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.

References:

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

Mosses of Central Florida 13. Philonotis longiseta

Philonotis longiseta clings to rocks along the edges of babbling
brooks. Specimen preserved as Essig 20150402-1 (USF).

[For other mosses in this series, see the Table of Contents]


Philonotis longiseta (Michx.) E. Britton  (Bartramiaceae) is a moss of wet places.  It is most often found on rocks or banks of streams, where it is constantly splashed and misted by rapidly moving water. It was said by Reese (1984) that it rarely forms sporophytes in the coastal Gulf of Mexico region, but I recently found it beside the artificial stream at the USF Botanical Garden in Tampa with abundant ripe spore capsules.  What exactly about this spot favors reproduction is not clear. Perhaps, the species requires a very stable situation, without drying or submergence, and such conditions are rare along Florida waterways.  In the constantly flowing artificial stream, however, the plants are continuously misted.


The midrib, or costa, extends as a sharp tip at the end of the leaf.  Cells are
rectangular and more elongate in the central part of the leaf. 

The leafy shoots are upright, forming bright green spongy masses clinging to the vertical rock surfaces. Leaves are stiff, long-triangular and have a prominent midrib that extends as a point beyond the tip of the leaf.  Leaf cells are elongate-rectangular, with usually a small papilla (hard, clear bump) at the upper end of each cell.


The spore capsules are frequently surrounded by
drops of water that form from the continuous
 misting.

The most distinctive feature of the genus, and others in the family that are not found in Florida, are the very round spore capsules, causing them to be generally known as apple mosses.  They form at the ends of long stalks rising from the bases of the leafy shoots.

Several other species of Philonotis have been reported from Florida, but are rare, and differ in minor ways.  P. longiseta occurs throughout eastern North America, the West Indies, Central and South America.



Reference:
Reese, W. D. 1984. Mosses of the Gulf South: From the Rio Grande to the Apalachicola. Louisiana State University Press.

Mosses of Central Florida 12. Forsstroemia trichomitria

Forsstroemia trichomitria forms a shaggy mat on the
trunk of a Liquidambar tree along the Hillsborough River
in west-central Florida. Photograph by Alan Franck.

[For other mosses in this series, see the Table of Contents]


Forsstroemia trichomitria (Hedw.) Lindb. (Cryphaeaceae) is an epiphytic moss that forms luxurious mats on hardwood trunks in moist forests.  It is found throughout eastern Asia as well as eastern North America and northeastern Mexico.  It is in the same family as Cryphaea glomerata, recently featured here, and has a very similar habit.  I am grateful to my colleague, Alan Franck, for the new collection and photographs of this species in the field. Alan was recently appointed curator of the USF Herbarium, and shares my interest in mosses and other "cryptogams."

The orange capsules are conspicuous among the upturned
leafy shoots. Photograph by Alan Franck.

The leafy shoots spread out from the trunk and curve upward slightly.  The leaves are ovate to long triangular, and are said to usually have a midrib.  In this specimen, the midrib is weak and disappears in mid-leaf.  The cells of the leaf are elongate and curved, but not as long as in Isopterygium and its relatives, with thick clear walls between them.  The leaf cells of Cryphaea are short and roundish.

The ovate-triangular leaf of Forsstroemia trichomitria has distinctive folds
along both edges.  A weak midrib can be seen
toward the torn bottom of the leaf. From Franck 3785 (USF).

The cells in the central and upper part of the leaf are
elongate, tapered, and slightly curved, with conspicuous
clear cell walls between them.

In both Cryphaea and Forsstroemia the stalks of the spore capsules are short, presumably due to their epiphytic habit.  They are elevated on the trunks and branches of trees, and so only need to drop their spores to get them into the air.  Mosses that live on the ground need long stalks to get their spores into a good launching position.  The stalks of Forsstroemia are a somewhat elongate and their capsules are fully exposed, while  those of Cryphea remain hidden within a nest of bracts.

The orange capsules are pushed out a short distance from the leafy shoots.
A ring of yellow teeth around the capsule mouth serves to push spores out 
as it alternately moistens and dries out.  
Photo from the dried specimen (Franck 3785, USF)

Mosses of Central Florida 11. Haplocladium microphyllum

Haplocladium microphyllum is frequently found growing in wet, sandy soil
in lawns.

[For other mosses in this series, see the Table of Contents]


Haplocladium microphyllum (Hedw.) Broth. (Leskeaceae)  is a creeping, mat-forming moss that is widespread globally, across much of Eurasia, South America, and in North America from Florida to Texas and up to southern Canada.

This species superficially resembles Isopterygium tenerum.  They are both prostrate, mat-forming mosses growing at the bases of trees, but Haplocladium is more likely to be found on soil, frequently in lawns, and often found on or near concrete or other alkaline materials.  Their capsules are essentially indistinguishable from those of Isopterygium, without examination of minute technical details.

Haplocladium colonies can sometimes be seen creeping
up on bricks or concrete surfaces.

The capsules of Haplocladium are curved to the
side as in Isopterygium.

The leaves of Haplocladium are short but slender-tipped, and
have a prominent midrib.

It is in their leaves that Haplocladium can be most easily distinguished from similar species.  The leaves of Haplocladium are shorter, but with more prolonged tips, and more pressed against the stem than those of Isopterygium, especially when dry.  The leaves also have a strong midrib, lacking in Isopterygium, and the cells are roundish to square, instead of long and worm-like.  The cells are also papillate, i.e. with short, hard, pimple-like projections.

The leaf cells are squarish and have a single hard
papillum (seen as a bright spot on the out-of-focus
cells to the sides). Part of the midrib is on the left.

Species in the genus Entodon have a similar creeping habit, but their capsules are upright and cylindric.

