Basic groups 2: Plants

Next in line, I think, should be plants and similar multicellular organisms, i.e. rhodophytes (red algae) and phaeophytes (brown algae).

These are the photosynthetic multicellular organisms. They produce organic molecules (and oxygen) from sunlight, water and carbon dioxide, in special organelles called chloroplasts, using the green pigment chlorophyll to absorb the sunlight. They also use various accessory pigments (to help with the absorption), which can give them different colours; this is why rhodophytes are mostly red, and phaeophytes have a green-brown-ish colour.

To fully grasp what is unique about the red and green algae, let us first look at the more familiar green plants.

Kingdom Plantae, also called Viridiplantae, comprises all land plants and the unicellular green algae, which we covered in Part 1; so, now we will focus on the land plants, the embryophytes.

I think we all have a pretty good idea of what a land plant is, and the details, although incredibly fascinating, might just be a bit too much for this quick guide-through. All I should say is that land plants have essentially two main life stages, i.e. two main forms throughout their life cycle. So, individuals of the same species may look very different, if they are in different life stages.

Land plants can broadly be divided into vascular and non-vascular plants. Non-vascular plants lack vascular tissue, specialised structures designed to transport water and/or nutrients throughout the plant. These include mosses (bryophytes), hornworts and liverworts. In addition to lacking vascular tissue, they also lack leaves, true roots and true stems. Therefore, they are typically small, not growing higher than a decimetre or so, and cannot grow in dry places, requiring wet soils to sustain themselves.

A moss. Image from http://www.mossandstonegardens.com/blog/the-truth-about-moss-dispelling-moss-myths/

A hornwort. Image from http://www.anbg.gov.au/cpbr/program/ha/ha-liverwort-hornwort.html

A liverwort. Image from http://www.aphotoflora.com/liverwort_marchantia_polymorpha.html

Vascular plants, or tracheophytes, includes primitive vascular plants, such as lycophytes (club mosses), arthrophytes (horsetails), ferns (pteridophytes), and seed plants (spermatophytes). Lycophytes have stems and roots (I think!), but no leaves. Instead, they have microphylls: leaf-like structures, but not true leaves, because they grow directly out of the stem, without the petiole (leaf stem) in between. Arthrophytes have true leaves (macrophylls), which are needle-like (much like those of conifers), but also have a green stem that carries out photosynthesis, instead of just the leaves. These are both rather rare plants; I don’t know if I have ever seen any of them in real life. Ferns, on the other hand, are far more familiar. They can be recognised by having their spore capsules attached to the underside of their leaves, which by the way also have a very characteristic shape (though I don’t know how to describe that in words…).

A lycophyte. Image from http://www.hermonslade.org.au/projects/HSF_07_4/hsf_07_04.html

An arthrophyte. Image from http://www.online-utility.org/image/gallery.jsp?title=Equisetum+telmateia

 

A fern. Image from http://www.fanpop.com/clubs/flowers/images/28504493/title/fern-photo

Spermatohpytes represent the next step in land plant evolution: the seed-bearing plants. They are divided into the gymnosperms and angiosperms. Gymnospermophyta includes conifers and several other ancient linages (e.g. cycads, ginkgos), many of which are extinct now, or only exist as a few species. Their seeds are ‘naked’, exposed, hence the name. Angiospermophyta, the flowering plants, on the other hand, have their seeds ‘hidden’ or enclosed in something, usually a fruit.

Conifers are characterised by their cones, which are their means of reproduction, i.e. where their seeds are carried, and by their needle-like leaves. Angiosperms are identified by their flowers (hence the colloquial name ‘flowering plants’) and fruits (which contains their seeds), although these are not always easily seen, for example in grasses. But, basically, most land plants today are angiosperms, the needle-leaved ones are conifers, the second-most abundant, and then there are ferns in third place, and mosses too… The rest are quite rare.

A conifer (gymnosperm). Image from http://www.jeannerose.net/articles/Conifers_EO.html

An buch of angiosperms. Image from

 http://www.gambassa.com/public/project/4448/JillianandStephanie%27sTimeline.html

In the writing moment, I realise that this is rather scattery; the information might make little sense in the big scheme of things unless you have some background in plant biology and/or evolution. I apologise for this, and am considering writing a supplementary background entry later, maybe by the end of next week.

But now, let us jump to the red and brown algae. Both are aquatic (water-living), so they don’t need stems like land plants, as the water gives them enough buoyancy to stay upright and reach the sunlight. They differ in their accessory pigments, giving them different colours, and in other factors. Overall, many rhodophytes grow like sheets over the surface, i.e. not that tall, while phaeophytes tend to be primarily seaweed forms, with ‘roots’ (called holdfast), a ‘stalk’ (stipe) and ‘leaves’ (blades).

A rhodophyte. Image from http://www.jonolavsakvarium.com/blog/200711/blog200711.html

A phaeophyte. Image from http://en.wikipedia.org/wiki/Brown_algae

I’m sorry for this post, I feel the quality is so low I am hesitating of whether to publish this or not; I am frankly too demotivated to improve it or start over. I think in order to break this evil circle, I shall publish it and seek to come back with a supplementary post later, when I feel more energised. I don’t know what came over me today, but I’m sure it will pass! I promise the next one will be more enthusiastic!