Friday, September 25, 2015

Dick Brummitt - champion of the paraphyletic

Botanist Dick Brummitt passed away on September 18, 2013.  I missed the opportunity to post a tribute to him at the time, and September of last year also flew by, so in the waning days of September 2015, I will at least mark the second anniversary of his passing.  He is someone we should remember.

Who was Dick Brummitt?  He had a long productive career as a plant taxonomist, primarily at the Royal Botanic Gardens at Kew.  He did what taxonomists do.  He went into the field collecting specimens, did generic revisions and worked on floras.  He did a lot of important work in Africa.  He is more widely known, however,  for his passionate and dogged defense of the peculiar beasts referred to as "paraphyletic taxa."   In a decades-long battle, he pitted himself against the advocates of the new phylogenetic taxonomy, who insisted that only monophyletic groups of organisms could be recognized as formal taxa.  I dedicated two earlier posts to this subject "the great botanical butter battle" and  "making the ancestor problem go away," so I will just briefly summarize it here.

A monophyletic taxon, you may recall, consists of a complete branch, or clade, of a phylogenetic tree: a common ancestor and all of its descendants.  In one of his most memorable essays, Brummitt declared himself to be a "bony fish."  This remark stemmed from the new phylogenetic taxonomy of vertebrates, in which amphibians were a subclade of the bony fish, reptiles (amniotes, if you prefer) a subclade of amphibians, and mammals and birds both subclades of reptiles.  "Mammals" and "bony fishes" are no longer equivalent classes as they were in previous classifications.  The new classification reflects better the evolutionary history of organisms, but raises some difficult practical questions.

What really irked Brummitt was the difficulty of naming ancestral groups.    If we wish to refer to just the bony fishes that did not become amphibians, reptiles, etc., what name can we give them? We cannot put them into a formal category, such as a class, phylum, or family  because they are a paraphyletic taxon.  Such a taxon contains a common ancestor (the first bony fish) and some of its descendants (bony fishes that remained bony fishes), but not all of its descendants (amphibians, etc.), so it is not a complete clade.  If we decide that the bony fish clade is a formal "class" then what are the mammals or birds - sub-sub-sub classes?

Brummitt argued that, taken to its logical conclusion, this situation would lead to a collapse of the taxonomic system, because, after all, bony fishes were just a subclade of an earlier group of vertebrates, the early vertebrates were a subclade of a still earlier group, and so on back to the first organisms.  Recognition of the branching clade structure of life is extremely valuable, but it has made the application of formal ranks difficult, inconsistent, and increasingly less useful.  Brummitt was right on that, and most taxonomists now avoid higher level taxa such as phyla, classes etc., referring instead to stem groups, clades, grades, and other references to portions of the phylogenetic tree.  Supporters of the "phylocode," advocate a system for naming clades without trying to stuff them into formal ranks.  This was a de facto recognition of one of the problems of phylogenetic classification perceived by Brummitt and other like-minded taxonomists.
The groups that we recognize as genera have arisen
sequentially from earlier genera.

With respect to lower level taxa, such as genera and species, we're kind of stuck, however.  We need to give organisms (including fossils) names, and there seems to be no good way to do that other than the traditional binomial: the genus plus the specific epithet.  So we need genera, and we need to be able to group organisms into genera even if they were ancestral to other genera.

For example, the very first species of our own genus, Homo, most certainly evolved from members of an earlier genus traditionally known as Australopithecus, making the latter genus paraphyletic. The same is true for every known genus and the genus that preceded it,  A genus that is monophyletic today might in several million years become paraphyletic by giving birth to new genera. Paraphyletic genera are therefore unavoidable.

Phylogenetic taxonomists have tried to avoid recognizing such genera - sometimes lumping paraphyletic genera with the nearest monophyletic genus, sometimes splitting paraphyletic genera into smaller monophyletic units, or by recognizing unavoidable groups of ancestral species (a"stem group" ) as some kind of special category.  All of these diminish the meaning of genera as comprehensible units of biodiversity.

Brummitt and others (see Hörandl  & Stuessy 2010) argued that paraphyletic genera (and other ranked taxa) should be simply recognized and named in our taxonomic system because they are unavoidable, natural units of evolutionary history.  

Literature cited:

Hörandl  & Stuessy. 2010. Paraphyletic groups as natural units of biological classification. Taxon 59 (6): 1641-1653.