Plant Life: a Brief History

If you've enjoyed these botany professor essays, you'll probably also find "Plant Life: a Brief History" of interest.  It was written in the same spirit, but provides a more complete narrative of the evolution of photosynthetic organisms from their ancient beginning as cyanobacteria, to their dizzying diversity as flowering plants.

I wrote this book to provide a different way of looking at the the fundamental features of plants, as well as to trace their evolutionary history.  In this chronological approach, the critical features and processes of plants are presented as they arose in adaptation to new habitats or lifestyle opportunities.  Adaptation is the key process of evolution and also provides  explanations for why plants are the way they are and why they do what they do.

Plant Life: a Brief History showcases some of the best
botanical line art from  classical botany texts.  This
illustration of different forms of mosses is from the classic
"Natural History of Plants," by Kerner & Oliver, 1895.
A. Polytrichum commune; B. Bryum caespiticium;
C. Hylocomium splendens; D. Sphagnum palustre.

Photosynthesis was an adaptation, a series of adaptations in fact, for taking advantage of the huge untapped source of energy coming from the sun.  The cyanobacteria that perfected it proliferated into vast numbers and dominated the Earth's ecosystem for two billion years.  And so the story began.

Everything else, from why green algae have flagella and red algae don't, to why monocot leaves have parallel veins, can be explained as adaptations to particular environmental challenges or opportunities.  While we don't know for sure the details or circumstances of some of these adaptations, we can be sure that nothing about plants came about accidentally or without value to their survival. For the adaptive events of plant evolution that are still shrouded in mystery, I provide some plausible scenarios, including some that are still controversial.

The detailed structures of Psilotum and Tmesipteris
are crystal clear in this illustration from the "The Plant
Kingdom," by William F. Brown, 1935.  Reprinted
with permission from the Brown family.

This book also highlights another aspect of history: the marvelously detailed line drawings of the great botanical texts written in the 19th and early 20th centuries.  There is a vast treasure of such illustrations out there, and I have chosen a number of them to illustrate the various plants we encounter in the book.

The book is designed as a supplement for students, a refresher for teachers, a primer for non-botanists whose research or teaching touches upon plants, and as an introduction to plants and their evolution for the general reader.  Because of this broad intended audience, technical jargon has been kept to a minimum.

A preview is posted on the Google Books website, which includes the table of contents, the introduction, and the first chapter. It is available from Oxford University Press USA or U.K. and other major booksellers.  It is also available as an eBook from many online booksellers.

If you can't purchase it at this time, please ask your local or school library to obtain it.

In the meantime, we will continue with more adventures on this blog site.  Stay tuned!

Mosses of Central Florida 10. Cryphaea glomerata

Cryphea glomerata grows on branches of hardwood trees. Photographed in
Hardee County by Alan Franck, USF Herbarium, 

[For other mosses in this series, see the Table of Contents]

Cryphaea glomerata Schimp. ex Sull. (Cryphaeaceae) is unusual in several respects.  It grows relatively high on the branches of hardwood trees in moist forests, and the sporangia are on very short stalks.  It is found throughout the eastern US, from New Jersey to Florida and west to Texas and Oklahoma.

Cells of the leaf are small and roundish, with thick walls.
From Franck 3699 (USF).

The stems are horizontal with leaves more-or-less flattened in a plane.  Leaf cells are flattened-roundish, with thick walls, and a midrib extends most of the way through the leaf.

Sporophytes are produced sporadically along the stems and are sessile (lacking a long stalk), remaining embedded within a cluster of sharp-tipped bracts. The unusual positioning indicates that long stalks are of no advantage to mosses that spread their branches from high vantage points, allowing them to provide greater protection for the sporangia.

A related species, Cryphea nervosa, is also found in Florida, and differs only in its more prolonged, sharp-tipped leaves, and the capsule teeth being more deeply embedded.

The family Cryphaeaceae is closely related to the Leucodontaceae, and not well-distinguished, making the genus Forsstroemia the most closely related genus locally.

Sporophytes (arrows) are produced sporadically along the leafy stems
and remain embedded within a nest of protective bracts.
From Franck 3699 (USF).

Mosses of Central Florida 9. Sematophyllum adnatum

Sematophyllum adnatum forms tangled mats of elongate leafy
stems.  It is distinguished from mosses of similar habit
by the leaves that curve strongly to one side.
(Essig 20060516, USF)

Revised 2018 Mar 20, with new photos of leaf cells.

[For other mosses in this series, see the Table of Contents]

Sematophyllum adnatum (Michx.) E. G. Britt. (Sematophyllaceae) is one of the most common mosses in Florida, and also occurs westward into Texas, northward to New York and Ohio, and in tropical America.

It resembles other common mosses, such as Isopterygium tenerum, in that it forms thick mats of straggling leafy stems primarily on the bark of tree trunks, particularly live oak trees, and fallen logs.  It differs from other straggling mosses most conspicuously in the way the leaves curl to one side, particularly as they dry out, and in the capsules, which are slightly asymmetric, but erect, not strongly curved the way they are in Isopterygium.  

Leaf cells of Sematophyllum are narrow, tapered, and somewhat
worm-like, but with thinner walls than in Isopterygium.  From
a dried herbarium specimen (Essig 20160119-2, USF)

The leaves gradually taper to a narrow point. 

The leaves are similar to those of Isopterygium and other members of the Hypnaceae, with slender, worm-like cells, and lacking in a costa, or midrib.  The leaf tips are generally not toothed like those in Isopterygium, and cells at the basal corners are more inflated. The capsules are also similar to those in the Hypnaceae, with two rows of teeth around the mouth.  Sematophyllum is presently separated in the Sematophyllaceae, which is scarcely distinguishable from the Hypnaceae.