How the Earth Turned Green

How the Earth Turned Green: A brief 3.8-billion-year history of plants

Joseph E Armstrong. The University of Chicago Press, 2014.

Spoiler alert! This book could seriously change your view of what a textbook can be(!)

I like Joseph Armstrong’s How the Earth turned green [hereafter referred to as HETG], particularly his style. Whilst readers may already be aware of Armstrong’s ‘style’ from his “comments on plants, the foibles and fun of academic life” on his blog site [under his nom-de-blog of ‘Phytophactor’], it will probably come as a surprise that the candid commentary one may equate with the socially informal medium of the blogosphere has made its way into the normally hallowed pages of textbooks. And HETG is a textbook (the publisher’s blurb on the back cover tells us so), of botany. Why the publisher allowed Prof. Armstrong to experiment with this refreshingly different approach is a question that isn’t answered in the book. But this latitude is appreciated by the author (and this reviewer), and, having been allowed this privilege, the world of plant science publishing is the richer for it.

You could easily imagine HETG’s text – which deviates from the approach of botany textbooks by “concentrating on what most books omit” (!! Preface, p. xii), offering explanations of basics of biology which are “seldom well explained in textbooks” (Preface, p. xii), and with its digressions “from the main narrative to explain and demonstrate how science operates” (Preface, p. xiii) – to be the transcripts of lectures delivered by the award-winning teacher who has penned that very prose (and who has been highly rated by his students).

In overview, HETG spans 3.8 billion years of Earth history, from an unashamedly photoautotrophic point of view, and comprises 11 chapters that document the evolutionary advance of plants, from Chapter 4’s algae in the sea to angiosperm supremacy in Ch. 10, via invasion of the land (Ch. 6) and Ch. 9, wherein the seed habit is considered. Finally Ch. 11 considers modern-day vegetation and plant-people interactions.

In a little more detail…

Ch. 1 ’A green world’ tackles the issue of what is a plant early on – and rightly so! Along with such matters as taxonomy and kingdoms, and DNA and fossils, it also provides essential scene-setting in both a geological and cosmic context. Thus, the grandest of stages has been set for the telling of one of Earth’s greatest dramas, admirably summarised by 8 major events in the history of green organisms on the chapter’s concluding page.

Ch. 2 ‘Small green beginnings’: Although acknowledging the importance of plants, Armstrong rightly recognises that underlying this is an absolute dependence on microbial processes – as exemplified in the development of chloroplasts from intracellularly-engulfed/enslaved microbes. As befits its title, Ch. 2 is a ‘microbe-fest’ and covers much of microbial relevance – biochemistry, pigmentation, recycling, symbiosis – and reminds us that Earth’s biology is very much a microbial ecology. HETG also states that tiny green organisms [phytoplankton] “may conduct more photosynthesis on a worldwide basis than all the big green organisms, like trees and grasses, combined”. Fine, but I would like to see the Ref. that supports it. Maybe Armstrong couldn’t find it, hence use of the word ‘may’? But, it would be nice to cite Refs to back-up such claims; that’s only responsible pedagogy/good teaching after all (and notwithstanding the fact that on p. xii we are told that Refs are neither exhaustive nor comprehensive). Intriguingly, we are told about infra-red-using photoautotrophs in deep-sea vents. And for this interesting bit of information we do get a Ref. (but not until the same notion is repeated and expanded upon several pages later – Beatty et al. (2005) [see also Molloy (2005)]). This phenomenon was a revelation to me and underlines the usefulness of Armstrong’s tome in handing out these tit-bits that can be incorporated into one’s own teaching, etc.

Non-American users of HETG sometimes have to work a little harder than US readers. A good example of this is mention of Rube Goldberg machines. I had no idea what they were other than assuming this to be a reference to American popular culture. Having subsequently ‘googled’ the term I learn that they are indeed an American concept, and Goldberg variations on the UK side of the Atlantic are the gadgets created by Prof. Brainstawm and Heath Robinson’s amazing contraptions. That cultural bafflement is countered by the masterful analogy that equates reasons why plants have kept the inefficient carbon-fixing enzyme RubisCO with humankind’s adherence to the QWERTY keyboard layout (which typewriter/computer allusion is surely globally understandable, even if alluding to photorespiration as the process in which “rubisco respires an organism’s sugar molecules … releasing carbon dioxide” seems a little over-simplified…). Otherwise, in Ch. 2 there there are lots of common-sense explanations about how evolution works, especially in terms of developing complexity in an organism’s structure or of biochemical pathways and processes. And there’s a dénouement early on in the book of why land plants are green (no, I won’t spoil it for you by giving away the answer!).