Wednesday, August 26, 2015

The evolutionary perspective in teaching botany

[This essay is modified from one that I recently posted at the Oxford University Press blog site]
Many of us involved in teaching botany feel a sense of urgency in our profession.  Botany departments, botany majors, and botany curricula have gradually shrunk or disappeared from most colleges and universities in the US, and I suspect in many other parts of the world as well.  Too many students are graduating with little or no understanding of the unique ways in which plants meet the challenges of survival and reproduction in the Earth’s diverse ecosystems.  Biology faculty who don’t have training or experience with plants are often ill-prepared to relate to or take advantage of the unique contributions plants might make to their own teaching and research.

So if we have only a semester, or worse only a week or two, to teach the fundamentals of plant life, and to pass on the exhilaration we feel in the face of their diverse adaptations, how do we do it?  If our non-botanical colleagues or teaching assistants have been assigned to teach a beginning level segment on plants, how do we help them understand the basics and develop some enthusiasm for the subject matter? 

Some teachers prefer an ecological approach, emphasizing the pivotal and diverse roles of plants in the ecosystem.  Others prefer an approach emphasizing applications to human technology, agriculture, nutrition or medicine.   All of these approaches are useful in developing interest, but may end up being too superficial with respect to fundamental structure and function.  Traditional botany texts tend to be dry and encyclopedic.   Non-majors texts may be more appropriate for most of today’s audience, but they still tend to avoid a side of biology that I call the “why” questions. 

One must have the “what” before the “why,” but it is the latter that gives some context or meaning to the former.   The “what” is the factual material one finds in a textbook.  The “why” is the explanation of the “what.” For example, textbooks typically contain a little section on the differences between monocots and dicots (or now monocots and eudicots, awkwardly ignoring magnolids, waterlilies and other basal angiosperms).  We are told that dicots typically have net-veined leaves, vascular bundles arranged in a ring in the stem, and secondary (woody) growth, while monocots typically have parallel-veined leaves, vascular bundles scattered within the stem, and no secondary growth.  That is the “what,” at least in a simplistic sense, but there is typically no “why” to follow it.
The sword-shaped leaves of cat tails, have parallel
veins because new tissues are added at their bases,
 pushing them upward from their underground stem
systems, This lengthens each vein as the leaf lengthens.
The corresponding suppression of woody tissues in
the underground stems occurred as the stems adapted
 for clonal spreading rather than vertical growth. 

  Monocots are the newer invention in plant architecture, having developed their unique structures and way of growth as they split from ancient dicots.  Why do their leaves have parallel veins?  Why do they not have secondary growth?  How do they interact differently with the world than dicots, and how did their innovative structures come about? (Hint: it has to do with ancestral monocots going “underground.” ) See the caption to the right, and for a more extended exploration of these questions see my blog post: How the grass leaf got its stripes.

“Why,” in scientific terms, has to do with the process of adaptation. It’s the story of origins, of plants facing environmental challenges and evolving innovative ways to cope.  This is what makes botany interesting.  It is also a way to make sense of the fundamental features of plants, some of which may be dismissed as obscure and unimportant, but which are loaded with both meaning and utility. 

For another example, let’s take everyone’s favorite: life cycles.   Students already sophisticated enough to know that sperm and egg in animals are produced through the special kind of nuclear division called meiosis are truly puzzled by why that does not happen in plants.  Others are surprised that plants produce sperm and egg at all. Meiosis mixes chromosomes and reduces a double set (diploid) into a single one (haploid) in each of the resulting cells.  In animals the haploid cells combine into a diploid zygote, which develops into a new diploid individual  In algae and plants, however, it's more complicated. Bear with me, even the short version is convoluted!

In the evolutionary story of sexual reproduction in plants, we find that the algae similar to those that gave rise to land plants, and simpler land plants themselves, are haploid and do produce sperm and egg directly.  In both cases, however, the joining of sperm and egg does not result in a new plant, but rather in a short-lived diploid zygote that produces spores through meiosis. Spores are adapted for long-distance dispersal, and germinate to form new haploid plants that will eventually produce gametes.  So spores, not gametes, are produced through meiosis in plants.

