The evolution of transport systems in plants
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Introduction
Xylem
Phloem
Summation; integration
http://www.xs4all.nl/~steurh/eng/rhynia.html
evolution or revolution ?
In the beginning………….
carbon
oxygen
ozone
nitrogen
light
hydrogen
migration
air
hydrocarbons
stress
land
water
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The symplast and apoplast
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Symplast - Defined as that region of the living plant, that is bounded
(enclosed by) a plasmamembrane. The cells forming this group are
called a DOMAIN, and DOMAINS are connected via PLASMODESMATA.
next
Apoplast. By definition, all regions of living plant cells NOT bounded by
a plasmamembrane. This must include the CELL WALL as well as
intercellular spaces. These two constitute FREE SPACE in the plant.
Apoplast thus involved in the (free) movement of substances and the
principal conduit in this case, must be the xylem or xylem-associated cells
that LACK a plasmamembrane.
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xylem and phloem - the parallels
Phloem and xylem have distinct physiological functions, and their
distinctly-different functions give them a unique parallelism.
The xylem conducts water, usually acropetally, from root to shoot and the
leaf, (in a sense, from source to sink) whilst the phloem in the crudest
sense, transports carbohydrate and other substances from a site of
manufacture (source) to a site of utilization (sink).
In both cases, tubes are involved – sieve tubes in the phloem, and
xylem vessels and narrower trachieds in the xylem. Both systems
are associated with parenchymatous elements, and both are relatively
delicate structures.
It is now more than 160 years since, for example, the publication of
Hartig’s descriptions of the bark of trees. There is much new
information concerning structure and function, but also, we have
become more aware of the fossil record, allowing us to trace the
evolution of these remarkable conduits through some 500 million years
of evolution.
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phloem - early work
1. In 1837, Theodor Hartig published his work on the organization of
the trunks of forest trees, and commented on the then “completely
uninvestigated elementary organs” of what was called the ‘sap skin’
(Safthaut) of trees.
2. Significantly, Hartig described three types of cell – Siebfasern,
Siebröhren and keulenförmige Saftröhen – in English, the
sieve elements and laticifers common in Euphorbia for example.
3. The Siebröhren and Siebfasern correspond entirely to what we
describe today as sieve tubes and in the latter case, to sieve cells,
associated cambial cells, phloem parenchyma as well as
sclerenchymatous elements.
4. In 1858, Nägeli coined the term ‘phloem’ top describe the food
conducting systems in higher plants.
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Moving on.
Developing the xylem transport system
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Initially, transport of water in plants that had just emerged onto
land, would have been a physiological necessity –
Why?
Ancient origins
Indeterminate hydroid or stromatoporoids in Starostinella nordica
Upper Permian ca 280 MY
Since then many examples in which hydroids appear.
Stelar arrangements and tissue organisation
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All comparisons serve to underscore the fundamental relationship
in early vascular plants between the evolution of increased
complexity in stelar architectures and the evolution of
complex lateral branches and leaves.
water transporting function is the crucial factor in the evolution of the
various steles.
Vascular plants seem to be the ideal study group for integrating
developmental process directly into analysis of homology and
estimation of phylogenetic relationship.
summary: main evolutionary trends
TRACHEID DIMORPHISM 1
•pit membrane diameter decreases
•reduction in size of borders on pits
•fewer pits
tracheid
fiber tracheid
libriform fiber
summary: main evolutionary trends
TRACHEID DIMORPHISM 2
tracheid
fiber tracheid
libriform fiber
SPECIAL EVOLUTIONARY TRENDS
tracheid dimorphism:
vasicentric tracheids +
septate fiber tracheids
fiber tracheid dimorphism:
fiber tracheids +
vasicentric tracheids
fiber dimorphism: parenchymalike fibers (or parenchyma) +
libriform fibers
Narrow is best ?
Tracheids in their various forms are thus the principal water conducting cells in
gymnosperms, as well as in vesselless dicotyledons.
They present many advantages in terms of transport.
Think for example, of the boreal forests – what is the species composition?
click
Mostly gymnosperm, some vesselless angiosperms
Why?
Vessel bearing dicotyledon
Vessel elements and tracheids much shorter than those in vessel-less wood
Transition from vessel-less to
vessel-bearing wood. Qualitatively,
a marked drop in the length of
the tracheid to primitive vessel
occurs.
Vessel end retains remnant primary wall strands
Scalariform perforation plates
Scalariform perforation plates are indicative of the
evolutionary level of the species, just as shortening of the
vessel elements is a (reliable) factor indicative of
evolutionary advancement, so to is the shape, structure
and hence, morphology of the scalariform perforation
plate associated with the end wall perforation of a vessel.
