From moss to flowers
Summary
The nearly 400,000 different species of land plant on the planet can be largely organised into just four groups: bryophytes (mainly mosses), pteridophytes (mainly ferns), gymnosperms (mainly conifers) and angiosperms (flowering plants). Bryophytes are non-vascular plants, the rest are vascular plants. Bryophytes and pteridophytes reproduce by means of spores; gymnosperms and angiosperms make use of seeds. The seeds of gymnosperms are naked, whereas those of angiosperms are contained within an ovary when developing, which matures into a fruit.
There are nearly 400,000 different species of (land) plant on the planet; and that’s before we even get started on the varieties (naturally occurring variations within a species) and cultivars (‘cultivated varieties’ - variations deliberately selected by plant breeders). So it might seem incredible that all of this immense variety fits into just four broad groups.
Bryophytes
The simplest of plants are the “bryophytes”, a group composed of approximately 20,000 species of liverworts, hornworts and mosses (I’ve included some photos of liverworts and hornworts below for those of you, like me, who have no idea what these are…). These plants are simple because they don’t produce either flowers or seeds, and don’t have vascular tissues for transporting water and nutrients upwards against the force of gravity.
Bryophytes reproduce either sexually (meaning that the offspring has genetic material from more than one parent, and is genetically distinct from either parent) through spores, or asexually (a single parent, with the offspring being a clone of the parent) by fragmentation.
Without vascular tissues, the vast majority of bryophytes are severely limited in height; the forces that move water vertically through the plant (capillary action) are unable to counter the force of gravity beyond a few centimetres. A few species have evolved specialised structures to move water and nutrients, but far less efficiently than in vascular plants (the largest being Dawsonia superba, a moss native to Australia, New Zealand, New Guinea and Malaysia, which can reach heights of 60cm).
However, the absence of vascular tissues means that bryophytes don’t need roots to grow; they can obtain nutrients directly from water on the surface of bare rocks or bare compacted soil. This is why moss thrives in damp, humid conditions on rocks and in lawns (particularly in the shade, where grasses struggle, and where the soil is compacted).
(A) Common liverwort (Marchantia polymorpha); (B) Smooth hornwort (Phaeoceros laevis); (C) Alpine clubmoss (Diphasiastrum alpinum); (D) Kraus’s spikemoss (Selaginella kraussiana); (E) Tuckerman’s quillwort (Isoetes tuckermanii)
Pteridophytes
Next up in terms of complexity are the “pteridophytes”, made up of around 13,000 species of ferns, horsetails, clubmosses, spikemosses and quillworts (once again, some photos, this time just above…).
The big difference (and in this case the difference really is big, since the largest pteridophyte, Cyathea cooperi, the Australian tree fern, can grow to a height of over 6m) is that pteridophytes are vascular plants. In other words, they have specialised tissues (xylem and phloem) that can transport water, minerals, carbohydrates and other molecules around the plant; in particular against the force of gravity. The maximum height reachable by a vascular plant is not known (the tallest, a Sequoia sempervirens or coast redwood - not a pteridophyte, is over 116m tall). However, the forces that can be generated by vascular tissues may be sufficient to lift water as high as 500m from the soil! Very different to the bryophytes,
Pteridophytes resemble the other plants we are more familiar with; they have clearly differentiated roots, stems and leaves. However, like bryophytes, they reproduce by means of spores and produce neither seeds nor flowers. The details of spore-based reproduction are fascinating, and worthy of an entire blog post…
From a gardener’s perspective, the key difference between pteridophytes and other plants we choose to cultivate is in how we seek to propagate them. Since pteridophytes don’t have seeds, we must either propagate them vegetatively (by dividing them or by taking cuttings), or we must learn to collect spores and grow them on (I may have to write that blog post…)
Gymnosperms
Our next grouping is actually the smallest, with only around 1,100 species, but with some of the mightiest trees on the planet. The “gymnosperms” are vascular plants that propagate themselves by means of seeds, but have no flowers. This group consists of the conifers, cycads (plants that look like palms but are not - palms are flowering plants), gnetophytes (a grouping of plants that are genetically related, but have few common characteristics - making it difficult to find a good representative photo…) and the unique Gingko genus (which also has only one species, Gingko biloba, and is unchanged since the time of the dinosaurs).