A moss colony, such as this Isopterygium, is
both photosynthetic and gamete-producing (the
gametophyte generation).  It holds water within 
its spongy matrix, which sustains the  life of the
vegetative tissues and also provides a watery
pathway for sperm cells in search of eggs.
Because of this mode of reproduction, mosses
must remain small and close to the ground.  The
sporangium and its elongate stalk constitute the
sporophyte generation, a separate individual
resulting from the fertilization of the egg.
Spores will germinate to establish new 
genetically mixed moss colonies.  
The production of spores in green algae mostly occurs within individual cells, but in land plants, a small, diploid, multicellular body, technically a separate plant called a sporophyte, develops for that purpose.  The fact that plants alternate between haploid gamete-producing plants and diploid spore-producing plants is the “what” of plant reproduction.  Students might memorize dozens of life cycle diagrams, but won’t know “why” such things exist, or why they have to bother with such tedia, until the adaptive story is told.     

That story has primarily to do with the fact that plants cannot move around to find mates, and that if they simply released sperm cells to go off and find an egg on their own, it would lead at best to severe inbreeding.  Such a strategy works well enough in some marine invertebrates, like sea stars, where currents can help disperse the sperm cells, but on land, these tiny, fragile cells just don’t get very far.  Spores do the traveling for  plants, taking the place of mate selection in mobile animals.  Genetic diversity in plants depends on spores from different genetic backgrounds landing close to one another, so that when they develop into gamete-producing plants, suitable mates will be next to one another. 

In the early vascular plants, the diploid sporophyte
generation became the dominant part of the life cycle,
lifting spore-producing structures high into the air. 
Some of their descendants, including the ancestors 
of these giant douglas firs, evolved seeds and pollen 
grains - the more complex spore-derived vehicles that 
bear tiny egg- and sperm-producing individuals. 
Spores are launched best from an elevated vantage point, and so sporophytes tend to be stretch upward as much as possible.  In mosses, this can be only a few centimeters (see "Why are there no moss trees?") but this suffices to get spores above the low-growing moss foliage. In the land plants we call vascular plants, however, sporophytes became larger and larger, and in fact the trees, herbs, and grasses we see today are actually the sporophyte generation of the plant life cycle.  The egg- and sperm-producing “plants” (gametophytes) - the equivalent of the algal or moss colonies, are hidden within the embryonic seeds and pollen grains of these more advanced plants.

Yes, it’s complicated, but if the story unfolds from the perspective of how and why it evolved, it does make sense. And it is an important story.  Understanding how plants and algae reproduce impacts both agriculture and ecology.  

Plant Life – A Brief History,” provides the adaptive perspective of plant features for students, instructors, and others interested in the biology of plants.

Wednesday, July 8, 2015

In Defense of Plants

 "In Defense of Plants" is the title of a fairly new blog site that has come to my attention.  It is written by Matt Candeias. The blog postings are thoughtful, informative, and always interesting.  The photography is excellent.  It is very encouraging to see members of the newest generation of thinking adults (what do we call them - millennials?) take a passionate and informed interest in plants.  Hopefully Matt will maintain this effort for many decades to come.  I'm putting it on my favorites list, and I hope you will too.
The following excerpts are from an article Matt did for the online magazine "The Wildernist,"  edited by John Jacobi:
"Towards the end of my undergraduate career I took a job restoring abandoned quarries throughout western New York. The goal was to take possibly the most destructive form of land use and attempt to coax something resembling a habitat out of it.
My favorite project took place in an old sand pit way out in the country. Spending time there was rewarding enough, as the surrounding wilderness was already beginning to reclaim what humans had taken from it. We were attempting to reintroduce an endangered butterfly to part of its former range, and to do so, we needed to establish a robust population of its host plant. The butterfly in question is the Karner blue (Lycaeides melissa samuelis) and its host plant the blue lupine (Lupinus perennis). Karner blue caterpillars feed on nothing else.
The Karner Butterfly
Following the end of the Pleistocene, L. perennis took advantage of the well-drained soils left in the wake of the retreating glacial ice sheets and spread from coastal New England all the way to Minnesota. It specializes on nutrient poor, sandy soils. In fact, these plants were once thought to be bad for the land, robbing it of life and vitality. As such, they were maligned. The generic name “Lupinus” has its roots in another Latin word and was given to these plants because early botanists associated them with another creature that haunted their nightmares and left the land impoverished—the wolf (Canis lupis). As with the misappropriated hatred towards the wolf, the idea that Lupine was bad for the land was far from true. Being a legume, it is able to fix atmospheric nitrogen, thus bringing life to barren soils. But, as is human nature, facts never seem to trump emotions, and L. perennis has seen a 90% decrease in its numbers in the wild. With it went the Karner blue butterfly."
".... plants have this amazing ability to absorb energy from our sun and turn it into food, a fact that with the exception of deep sea thermal vents, every organism on this planet relies on in one form or another. They have been at it for a long time too. The botanical world is full of survivors. Far from being boring and nonreactive, plants are living, breathing organisms capable of some amazing biological feats, which include chemical warfare that the UN would seriously frown upon. They have been at this whole survival game for much longer than any of our ancestors have. Each species has its own story, its own ecology, and its own way of interacting with the world around it. Plants aren’t here for us. We are here because of them. Everything is. We define entire ecosystems by the types of plants that grow there. We simply cannot understand the living world without first considering the flora that shaped it."

