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.
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.

Thursday, April 23, 2015

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).
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.

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

Friday, April 10, 2015

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.
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)

Monday, March 16, 2015

Mosses of Central Florida 11. Haplocladium microphyllum

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

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 tenereum.  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.