seeds in the conifer division. Sexual reproduction and seed formation in the other three gymnosperm divisions is a similar story but varies according to species. Even within the conifers there are differences. For example, the “seed cones” of several gymnosperm species can actually be quite fruit-like, which serves to attract animals to aid in seed dispersal. Also, the pollen of gymnosperms is often thought of as being wind dispersed (and occasionally water dispersed in the case of Ginkgo biloba and some cycads); however, researchers are continuing to discover the pivotal role that insects play in the transfer of pollen for many cycad species, just as they do for so many species of angiosperms.

All of this to say that Botany 101 is simply a window into what is undoubtedly an incredibly diverse and endlessly fascinating group of organisms, and that, as with all branches of science, there is still so much to discover.