Bordered bars in primitive species vary but all are
subdivided as in this example.
Overview of variation and evolutionary lines of
scalariform perforation plates
evolutionary paths towards
simplified perforation plates.
Does this suggest
environmental and
cliamatalogical influences?
Variations and evolutionary lines of scalariform perforation plates 1.
scalariform = ladder-like structure
Perforation plates become simpler, but still flake- or
strand-like. Advantages? Disadvantages? Climatology?
Variations and evolutionary lines of scalariform perforation plates 2.
coalescence of pores into
larger structures.
Advantages? Disadvantages?
Climatology?
Lateral wall pitting, in tracheids and vessels
scalariform
transitional
opposite
alternate
The bordered pit – micro-engineered flow control
Once the pit membrane pores
are blocked, flow is impeded
minimum air bubble size
is larger than the size of
the pit membrane pores
weakening of the wall is
lessened by overarching
of the pit border.
The combined flow capacity of
the pit membrane pores, equals
that of the pit aperture; in other
words, efficient traffic through
the bordered pit requires that
the sum of the area of the
perforations in the pit equal
the area of the border
Phylogenetic change: foreshortening vessels, dicotyledonous wood.
Vessel members usually have
perforate end walls, but in steeplyinclined walls, these perforations are
really on the side of the vessel
element.
Short, wide vessels are very efficient
transporters, but extremely prone to
embolism.
Phylogenetic change: monocotyledonous wood
Important to remember
that vessels first
appeared in the root, only
later in stems and leaves.
Specialization of these,
followed the same pattern
summary: main evolutionary trends
TRACHEID DIMORPHISM 3 loosing vessels
Vessel dimorphism leads to the formation of libriform vessel elements, and
eventually, vasicentric tracheids. Tracheid dimorphism and fiber tracheid
dimorphism
Defn: vasicentric =
Trends in the evolution of vessel elements
Evolutionary adaptation
SAFE SYSTEM
vessel element with scalariform
perforation plate; plus tracheid
vessel element with nearly
simple perforation plate and a
fiber-tracheid
vessel element with a simple
perforation plate; accompanied by
libriform fiber
IMPROVED CONDUCTIVE EFFICIENCY
IMPROVED MECHANICAL STRENGTH
DECREASED SAFETY
libriform fibre which has pits with no border and a slit-like
aperture on the outer face.
towards conductive efficiency and safety #1
vessel element with
scalariform perforation plate;
plus tracheid, safe wood
Scalariform = ladder-like
Vessel element with simple perforation plate
provides conductive efficiency. True tracheids
provide & retains conductive safety
towards conductive efficiency and safety #2
vessel element with nearly simple
perforation plate and a fiber-tracheid
Vessel element with simple perforation plate provides
conductive efficiency. vasicentric tracheids and
fiber tracheid provide & retains conductive safety
towards conductive efficiency and safety #3
vessel element with a
simple perforation
plate, accompanied by
libriform fiber
libriform: In secondary xylem,
with few, simple pits. Slit-like
aperture in the outer face
Vessel element with simple perforation
plate provides conductive efficiency.
vasicentric tracheids and libriform
fiber provide & retains conductive
safety
pumpkin
sunflower
Tracheary element evolution –
(& the vesselless dicotyledon)
Narrower tracheids have circular pits on end and lateral walls
Wider tracheids may bear scalariform pits on overlap areas (end
walls), as well as on their lateral walls.
Tracheid end wall. These are really micropores in the pit
membrane (scalariform perforations develop from this).
Are there advantages? disadvantages? Much argument still.
Example? Winteraceae resist freeze-thaw conditions, 0 – 6% loss of
hydraulic conductance only, vs >20% for vessel-containing dicots.
Clearly an advantage. Reasons for the possible loss of vessels and
the ecological events underlying this process remains mysterious!
50 µm
pine, TS
50 µm
pine, LS
Whose got xylem?
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Division Psilophyta: Psilopsids, characterised by presence of microphylls. No
differentiation of shoot and root.
Division Lycophyta: Have microphylls.
Division Sphenophyta: The horsetails Single genus (Equisetum) – jointed
stems, conspicuous nodes. Leaves scale-like.
Division Pterophyta: Ferns. All possess a megaphyll.
Division Coniferophyta: Conifers. Gymnosperms with active cambial growth
(secondary tissue) simple leaves.
Division Cycadophyta: Cycads. Gymnosperms with sluggish cambial activity.
Division Ginkophyta: Considerable cambial growth, fan-shaped leaves. Open
dichotomous venation.
Division Gnetophyta: Gymnosperms with many angiosperm-like features.
Only Gymnosperms in which vessels occur.
Division Anthophyta: Flowering plants. Megaphylls, secondary growth.