If chapter 2 was quite fun and relatively easy to read, Ch. 3 – ‘Cellular Collaborations’ – requires some considerable effort to get the most out of it, even if it is the section wherein sex is giving a good airing.

Whilst it’s good to see algae so well served in the book – in Chs 4 ‘A big blue marble’, and 5 ‘Down by the sea (-weeds)’ – it seems such a pity that together those two chapters are only 31 pp., which is considerably less than e.g. the 55 pp. of Ch. 3. But Ch. 5 has some interesting thoughts on why plants are green – because they are derived from green algae (and not red or brown if they’d had red or brown algal ancestors) – and has set the scene for the invasion of the land by aquatic (not marine!) photosynthesisers.

Ch. 6 ‘The great invasion’; surely it’s inaccurate to say “only once has a land plant (eel grass…) made the transition back to a marine aquatic environment”. I can’t comment on frequency of land plants returning to their watery ancestral home, but there are many spp. of marine angiosperms of which only some are eelgrasses – Zostera spp. – per se. So, I’m guessing Armstrong is using ‘eel grass’ as a general name for all marine angiosperms? But it’s unclear and would benefit from clarification. And, arguably, seagrass is a better, more general term for all marine angiosperms. But, maybe we can allow Armstrong that latitude when his stated primary expertise is “explaining science to non-scientists” (Author’s Preface p. x). However, Armstrong also states that CO₂ “as a gas diffuses up to 1,000 times faster in air than when dissolved in water”. This caused me to stop and wonder: Didn’t this use to be 10,000 times faster in my own dim-and-too-often-distant early botany education days? This matter niggled me, especially since no Ref. was cited in support of Armstrong’s assertion, and I did some checking. The post-Google consensus is that 10,000 is correct (e.g. Armstrong (a different Armstrong to HETG’s author!), 1979; Armstrong and Drew, 2002). On the one hand, this may be seen as good because it makes one think, and encourages researching and checking. But on the other it’s not so good if you are a novice botanist expecting true statements and don’t have any reason to question such ‘facts’. The multi-page section dealing with the hallmarks of land plants was great and would make a lovely stand-alone lesson (as would so many of the other ‘essays’ throughout HETG…), and in which we learnt why bryophytes are not proper land plants, and why Cooksonia – an extinct plant from the lower Devonian – is the first unequivocal true land plant. Finally, there is a nice introduction to, and consideration of, alternation of generations (though the phenomenon is not actually called that until several pages in to this bit of biology). Despite my grumbles this was a great chapter! [And I’m optimistic that any deficiencies will be made good in a future printing/edition… But whilst we’re thinking of errors, the following ‘typos’ were noted: p. 55, mitochondrium; p. 47, 0 used for O in the chemical formula of oxygen; pp. 2, 305, understory; p. 306, lightening; p. 394, Gingkoes…]

Ch. 7 ‘The pioneer spirit’ is a tour-de-force of bryophyte evolution and biology, which also reveals the original function of stomata…

Ch. 8 ‘Back to the Devonian’ is – appropriately – devoted to the Devonian period, approx. 419 – 359 million years ago, by the end of which we are reminded that every land plant lineage – save angiosperms – had arrived. It was accordingly a very busy time in the green history of Earth, and those early vascularised plants – now known only as fossils – left a lasting impression upon the planet. We are also faced with the statement that “around one-quarter of all vascular plant species (ca. 400,000) living as trees” (by which is meant – lest there be any ambiguity – that approx. 100,000 of the 400,000 vascular plant species are trees – Raven & Crane (2007)): I hadn’t appreciated quite how numerous tree species were (and this admission from one who spent many years studying the cell biology of wood formation in trees – but thereby proving that we can all still learn stuff!). Appropriately, we also get a good explanation of the tree habit, its prevalence and origin, alongside insights into the impetus for development of leaves, and development of xylem and water conduction, and branching. We also learn what cuticle was originally for, and that what we nowadays call a hypothesis was originally termed a theory. Altogether another really impressive chapter!

