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.

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Winter Interest

We are well into winter in the northern hemisphere, and the plants in our landscapes have been dormant for weeks now. Trees and shrubs have dropped their leaves, grasses have gone brown, and perennial forbs have died back – their roots harboring the food they will need to return to life in the spring. What little green that is left is provided mainly by evergreen trees and shrubs, but even they are resting – metabolizing slowly and putting off further growth until warmer temperatures return. The view outside may appear largely bleak and dreary, but there is still beauty in a frozen landscape, and much of that beauty is provided by the same things that brought color and interest during the warmer months.

Many plants, though appearing dead, remain attractive throughout the winter. From fruits and cones to seed heads and seed pods, there are various structures that remain on certain plants even after leaves fall that provide winter interest. Deciduous trees and shrubs show off their branches in the winter months, which when freed from the camouflage of leaves are like sculptures – art pieces in their own right. Perennial grasses can continue to provide structure to a garden bed when left in place and upright, and color is provided by evergreen foliage and colored bark, such as the red and yellow bark of some dogwoods (Cornus spp.).

Beauty surrounds us, even in unlikely places. Things are quiet and frozen now, and foggy, dismal days abound. But winter won’t last forever. Plants can remind us of that. In them we find remnants of brighter days and an assurance that there are more to come.

alnus viridis

Male and female cones on Sitka alder (Alnus viridis)

ericameria nauseosa

Seed head on rubber rabbitbrush (Ericameria nauseosa)

SAMSUNG

Flower stalks on strict buckwheat (Eriogonum strictum)

sorbus scopulina

Cluster of berries on Cascade mountain-ash (Sorbus scopulina)

maclura pomifera

Ice crystals on the branches of young Osage-orange (Maclura pomifera)

rosa pisocarpa

Rose hips on cluster rose (Rosa pisocarpa)

sedum sp. seed head

Seed head on showy stonecrop (Sedum telephium ‘Autumn Joy’)

All photos were taken at Idaho Botanical Garden in Boise, Idaho.