Monday, June 15, 2015

Mosses of Central Florida 14. Hyophiladelphus agrarius

This specimen of Hyophiladelphus was  found
growing on imported red pumice used in a
landscaping in Pinellas County
(Essig 20120818-1 USF).  In Florida, it is typically
found on limestone.
[For other mosses in this series, see the Table of Contents]

Hyophiladelphus agrarius (Hedw.) R. H. Zander (Pottiaceae) is one of many Florida mosses that is found only on limestone or other calcareous materials.  The single species in this genus is found throughout the southeastern United States, from Texas to South Carolina, and throughout tropical America.  It has at times been included in the related genus Barbula, but differs in its more compact, rosette-like shoots, and the lack of papillae (small, hard, round bumps) on the leaves.  Another related genus, Tortella, has a distinctive V-shaped pattern of clear cells at the base of the leaf, by which it can be distinguished.

All of these genera, and a few others in the family, have long, hair-like teeth, twisted around the opening of the capsule.

A colony of Hyophiladelphus, dried out at the time of the photograph,
occupies pockets in the eroded surface of a limestone boulder. (Essig 20150402-2, USF)
Hyophiladelphus shoots are upright rosettes. The broad leaves
are nearly flat when hydrated, and have prominent, thick
midrib.  The shoots are green to dark green, or almost blackish
when dry. (Lewis 20061028-1, USF)

The long, hair-like teeth  (peristome) around the mouth
of the capusle are twisted around each other. (Essig 20060931-1, USF)

At the base of the leaf, the cells are greatly enlarged, more or less rectangular,
and sometimes brownish. (Essig 20150527-2, USAF)
Cells in the upper part of the leaf are compact and roughly
squarrish to rectangular. (Essig 20120818-1, USF)
When dry, the leaves of Hyophiladelphus roll into a curved, tube-like configuration.  In these old capsules the long, twisted
peristome teeth have mostly fallen off. (Essig20150404-2, USF)

Wednesday, June 3, 2015

Minding your stems and crowns

[The following essay is re-posted, with some minor revisions, from one I did for the Oxford University Press blog.  The OUP site features pieces done by authors of OUP books.  I encourage you to check this interesting site.]

Since evolution became the primary framework for biological thought, we have been fascinated, sometimes obsessed, with the origins of things.  Darwin himself was puzzled by the seemingly sudden appearance of the angiosperms (flowering plants) in the fossil record.  In that mid-Cretaceous debut, they seemed to be already diversified into modern families, with no evidence of what came before them.  This was Darwin’s famous “abominable mystery.”

Birds arose around the same time, but for them we have a detailed fossil record documenting the evolution of their feathers, wings, and specialized skeletal features.   For plants, there is still a huge gap between living angiosperms and fossil groups that might be related to them, but we do have tools for whittling away at the mystery.   

Figure 1.  The angiosperm stem group consists of extinct 
seed plants that branched off after the common ancestor
 with other living seed plants (the gymnosperms), but before 
the common ancestor of known angiosperms (the crown group). 
The distinctive features of the angiosperms evolved in the
stem group. Source: modified from Wikimedia Commons, 
licensed by Creative Commons.
The “top-down” approach uses modern methods of DNA-based phylogenetic analysis to build accurate trees of the living angiosperms, identify the most archaic taxa among them, and from their characteristics make predictions about their common ancestor.  By definition, the living members of a group of organisms, their common ancestor, and any extinct species, or “dead ends,” among them, constitute a “crown group” (Fig. 1).