Contain dicotyledons (cambium) and monocotyledons (no cambium).
50 µm
200 µm
Helianthus
pine
maize
Summary
First land plants had hydroids – simple in structure – may have been
lignified
Later evolution of more complex wall structure
Evolution of tracheoids
Tracheids become the principal water conducting elements
Vessels – essentially complex, with compound perforations (scalariform
common)
Mesomorphs/subtropical to tropical wood – evolution of simple perforation
plates, some crassulae
Crassulae: Transversely oriented thickenings in tracheid walls of
gymnosperms accompanying the pit pairs. Also called Bars of Sanio).
Shortening and widening of vessels in warmer climates.
Xylem transport
The phloem
Definitions
Sieve element: The cell of the phloem that is specialized and
involved in the long-distance transport of food substances; sieve
tubes are further classified into sieve cells and sieve tube
elements (or members).
Sieve cell: A long slender element, with relatively unspecialized
sieve areas, with tapering end walls that lack sieve plates; found in
the phloem of gymnosperms.
Sieve tube element (member): One of the component cells of
the sieve tube; found primarily in the flowering plants and
typically associated with companion cells.
Sieve plate: That part of the wall of sieve tube elements (members)
bearing one or more highly differentiated sieve areas.
Sieve element evolution, brown algae
The micrograph at left, shows a
longitudinal section of the phloem
of Desmarestia ligulata. Sieve
elements are trumpet-shaped
where end walls join.
This plant belongs to one of the
most dominant groups of
seaweeds occurring in the
Antarctic ocean.
Nereocystis lütkeana
This LS shows adjacent sieve
elements of the pneumatocyst,
in which files of sieve elements
are separated by large spaces
organized transport in seaweeds sieve cells or sieve tubes
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1. The perforated end wall is one of the most characteristic features of
the phloem in the marine algae. The size of the perforations are
species dependant.
Pore sizes range from as small as plasmodesma (100 nm or so) or as
wide as the sieve pores in vascular plants.
2. Phloem systems tend to be well-developed for long-distance
transport of organic and inorganic nutrient.
3. In Laminariales, cross-connections exist, interconnecting longitudinal
strands either via cross-connecting sieve elements, or via lateral
sieve areas. In Macrocystis these lateral sieve area pores are smaller in
diameter than those occurring in the cross walls, thus one could define
the sieve elements as true sieve tubes and the perforated end walls
are thus true sieve plates.
the phloem lower orders, Gnetophytes
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Within the cycads, the phloem
consists of sieve elements and
narrow parenchyma cells.
The Cycadophytinia represent a
group of plants, extant since the
Jurassic (208my), with strong links
to the angiospermae.
Note: The term ‘sieve element’ is
used here, to describe more
primitive sieve cells and more
advanced sieve tube members.
Longitudinal sections through part of the
phloem in Cycas revoluta, showing
differentiated (SE) and differentiating sieve
elements (DSE)
primitive dicotyledonous plants
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The photomicrographs to the
left, show details of the
phloem tissue in the phloem
of Magnolia kobus in which
the cell wall of the sieve
elements, can be see to be
thickened, and of wavy
outline. This wall is termed
the nacreous wall, which is
less cellulosic and pectin-poor
compared with the outer wall
layer. These inner walls have
been shown to be
polylamellate in many species.
higher plant sieve plate pores
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0.5 µm
As seen in this example from Beta
vulgaris, the sieve plate pores in
mature sieve elements (bottom
micrograph) are large, and if
prepared properly for electron
microscopy, are devoid of callose.
They are approximately 0.5 µm in
diameter.
unusual wall thickenings
This electron micrograph shows sieve
elements in the monocotyledon,
Heterozostera tasmanica Note the
thickened sieve tube walls, termed
nacreous thickening
fossil records, carboniferous (362my) gymnosperms
Left: Cordaixylon, ts outer
phloem. Parenchyma cells
appear smaller than the
sieve cells (S).
Right: Callistophyton, showing
alternation of sieve cells
with parenchyma cells. R =
Ray tissue
EL Taylor Phloem evolution: Ch 14, Behnke and Sjoland Sieve Elements
Evolutionary process, fossil records -- Lycopods,
Sphenophyta and ferns
Taxon
Details of Phloem
Lycopods
Sieve elements, parenchyma; ca 15.7 µm xs; end wall
horiz/obl;
Sphenophyta
Sieve elements; 12.5-21 µm xs; end wall horizontal
Primary and secondary sieve elements + parenchyma +
interfasicular parenchyma.
Spenophyllum
Ferns
Ankyopteris; Botryopteris;
Elapteris; Psaronius;
Sauropteris
Adaxial & abaxial sieve elements; small & large sieve
elements, (small = 10 27 µm; large = 48-120 µm).