The word gymnosperm is a composite of two Greek words: gymnos meaning ‘naked’ and sperma meaning ‘seed’. They are literally plants whose seeds are naked; unlike flowering plants, whose seeds are always encased in a protective structure. Gymnosperm seeds develop on the surface of the plant’s leaves (often modified to form cones - conifer cones are just heavily modified leaves), or sometimes directly on the stems themselves (as in Gingko biloba and Taxus baccata, common yew).
Without flowers, gymnosperms typically rely on the wind for pollination (albeit some are insect pollinated); pollen being blown from the male parts (“pollen cones”) of one plant to the female parts (“ovulate cones”) of another. This process is inefficient, since the majority of pollen will be blown away on the winds and never come into contact with another plant of the same species. As a result, gymnosperms typically produce large volumes of pollen, which can coat the floor of a conifer forest in spring time.
Angiosperms
The final grouping of plants is by far the largest, with over 300,000 species in over 13,000 genera. This group accounts for nearly all plant-based food, and for the vast majority of plants we grow in our gardens. These are the “angiosperms”, the flowering plants.
Angiosperms are also vascular plants (though their vascular tissues have a different anatomy to those of gymnosperms and pteridophytes). They are distinguished from gymnosperms by having flowers, and by having fruits that completely encase their immature seeds; albeit these are ‘fruits’ in the botanical sense, not in the everyday sense, being the fleshy or dry ripened ovary of a flowering plant.
Angiosperms have a wide variety of methods for pollination, whether by insects, by wind or by water. All of these rely on pollen produced in male flower parts (“anthers”) being transferred to female flower parts (“stigmas”), with combinations of anatomical features, colours and scents being used by the plants to achieve this.
Angiosperms have been incredibly successful from an evolutionary point of view, having evolved to fill the vast majority of available growing niches (hang on in there, bryophytes…the angiosperms still need their roots and can’t grow on bare rock). This includes plants adapted to harsh desert environments, to extremely acid or alkaline conditions, to deep shade, to boggy ground, and to a huge range of other habitats.
To achieve this, angiosperms have developed lots of different approaches. Some (“ephemerals”) complete their entire life cycle in just a short time, allowing many generations in a single growing season; some live a single growing season (“annuals”); some for two seasons (“biennials”), and some for three or more (“perennials”). Some develop strong tissues that can survive winter cold and form a basis for further growth in subsequent seasons (“woody” plants). Others are “herbaceous” (non-woody), often (but not always) dying back to the ground in winter, but resprouting from their roots in the following spring. Gymnosperms lack this diversity; all are woody perennials.
Many have developed adaptations to enable them to survive in otherwise hostile environments, such as underground storage organs (e.g. potatoes); defensive structures (e.g. spines on cacti); air-filled tissues (many aquatic plants); tendrils for climbing (e.g. clematis); and many others.
These four groups are a simplification of a more complex classification of plants based on their evolution. Genetic analysis is used to trace when different groups of plants last had a common ancestor, and groupings determined by when these evolutionary histories diverged. This more complete classification includes all four of these groups, but also others, such as the “Amborellales”, a grouping that contains only a single species, Amborella trichopoda, native to the main island, Grande Terre, of New Caledonia in the southwest Pacific Ocean. Genetic analysis shows that this was the first flowering plant to evolve, separately from all other flowering plants. Fascinating…but this particular rabbit hole goes too deep even for Getting Dirty!
Image credits
The images of bryophytes and pteridophytes in this post have been shared under Creative Commons Licences as follows:
(A) Credit: Ryan Hodnett. Image source. Creative Commons Attribution-Share Alike 4.0 International Licence.
(B) Credit: Nk. Image source. Creative Commons Attribution-Share Alike 3.0 Unported Licence.
(C) Credit: Ryzhkov Sergey. Image source. Creative Commons Attribution-Share Alike 4.0 International Licence.
(D) Credit: David Stang. Image source. Creative Commons Attribution-Share Alike 4.0 International Licence.
(E) Credit: Gilles Ayotte. https://commons.m.wikimedia.org/wiki/File:Isoetes_tuckermanii_15-p.bot-isoe.tucke-010.jpg. Creative Commons Attribution-Share Alike 4.0 International Licence.