The “bottom up” approach analyzes the available fossil record, to identify which extinct species might be most closely related to the crown group, and which of their structures might have been transformed into the characteristic features of the crown group.  In the angiosperms, this means particularly the  flower parts.  The extinct organisms leading up to the crown group are referred to as the “stem group,”  which, by definition, extends backwards to an earlier ancestor shared with the next most closely related group of living organisms.

For example, the closest living relatives of birds are the crocodilians, and the bird stem group includes all of the dinosaurs (!).  That may sound like the tail wagging the dog (we can alternately call birds a subgroup of dinosaurs), but it is during the long line of dinosaurian ancestry that we see the evolution of feathers, wings, and flight, along with other features shared by birds and dinosaurs but not found in crocodilians.  Similarly, the stem group of amphibians is where fish turned into land animals, and the stem group of reptiles is where amphibians turned into reptiles, with advanced (amniotic) eggs that could be laid on land.  The stem groups are where all the fun is!

Figure 2. Like this modern Anemone, the first 
true flowers consisted of tepals, stamens, 
and a central cluster of carpels.
But who were the “dinosaurs” of the angiosperm story?  The stem group of the angiosperms goes back some 300 million years to where it split from the ancestor of the living gymnosperms – conifers, cycads, etc. (the “crocodilians” of the angiosperm story) (Fig. 1).   The common ancestor of both gymnosperms and angiosperms, which lived some 300 million years ago, was some kind of seed fern, a plant that bore seeds and pollen on its leaves.    The first full flowers, which may have come into existence around 140 million years ago were bisexual, with distinctive closed carpels, flattened stamens with 4 pollen sacs, and embryonic seeds (ovules) that were “bent” and contained by a double envelope (integument) (Figs. 2 and 3).  The plants that might tell how, why, and where ancient leafy structures were transformed into these distinctive organs are not only extinct, but also largely missing from the fossil record. 

Among known members of the angiosperm stem group, one bright spot lies within the extinct order Caytoniales.  Some phylogenetic analyses of fossil Mesozoic seed plants reveal this group to be the most closely related to the angiosperms.  This supports an older hypothesis promoted by evolutionary botanist G. L. Stebbins that the peculiar bent angiosperm ovule was derived from the seed-bearing cupule of the Caytoniales (Fig. 3).  Known members of the Caytoniales, however, provide little information about the evolution of modern stamens and carpels. 
Figure 3. The bent ovule with a double integument 
characteristic of the angiosperms (C) may have 
evolved from the seed-bearing cupules of the
 Caytoniales (A), as the number of ovules within 
was reduced to one (B). Source: redrawn after 
Brown, 1935, The Plant Kingdom, Ginn & Co., 
Boston and New York, with permission.

Why should there be a gap in the crucial part of the record?  The various Mesozoic seed ferns left a fair number of fossils; why not those leading up to the first angiosperms?  Aside from the lower fossilization rate of plants in general, it may be that the pre- and proto-angiosperms evolved in habitats where fossilization was particularly unlikely.  For Stebbins and others, that habitat was semi-arid subtropical uplands.  Stebbins felt that the patchy physical environment and seasonal, marginally sufficient rainfall in such environments provided the maximum stimulation for evolution of new growth forms, and in particular for the short reproductive cycle that is characteristic of the angiosperms.  Such environments are the primary hotbeds (“cradles”) of angiosperm innovation and diversity today, while wet tropical forests serve more as refugia or “museums” for archaic angiosperms. 

The study by Taylor Feild and his colleagues in 2004, which included analysis of the anatomy, physiology, and ecology of archaic living angiosperms, resulted in a very different hypothesis about the crown group ancestor: that it was adapted to disturbed areas and stream margins in dark, damp forests, where there might be similar pressures for a more rapid reproductive cycle.  Who was right?

The answer depends on our reference point.   The top-down approach defines the nature of the crown group ancestor, while the bottom-up approach makes hypotheses about adaptive events along the long stem lineage.  Angiosperm precursors in the stem group may very well have lived in a variety of habitats, including upland, semi-arid habitats prior to moving into damp, disturbed habitats.

