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

———————

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.

Advertisements

What Is a Plant, and Why Should I Care? part one

I want to tell the story of plants. In order to do that, I suppose I will need to research the 4 billion year history of life on earth. And so I am. Apart from satiating my own curiosity, studying and telling the story of plants advances me towards my goal of creating a series of botany lesson themed posts. Botany 101 and beyond, if you will. An ambitious project, perhaps, but what else am I going to do with my time?

So what is a plant anyway? We all know plants when we see them, but have you ever tried to define them? They are living beings, but they are not animals. They are stationary – rooted in the ground, usually. Most of them are green, but not all of them. They photosynthesize, which means they use water, carbon dioxide collected from the atmosphere, and energy harvested from the sun to make food for themselves. No animal can do that (okay…a few sort of can). They reproduce sexually, but many can also reproduce asexually. They are incredibly diverse. Some grow hundreds of feet into the air. Some barely reach more than a few centimeters off the ground at maturity. They have discernible parts and pieces, but they can also lose parts and pieces and then grow them back. There aren’t many animals that can do that. They have been on this planet for hundreds of millions of years, colonizing land millions of years before animals. Plants helped pave the way, and if it weren’t for plants, animals may not have stood a chance.

I don’t mean to pick on animals, it’s just that for a long time, humans grouped living things into just two kingdoms: Plantae and Animalia. Stationary things that appeared to be rooted to the ground or some other surface were classified as plants. Green things that lived in the water were also considered plants. Thus, lichens, fungi, algae, and everything we consider to be a plant today were placed in kingdom Plantae. Everything else was placed in kingdom Animalia. This, of course, was before much was known about microorganisms.

Dichotomous classification was reconsidered as we learned more about the diversity of organisms in each kingdom, particularly as the theory of evolution came into play and microscopes allowed us to observe single celled organisms and chromosomes. Eventually, fungi was awarded its own kingdom, which includes lichens – organisms composed of both fungi and photosynthetic species but classified according to their fungal components. Most of the algae was placed in a kingdom called Protista, a hodgepodge group of unicellular and unicellular-colonial organisms, some of which are animal-like and some of which are plant-like. Two kingdoms were also formed for prokaryotic organisms (organisms with cells that lack membrane bound organelles): Bacteria and Archaea.

Illustration of one current itteration of kingdom classification system (illustration credit: wikimedia commons)

Taxonomic kingdoms as we currently consider them (illustration credit: wikimedia commons)

In short, the answer to what is a plant seems to be whatever organisms humans decide to put in kingdom Plantae. One problem with this answer is that some chose to include certain species of algae and others don’t. But why is that? It has to do with how plants evolved and became photosynthetic in the first place.

Microorganisms developed the ability to photosynthesize around 3.5 billion years ago; however, the photosynthetic process that plants use today appeared much later – around 2.7 billion years ago. It evolved in an organism called cyanobacteria – a prokaryote. Eukaryotic organisms were formed when one single cell organism was taken inside another single cell organism, a process known as symbiogenesis. In this case, cyanobacteria was taken up and the eukaryotic organisms known today as algae were formed. The incorporated cyanobacteria became known as chloroplasts.

Not all algae species went on to evolve into plants. A group known as green algae appears to be the most closely related to plants, and a certain subset of green algae colonized the land and evolved into modern day plants (also known as land plants). That is why some taxonomists choose to include green algae in the plant kingdom, excluding all other types of algae.

Common stonewort (Chara vulgaris, a species of green algae (photo credit: www.eol.org)

Common stonewort, Chara vulgaris, a species of green algae (photo credit: www.eol.org)

The term land plants refers to liverworts, hornworts, mosses, ferns, fern allies, gymnosperms, and flowering plants – or in other words, all vascular and non-vascular plants. Another all encompassing term for this large group of organisms is embryophytes (embryo-producing plants).

Still confused about what a plant is? Three main features can be attributed to all plants: 1. They are multicellular organisms. 2. Their cell structure includes a cell wall composed of cellulose 3. They are capable of photosynthesis. Many species of green algae are unicellular, which is an argument for leaving them out of kingdom Plantae. Certain parasitic plants like toothwort, dodder, and beech drops have lost all or most of their chlorophyll and no longer photosynthesize, but they are still plants.

Deciding what is and isn’t a plant ultimately comes down to evolutionary history and common ancestry. As Joseph Armstrong writes in his book, How the Earth Turned Green, “Our classifications of human artifacts are totally arbitrary, but to be useful scientifically our classification of life must accurately reflect groupings that resulted from real historical events, common ancestries.”

Obviously this is going to be a multi-part series, so I will have much more to tell you about plants in part two, etc. For now, this You Tube video offers a decent summary.