ent. Scattered across the ridges and valleys of the western slopes of the Sierra
Nevada Mountains, California, natural groves of these huge trees are dotted over
thousands of hectares. Well known to Native American tribes living in the region, long before their discovery by European settlers early in the nineteenth century, these massive photosynthetic organisms remind us of the extent to which plants
have truly conquered the business of living in thin air.
Understandably taken with these majestic trees, the producers of the BBC
Television series How to Grow a Planet opened each episode with a filming sequence showing dramatic sweeping panoramic views of the Sierra Nevada forests. The
series provided a companion to its sister series, Botany: A Blooming History, presented by Timothy Walker, former Director of the Botanic Garden at the University of
Oxford . In mischievously highlighting these commendable contributions by the BBC to the fight against plant blindness, I should disclose that the former series
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was based on my earlier 2007 book, The Emerald Planet. Both the book and TV series hoped to offer readers and viewers an extraordinary new perspective on the role
of plant life in shaping Earth’s history. History will judge whether this was the case—the point is that each episode showing Sierra Nevada’s rocky landscape
and its Sequoiadendron trees opened with the camera tightly focussed on the ebul-lient Scottish presenter Iain Stewart. Only when it zooms out do we discover
Stewart roped to the top of a giant redwood tree, where he proclaims the power of the plant kingdom in transforming the Earth over millions of years.
Yet an overlooked secret of the success of redwood trees only becomes evident by
scrutinizing the fine details. Zoom in close, past Iain Stewart, past the branches, past the feathery leaflets, and focus down onto the cellular details of a single needle. There, on the surface of each needle, are special elongated cells, lined up like soldiers flank-ing the central midrib from tip to base. These microscopic soldiers are paying close attention to the atmospheric conditions and receiving commands, via hormones,
from roots sensing the moisture of the soil, not to mention the electrical currents signalling changes in the water status of the needles themselves. Each soldier comprises a pair of specialized guard cells that flank an adjustable stomatal aperture or pore (from the Greek stoma for mouth). As weather conditions change, or the soil dries out, the tiny pores respond by opening and closing, tightly regulating the molecular exchange of gases between needle and the surrounding atmosphere to limit
excessive water loss from the battalion of cells that make up the rest of the leaflet.
The desiccation of delicate tissues is an ever-present danger for plants because
in the essential business of absorbing carbon dioxide from the atmosphere for
photosynthesis, leaves must open their stomatal pores. As they do so, water
immediately escapes, evaporating from the wet surfaces of cells inside the leaf and streaming out into the drier surrounding atmosphere. In fact, carbon dioxide
molecules have to enter the leaf against this outward flux of water molecules—
the transpiration stream. Mainly as a consequence of opening pores and exposing
the wet cell surfaces of the leaf interior to a drier atmosphere, it costs plants about 1 kilogram of water to synthesize a few grams of plant tissue from carbon dioxide.
The actual amount depends on weather conditions, especially atmospheric
dryness, and the photosynthetic mode of the plants.
For trees, and indeed virtually all terrestrial plant life, water is the price to be paid for the freedom of living in thin air. Yet the dilemma facing land plants is that water is also a precious commodity, essential both for running their metabolism and for keeping them upright by pressurizing cells to give tissues rigidity and drive cell
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expansion for growth. The pressures involved are enormous. Encased by rigid cell
walls, plant cells withstand pressures three times those found inside a bottle of cham-pagne. Not surprisingly, blind, ruthless evolutionary trial and error has engineered plants with exquisite capabilities for managing both sides of their watery economics—its supply and demand—as they refuel with carbon dioxide. Roots supply water
to the tree; water-conducting tissues in the trunk and branches deliver it to leaves; and stomata trade its evaporative escape for carbon dioxide in the glare of the Sun.
Consider the harsh realities of the situation facing early plant life when it began adapting to a terrestrial existence half a billion years ago. Survival depended on evolving a strategy balancing the imperative to grow against the risk of death by desiccation. Secreting a waxy impermeable outer coating, a cuticle, surrounding
the delicate shoots thrust upwards into the air provided part of the solution. Indeed, a lipid-rich cuticle may have been inherited from their freshwater algal ancestors, who needed it to avoid drying out when falling water levels left them stranded on muddy banks.1 But the impervious cuticle itself created a problem: it impeded the ability of simple early land plants to absorb carbon dioxide. Specialized miniature gas valves with adjustable apertures—stomata—provided the solution to the per-meability problem created by the cuticle. These remarkable cellular innovations
were actually sophisticated microscopic pores that proved pivotal to plant life as it expanded across the landscape to transform the planet’s continents.