The accumulation of the distinctive features of the angiosperms probably took millions of years, paralleling the progression from feathered dinosaur to true birds.  In fact, if we designate the first plant with closed carpels as the first angiosperm (“hidden seeds”), and if other standard features of the flower evolved either before or after that, then “angiosperms” and “flowering plants” are not exactly synonymous.  And the crown group ancestor refers to a still later reference point!   The crown group ancestor was not the first angiosperm, just as there were true birds prior to the bird crown group ancestor.  All we can say for sure is that it was a successful angiosperm, with all the standard floral features in place, and that it proliferated at the expense of other early angiosperms. 

Therefore, when postulating the origins of groups of plants, we must be careful to mind our stems and crowns!

Monday, May 18, 2015

Happy Fascination of Plants Day!

May 18 has been established as a day to celebrate the fascination of plants.  It is a time time remember how much plants give to us, from food and medicine to building materials and garden flowers.  It is their great diversity and the remarkable ways in which plants solve the problems of survival that provide botanists, gardeners, nature lovers, natural healers, and gourmet cooks with endless fascination.  It is important that all of us who care about plants share our enthusiasm with others around us who are not as aware of our rich botanical legacy.

Oxford University Press has posted paragraphs from several botanical authors, including yours truly, on their blogsite, which you might find interesting.

Thursday, May 7, 2015

Water potential explained

The short answer to the question "How does water get to the top of a redwood tree" was that trees function like gigantic, complex paper towels, and that a combination of capillary action and evaporation (transpiration) maintains a moving stream of water from the roots to the leaves.  But that's only half of the story.  The other half has to do with the living cells embedded within the "paper-towel matrix."  Their survival depends upon being able to draw water and minerals from the flow around them, but they also can move water and materials directly among themselves, creating an alternate path for moving fluids around the plant.

The movement of water into and out of living cells results from a balance of forces around them: primarily evaporation and solute concentration ("saltiness").  Evaporation and salt water can both remove water from cells, but living cells also contain enough solutes of their own to draw in fresh water from the soil.  Some other factors, such as gravity in tall trees, also influence the flow of water through a plant, but for most practical purposes we are concerned only with the roles of solutes and evaporation.

I should probably begin by providing a more proper term for that spongy mass of cellulose - the "paper-towel matrix" -  that permeates the plant.  That would be the apoplast, which translates as "outside of the cells."  Basically, the apoplast is the interconnected mass of cellulose walls that surround each cell, as well as the small spaces between them, and most importantly the empty tubes of the xylem tissues.  The apoplast is all non-living material secreted to the outside of the living cells.

Living plant cells are interconnected by tiny tubes of protoplasm called plasmodesmata, and water can  move directly from cell to cell through them.   This network is called the symplast.  On a sunny day, the net movement of water through both the symplast and the apoplast is upward toward the leaves, though it is much faster in the apoplast. Thus there are two parallel, cross-connected networks running through the plant.

Evaporation is the dominant force pulling water to the top of the plant, which is normally OK, but if excessive, can be a threat to the survival of the plant. Loss through evaporation has to be balanced by water absorbed from the soil.  If the soil dries out, it becomes an evaporative agent like the atmosphere, and can suck water back out of the roots.  Even if there is sufficient moisture in the soil, an excessively hot and dry atmosphere may pull water out of the plant faster than it can move up the plant.  Plants are generally adapted to the conditions of their native habitats, but can still perish in an extreme drought.

Now,  if you stick your favorite house plant into a bucket of saltwater, it is effectively the same as sticking them into dried-out soil.  The sodium and chloride ions in saltwater are solutes, the particles that can draw water across a cell membrane. The salt water would rise up through the apoplast, but in passing by the living cells, it would literally suck them dry.  The same thing happens to us if we drink salt water. This is due to osmosis: the movement of water across a cell membrane toward a region of lower water concentration (i.e. toward higher salt or solute concentration).  

Plants such as mangroves that grow in salty water have special adaptations to keep the salt away from their cells.  Some are able to filter out salt at the root surface, others have salt-excreting glands on their leaves, and still others accumulate salt crystals within their leaves, which are eventually shed from the plant. But ordinary plants without such adaptations will be killed by exposure to salt water.