Ch. 9 ‘Seeds to success’ deals with development of the seed habit and evolution of seed-bearing plants. It’s quite a hard-going chapter, but at least we get to discover the author’s nomination for the most familiar yet least understood biological entity*.

Ch. 10 ‘A Cretaceous takeover’ shifts to the Cretaceous (!) period (c. 145 – 66 MYA) and the ascendancy of angiosperms which ‘rapidly’ displace ferns and cycads from the ecological roles they had held for the preceding 150 million years. There is also much consideration of biotic interactions between angiosperms and animals whether to disperse pollen, or fruits/seeds – or entire plant populations/spp. as in the case of agriculture by humans. There is also a nice consideration of the advantages/disadvantages of anemophily vs zoophily, and an interesting comparison of biological factors that may have contributed to the success of angiosperms over gymnosperms to end the chapter. Plus, Fascinating Fact No. 17: endosperm provides 50-70% of all human calories (But, Phytophactor, please may we have a Ref. to support this important phytofact?). Note 28 to Ch. 10 also puts the record straight about Darwin’s abominable mystery’ regarding angiosperms’ appearance in the fossil record – which is often misunderstood in textbooks – and cites Friedman (2009) therefor.

Ch. 11 ‘All flesh is grass’ deals predominantly with events post the dinosaur-demise at the K-T boundary approx.. 66 MYA (and in case you’ve ever wondered why it’s K for Cretaceous, it’s because C has already been used for the Cambrian period…). Thus, the Azolla-induced global cooling event (which I suspect is not that well-known amongst a general – or indeed botanical – readership..?) is given an airing. And there are musings on the rise of grasses (and sunflowers…), and grassland communities, and the development of fire ecology which helps to maintain grasslands at the expense of woody biomes, and development of C4 photosynthesis. Continental land mass movements and mountain-building which led to development of different climatic regions, which in turn promoted different vegetation patterns, are also covered here. However, with quite detailed consideration of taxonomy, and angiosperm phylogeny in particular, this chapter does get a bit ‘heavy’. For one who is quite a stickler for accuracy (e.g. see Phytophactor on why pollen isn’t the plant equivalent of sperm), it’s a pity that HETG didn’t take the opportunity to correct the inaccurate notion that cotton is a fibre. It isn’t, it’s a hair, which is an outgrowth of an epidermal cell, not a separate cell as is a fibre. Otherwise, this is a very good, thorough consideration of human exploitation of, and dependency upon, the plant resource – to keep us clothed, happy and fed. And, ruing human-driven habitat destruction, we have probably the most poetic – but highly quotable – statement of the book: “We do not know how many strands of biological diversity can be severed before the fabric of life really starts to unravel”.

The remaining parts of HETG are a big Appendix (really big! approx. 150 pp. big) – wherein “the usual textbook material” is relegated (Author’s Preface p. xii) – with separate accounts dealing in alphabetical order (“for convenience”, not phytochronologically in order of appearance on Earth as might befit the evolutionary thrust of the main text…) such plant progenitors as brown algae, cycads, hornworts, phytoplankton, red algae, seed ferns (an oxymoron one might have expected Armstrong to make much of – but maybe not here in the ’proper textbook’ section of the book?), and – lastly – whisk ferns. What, no angiosperms? No: Why not? Approx. 19 pp. of Notes to the chapters, an 8 pp. Glossary (from Abiotic to Zygote (including undulopodia – look it up!)), and c. 19 pp. of References (with approx. 140 dated post-2000). Finally, there are c. 10.5 pp. of 2-columned Index (also, from abiotic to zygote), but which is otherwise rather idiosyncratic. For instance given their importance to the development of land plants it is surprising that there’s no entry for cuticle, lignin or xylem, yet there are 4 entries for beer, 3 for vinegar, 2 for QWERTY(!!), and 1 each for cheese and antimatter… But, and considering the green subject matter of the book, the most amazing omission is any Index entry for chlorophyll(!!). Neither is there an entry for evolution; maybe that could be excused on the grounds that it is evolution that permeates the whole of the book’s narrative(?). And a strong memory one gained from the book is that Armstrong is an advocate for evolution (as opposed to Creationism – 4 Index entries, or Intelligent Design – 3 entries!).

Comparisons?