The oldest fossil stomata unearthed to date are those from 418-million-year-old
Silurian rocks in Wales.2 They belong to the iconic fossilized land plant Cooksonia, the earliest known simple vascular land plant, which grew in clusters of slender
leafless stems the width of a needle, if that, and stood only a centimetre or so tall (Figure 12). View Cooksonia fossils under a scanning electron microscope and the wonderful detail of ancient stomata is visible, created by a pair of specialized
kidney-shaped guard cells that closely resemble those on the leaves of modern
plants. Indeed, for all early vascular land plants, the presence of stomata is the rule not the exception. Microscopic examination of their tiny fossilized remains,
dating to the Silurian from localities worldwide, confirms that stomata are dotted across the surfaces of their stems and reproductive structures (sporangia) held on the tips of slender stems.3 Written in stone, the anatomy of these fossils is testa-ment to the central role stomata have played in regulating the fundamental plant
processes of photosynthesis and transpiration for hundreds of millions of years.
By performing this important function, stomata, coupled with other innovations
we have already considered, including rhizoids to absorb nutrients and water
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Figure 12 A scanning electron micrograph image of an early Devonianian vascular plant’s ( Cooksonia) 410-million-year-old stoma composed of two kidney-shaped guard cells, much like those possessed by all living land plants (scale bar = 10 μm).
from the soils, and vascular tissues for transporting water through stems, proved vital in allowing Earth’s early land floras to breathe deeply and grow taller. The tallest trees on Earth, the Californian coastal redwoods ( Sequoia sempervirens)—
close cousins of the giant redwoods—illustrate how this may have happened long
ago.4 Shackled to the flimsiest branches of redwoods in the Humboldt Redwoods
State Park, north of San Francisco, and braving the high winds, intrepid plant
physiologists have revealed the secret role of stomata in permitting these trees to attain great heights of over 100 m (Plate 4).
In these tall trees, water escaping through the stomata at the treetops generates an enormous transpiration stream; the canopy of each tree loses hundreds of
kilograms of water every day
.5 This loss of water, the ‘cost’ of photosynthesizing on land, solves its own problem of supply. It helps ‘pull’ water into the rooting systems from the soil, through the hollow water-conducting cells of the trunks, to the very tops of the trees, to overcome the opposing forces of gravity and friction.
The water-conducting cells themselves are long, hollow tubes stacked vertically
through the stem, often with thickened cell walls for reinforcement. The
transpirational ‘pull’ of water, as the process is known, tends to be greatest near the top of the tree, where water is evaporating through stomata.6 In coastal
redwoods, water moves upwards at a stately pace, taking several weeks to travel
from the base of the trunk to the top of the tall trees, where it pressurizes cells and drives the growth of leaves by cell expansion. We can think of the continuous
column of water from roots to the treetop as being under tension between these
opposing forces of gravity and transpiration, and the column holds together
because of the adhesion of water to the walls of water-conducting cells and the
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cohesive bonds attracting water molecules to each other.7 This is the so-called cohesion-tension theory and since its introduction in the late nineteenth century, it has become widely accepted as the mechanism explaining the rise of water in plants.
The idea that ‘strings’ of water are lifted up through the plant by its evaporation from leaf surfaces is extraordinary, and it originated over a century ago in 1895.
When first proposed by Henry H. Dixon and his colleague J. Joly at Trinity College, Dublin, Francis Darwin (1848–1925), Charles Darwin’s third son, who had a great
passion for stomata, was incredulous. Darwin commented, ‘To believe that col-
umns of water should hang in the tracheals [water-conducting cells] like solid
bodies, and should, like them, transmit downwards the pull exerted on them at
their upper ends by the transpiring leaves, is to some of us equivalent to believing in ropes of sand’.8 Sir Isaac Newton (1643–1727), on the other hand, would presumably have been delighted with the idea. Newton penned his perceptive ideas about
how light knocks away fluid particles escaping from the shoot in an obscure
undergraduate notebook sometime between 1661 and 1665.9 He wrote that ‘by this
meanes [ sic] juices continually arise up from the roots of trees upwards’. So the father of gravity himself thought of an explanation for how plants oppose gravity that has more than passing resemblance to our modern understanding. And he did
so two centuries before botanists suggested the widely accepted cohesion-tension
theory explaining the ability of plants, from grasses to the Earth’s tallest trees, to transport water from roots to leaves.10 As Emily Conover of the international journal Science snappily remarked in commenting on the matter, ‘Although he didn’t quite get the details right, one thing’s for sure—Newton was no sap’.11
The tussle between Newton’s gravity and transpiration means that trees face
a problem as they grow taller. With increasing height, the effectiveness of
transpiration in overcoming the forces of gravity and friction diminishes, and this reduces the flow of water into the cells of leaves, making it difficult for them to expand, and slowing their growth. Those cells at the top of trees are very small
and produce thick, dense leaves that are less effective at photosynthesis, limiting the maximum height of the trees.12 The coastal redwood trees in the Humboldt
Redwoods State Park stand some 113 m tall and are over 2000 years old, and still
have some way to go before topping out at around 120–130 m. Reaching those
heights could take another 30 or 40 years. The paradox of how these trees, losing hundreds of kilograms of water every day, persist in the Californian climate with a long dry season is resolved by the realization that during droughts they switch
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to the substantial reservoir of water stored in the outer few centimetres of their trunks. The tree canopies also intercept and capture fog drifting in off the Pacific Ocean. As it condenses on the foliage, water droplets collect and drip onto the soil to provide another source of water that helps tide them over until the rains arrive.13
We can now begin to see how the collective activities of tiny gas valves play a crucial role in allowing Sequoia trees to grow tall by generating a transpiration stream pulling water into roots from the soil and through their trunks to the treetops. And think about how the colonnades of redwoods sprouting plumes of green foliage
rising tens of metres into the air, peppered with stomata, achieve this feat. Plants cleverly harness the power of the sunlight to drive evaporation and generate transpiration, without expending extra energy. It’s a similar story for all plants that make up the great diversity of Earth’s land floras. The packaging and longevity of the plants involved differs, but the fundamental physics of the processes remains the same.