Sieve plates horiz – oblique; have sieve areas and sieve
pores
Evolutionary process, fossil records - Progymnosperms
Taxon
Details of Phloem
Progymnosperms
‘aneurophyalean’
Sieve cells, 15-25 µm xs; very oblique end walls;
sieve pores v. small, 0.5-0.6 µm
Callixylon (Devonian)
Not understood, poor preservation
Calamopitys
Sieve cells (primary and secondary);50-60 µm xs;
oblique end walls
Callistophyton
Sieve cells; secretory canals; 20-25 µm xs; very
oblique end walls; v. long sieve cells (6mm).
Cordaites
Cordaixylon
Sieve cells 25 µm xs; cf 1.5 – 1.7 mm long; very
tapered ends.
Coniferales
Sieve cells; 25-30 µm xs
Cupressinocortex
Taxodioxylon
Sieve cells; 20 µm xs 50 µm long
Fossil records - cryptogram & gymnosperm phloem similarities
Tissue origin:
conducting cell:
Vascular Cryptograms
sieve elements (primary)
sieve elements and parenchyma
cells
Gymnosperms
sieve cells (primary and
secondary)
sieve cells, parenchyma
elements, fibers, sclereids,
secretory cells
tissue
composition:
Sieve elements:
rectangular to elongate
elongate
shape:
diameter:
length:
end wall:
sieve areas:
sieve pores:
callose:
10-40 µm (up to 120 µm a)
<600 µm (>2.75 mm)
horizontal to very oblique
elliptical-rectangular & small
approx 1 µm dia.
present in some
20 -100 µm
1 – 9 mm
horizontal to very oblique
elliptical, larger app. 10x30µm
approx 1 µm dia.
Phloem phylogeny #1
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Amongst fossil and extant species, the basic structure of the sieve
element is fairly uniform in the cryptograms,.
Primary phloem is very poorly known from fossil Gymnosperms and
the data presented in the previous two tables is based purely on the
secondary phloem.
Whilst phloem has been noted in all groups of fossil Gymnosperms, it
appears to have been researched in very few groups, including the
cordaites, conifers, and Palaeozoic seed ferns.
In all instances phloem alternates with bands of fibers which may
have interspersed axial parenchyma.
All in all the gymnosperms, the sieve cell is remarkably constant in
its structure, with the first Middle Devonian progymnosperm record,
being very similar to those in Carboniferous seed plants, and are
comparable to extant sieve cells being 2.5 -> 9mm in length, with
gradually tapering end walls.
Phloem phylogeny #2
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Regardless of the group within the Gymnosperms, there is an intimate and
regular relationship between axial parenchyma and conducting
elements.
Fibers tend to be present and are variable in distribution, from the
innermost (presumed functional) to the outermost regions closely spatially
associated with the bark.
Sieve areas are discrete, uniform in size and shape, with distinct pores
that can be counted (unlike cryptograms where pores require TEM or SEM
for elucidation).
Callose has been observed, both as a collar surrounding the pores, or as
definitive callose.
Phloem Phylogeny #3 – where did divergence occur?
MESOZOIC (245)
Cretaceous (145)
PALEOZIC, (570)
In cycadophytes such as Cycadeoidea sieve cells
alternate with parenchyma
Sieve elements in vascular cryptograms were considerably
different structurally, (perhaps also functionally? From
gymnosperms. By the early Carboniferous, these two cell
types had diverged.
Carboniferous (362)
Sieve cells and sieve tube members
(elements) diverge. Seed fern Medullosa
sieve cells alternate with parenchyma.
Middle Devonian (~ 415)
Slightly oblique end walls in cells occurring in
the position which should be occupied by
phloem in Rhynia and Trimerophyton.
So, why two systems?
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Even given that the xylem and phloem have distinct functions, with o
being confined in an apoplasmic environment, and the other in a
symplasmic environment, is this necessary?
One could argue that both functionalities could be contained,
maintained and carried out in the same tube – or could one?
It could be argued that one does not ‘need’ a confinement process
such as those typified by functional phloem, and functional xylem – or
could one?
specialization in leaves
Transection of the phloem in a vascular
bundle in Beta vulgaris leaf minor
vein tissue, showing the narrowdiameter sieve tubes compared with
the larger diameter parenchymatous
elements.
reading
Behnke, H-D and RJ Sjölund (eds) 1990. Sieve elements.
Comparative structure, induction and Development. Springer ISBN 3-54050783-3.
Carlquist, S.1988. Comparative Wood Anatomy. Springer ISBN 3-54018827-4. 583.04 Car.
Fahn, A 1967 Plant Anatomy. Pergamon Press. 581.3 Fah
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That’s it……….