The forces of solute concentration and evaporation can be quantified.  Water potential is the measurable tendency for water to move from one part of the plant system to another depending on the balance of forces around it. Water potential allows us to predict which way water will move and how fast it will move.  Water potential is expressed in negative numbers.  The highest water potential we find in plants is zero, and water will always moves into areas of more negative water potential.  The most negative areas of a plant are at the top where evaporation is occurring, and the least negative are in the roots.  So on a sunny day, the flow of water is upward from roots toward the leaves.

Pure water at sea level and average atmospheric pressure and temperature has a water potential of zero, measured in megapascals (MPa).  That's our reference point.  The water potential of a typical, well-hydrated soil is also close to zero, but is slightly negative due to some dissolved minerals in it. The atmosphere and salt water both have strongly negative water potentials sufficient to remove water from unprotected cells.

Plant cells contain minerals, sugars, and other solutes that make them more "salty" than the water in the soil. The water potential of living plant cells varies, but is generally about -0.2 in the roots. Progressing up the stem, the water potential decreases.  A typical figure in mid-stem might be around -.6 MPa.  In the leaves, where the cells are much closer to the site of evaporation, it can decrease to -1.5 or less.  All of this varies considerably depending on the height of the plant, the external conditions, and special adaptations of the plant for its particular environment.

 The atmosphere is usually pretty dry but that depends on the relative humidity.  Saturated air, on a damp, foggy night for example, will have a water potential near zero, and not much water will flow.  Typically though the water potential of the air will be -100 or lower.  Hot, dry desert air can have a water potential as low -300 or even -500 MPa.  This then sets up a gradient from the soil to the top of the plant that drives the flow of water.

Salt water (with salt concentration of 3%)  has a water potential of about -25 MPa (Tomlinson 2004), much more negative than the typical living plant cell.   Remember that water can flow either direction across a cell membrane, from whichever side has the higher water potential, as a result of osmosis.  Salinization of agricultural soil is a big problem in dry climates where irrigation water evaporates, leaving ever higher concentration of salts in the soil.  It becomes a necessity to seek more salt-resistant plants for continued productivity in such regions.

Something similar happens if you water a potted plant repeatedly without letting the excess water drain from the bottom of the pot.  Mineral salts in the water accumulate, making it harder for the plants to absorb water from the soil.

What about turgor pressure?  That is the positive pressure that builds up in healthy plant cells as a result of osmosis.  Turgor pressure drives many processes, such as cell expansion, phloem transport, and venus-flytrap closing (See "How plants do everything without moving a muscle").  It may seem contradictory that living plant cells maintain a negative water potential and at the same time a positive turgor pressure.  Turgor pressure is a direct result of water moving into a cell  because of its solute content (its "saltiness'), and does cancel out some of the overall water potential of the cell.  So the measured water potential of the cell is its negative solute potential plus its positive pressure potential (i.e. pressure potential minus the solute potential).

  At maximum turgor pressure, such as on a foggy night when there is no evaporation, the turgor potential and solute potential can balance out, resulting in a water potential near zero throughout the plant and no water movement.  But on a sunny day, evaporation creates a net upward flow of water  that runs through the symplast as well as the apoplast, so maximum turgor pressure is not reached.  This leaves the net water potential of the cells of the root negative enough to continue pulling water from the soil.

"Reverse osmosis" is a process for purifying sea water by applying sufficient pressure to overcome the solute potential of the seawater, forcing water molecules, but not salt particles, across a membrane similar to that which surrounds every living cell.  Such a membrane is called semipermeable.

Water moves freely through the apoplast by capillary action, and is drawn upward by evaporation in the leaves, especially in the xylem (left side of diagram). This is transpiration. Water is absorbed into cells by osmosis, particularly in the roots, which increases turgor pressure. Turgor pressure then pushes water through the symplast toward cells higher up that are losing water to evaporation, paralleling the flow in the xylem, but much more slowly. The tiny blue passageways between cells are plasmodesmata. Water does not evaporate directly from the xylem, but through leaf cells exposed to air chambers connected to stomata.  As water evaporates from the mesophyll cells of the leaf, their turgor pressure decreases, decreasing their overall water potential, and this causes them to continually absorb water from the xylem as well as from the living cells below.

Tomlinson, P. B. 2004.  The Biology of Mangroves.  Cambridge University Press.