Now consider the alternative situation—that facing the earliest plants making
the first tentative move to land without stomata (the proto-bryophytes, Chapter
Two). Living on thin soils with a limited ability to soak up and retain rainwater, perhaps similar to those found in the Mojave Desert today, and attached by simple
rhizoids, photosynthesis in these pioneering plants could only get going after
it rained. It could only continue provided the surface of simple soils on rock surfaces remained sufficiently moist to permit capillary forces to draw up water. This wick-ing process requires a continuous film of liquid water connecting one particle of soil to the next in an unbroken chain to sustain the flow, and is driven by evaporation on the surface by sunlight. The problem is that as water evaporates from the soil surface, air enters the larger pores between particles lower down, gradually breaking the continuity of the water film. Once the connection is broken, it creates a roadblock, preventing the shallow rhizoids from accessing liquid water deeper in the soil profile.14 Under such challenging conditions, early land plants lacking stomata
would have been unable to regulate water loss and unable to access soil water. These plants would have had no choice but to remain small, enter dormancy, and await
the next rains, like many bryophytes do today, to avoid desiccation.
The appearance of plants with stomata, on the other hand, marked the begin-
ning of a radical new mode of plant life.15 Thanks to this cellular innovation,
plants could sensitively control evaporative water loss from their shoots and pull water into their roots from soils. This meant they could access water deeper in the soil profile, and they gained the ability to survive on soils that dried out between
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rainfall events. It sounds like such a simple thing, but it proved revolutionary, because plant life had established a continuous watery connection between the
chloroplasts doing photosynthesis and roots in the soil. Plants could now stay
hydrated between rainfall events by regulating water loss and avoid going into
dormancy. Success, of course, hinged on the clever decision-making of stomata
that help keep the watery linkages intact while negotiating the optimal exchange
of water for the carbon dioxide needed to fuel photosynthesis.16
As land floras composed of stomata-bearing plants spread across the continents,
the burgeoning terrestrial biosphere began exerting a growing influence on
how water cycles around the Earth. For plants are really upside-down waterfalls,
showering the atmosphere with water vapour that eventually returns to Earth as
rain. At the global scale, the stomatal transpiration of modern forests, grasslands, and crops releases an estimated 32 billion tonnes of water vapour into the atmosphere each year, double th
e water vapour content of the atmosphere from other
sources. This is enough to play a significant role in the global water cycle17 and we should probably not be surprised to discover that roughly 40% of the rainfall
on land originates from the transpiring canopies of modern forests.18 In a world
without plants enriching the atmosphere with water vapour, the climate would be
drier, and the landscape more barren. But as plant life cloaked the land surface in green photosynthetic leaves perforated with pores, it gradually began re-tuning
the hydrological cycle, capturing an ever-increasing amount of precipitation
from the soil and recycling it back into the atmosphere as transpiration.
Simulations with global climate models illustrate how different things might be
without plants.19 In one simulation, a fully vegetated Earth (‘green world’) actively transpires water vapour, and in the other simulation transpiring vegetation is
removed (‘desert world’). Compare the two simulations and an idealized picture
of the role plants play in shaping global climate emerges. In ‘desert world’, rainfall over continental interiors dries up completely and summer temperatures are
nearly 20°C warmer than in ‘green world’. In other regions, land areas proved less sensitive to the absence of vegetation, especially around the margins of continents and in the tropics, where abundant precipitation fell even without plants. Could
these areas offer clues to where plants began to make landfall?
Whatever the answer to that question, we can speculate that by adding water
vapour to the overlying atmosphere, diversifying floras slowly created condi-
tions more hospitable to facilitating their continued colonization of the land.
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GAS VALVES RELEASE THE PRESSURE
ON DEVONIAN FISHES
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Plant life equipped with stomata and its revolutionary watery linkages
between roots and shoots soon spread into a new world of light and air, with
large trees and forests taking hold on the continents. The expanding biomass
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