HETG’s subject matter is reminiscent of Walker’s “A very short introduction to plants” (2012) and Willis and McElwain’s “Plant evolution” (2002), but covers a bigger range of material than both, and is written like no other textbook I know. There are plenty of books around nowadays that consider the current and future importance of plants, and many go as far back as the dawn/origins of agriculture – a human invention which depends upon plants and plant productivity. But I don’t know of many texts that consider that far older, more important aspect of plant history that shows how the Earth that we now see around us is dependent upon the role of and intervention by plants. HETG is arguably unique **.

Quibbles…

One thing that should be sorted, for consistency (if for no other reason) throughout HETG, is how many flowering plant species there are. Ch. 1, p. 13 speaks of 220,000 angiosperm spp. In Ch. 10 (p. 297) that has increased to >220,000 [although Note 2 thereto (p. 517) admits to 250,000 – 300,000, if one includes spp. not-yet-documented). Back to the main text and in Ch. 11 (p. 349) it’s increased to 250,000 spp. (although Note 16 to that chapter on p. 16 drops down to 235,000). It would be nice – and reader-centred – to choose a figure and stick with it. A widely-quoted number of angiosperm species is 352,000 (itself an approximation for the 352,282 in Paton et al. (2008)). I know that number has recently been increased to 450,000 angiosperm spp. (Pimm & Joppa, 2015), but that Ref. post-dates HETG’s publication date so is inadmissible for the point being made. But, it does underline the very valid point acknowledged by Armstrong that science progresses and “some of the book’s contents will be out of date” (Preface, p. xii).

But! And, for all its talk of Green things, it is indeed curious that there is no colour image to show off the verdant splendour of HETG’s subject matter. Not even on the book’s cover (though it does have a coloured image, of petrified wood…).

Does HETG achieve its goals?

What are they? According to the book’s back-cover, we are told that HETG is more engaging than a traditional textbook [Yes!],

with astonishing breadth [agreed],

which will both delight [I suspect that depends upon one’s point of view of what a textbook ought to do, and how it should achieve that goal…]

and enlighten [it certainly does that – even for one who thought he knew a lot about plants already!]

embryonic botanists [hmmm, you do need a certain level of understanding/prior knowledge to appreciate the text, so maybe impressionable botanical neophytes?]

and any student in evolutionary biology of plants [agreed].

Overall view

Whilst HETG is quite technical in places, that should be viewed neither as a negative nor a surprise – it is supposed to be a textbook (for undergraduates) after all! But its very informal style (which was most unexpected in – although refreshingly different for– a scholarly text) makes for a highly readable, educational account. One can only hope that its intended audience – “everyone else [apart from botanists(!!)]” (Preface, p. x) – appreciates it as much as this reviewer (who is also a Botanist…) did.

References

Armstrong W (1979) Aeration in higher plants. Advances in Botanical Research 5: 236–332.

Armstrong W and Drew MC (2002) In: Plant Roots: The Hidden Half, 3e. Waisel Y et al., editor. New York & Basel; Root growth and metabolism under oxygen deficiency; pp. 729–761.

Beatty JT, Overmann J, Lince MT, Manske AK, Lang AS, Blankenship RE, Van Dover CL, Martinson TA and Plumley FG (2005) An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent. PNAS 102: 9306–9310.

Essig FB (2015) Plant Life: A Brief History. Oxford: Oxford University Press.

Friedman W.E (2009) The meaning of Darwin’s “abominable mystery.” American Journal of Botany 96: 5–21.

Molloy S (2005) Down in the depths. Nature Reviews Microbiology 3: 582–582.

Paton AJ, Brummitt N, Govaerts R, Harman K, Hinchcliffe S, Allkin B and Lughadha EN (2008) Towards Target 1 of the Global Strategy for Plant Conservation: a working list of all known plant species—progress and prospects. Taxon 57: 602–611.

Pimm SL and Joppa LN (2015) How many species are there, where are they, and at what rate are they going extinct? Ann. Missouri Bot. Gdn 100: 170–176.

Raven J and Crane P (2007) Trees. Current Biology 17: R303–304.

Walker T (2012) Plants: A very short introduction. Oxford: Oxford University Press.

Willis KJ and McElwain JC (2002) The evolution of plants. Oxford: Oxford University Press.

* the seed…

** HETG covers material that Essig’s 2015 book “Plant Life: A brief history” also purports to deal with. I’ve yet to read Essig’s tome, but I suspect the style – and maybe the coverage – will be different to Armstrong’s…