Torrey Botanical Society
Structure and Function of Dimorphic Prop Roots in Piper auritum L.
Author(s): Nancy Greig and James D. Mauseth
Reviewed work(s):
Source: Bulletin of the Torrey Botanical Club, Vol. 118, No. 2 (Apr. - Jun., 1991), pp. 176-183
Published by: Torrey Botanical Society
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Bulletinofthe TorreyBotanicalClub 118(2), 1991, pp. 176-183
ofdimorphic
proprootsinand function
Structure
PiperauritumL.1
Nancy Greig and James D. Mauseth2
ofBotany,
DivisionofBiologicalSciencesand Department
ofTexas,Austin,TX 78712
University
ABSTRACT
ofBotany,
GREIG,N. ANDJ.D. MAUSETH (DivisionofBiologicalSciencesand Department
proprootsin
of dimorphic
and function
University
of Texas,Austin,TX 78712). Structure
on growth
PiperauritumL. Bull.TorreyBot. Club 118: 176-183. 1991.-Field observations
ofPiperauritum
weremadeoftherootsystems
on structure
formand laboratory
observations
sitesin humidlowlandareasofthe
L. (Piperaceae),a smallweedytreecolonizingdisturbed
withdifroots(proproots,stiltroots)ofP. auritumare dimorphic,
Adventitious
neotropics.
portions.
and subterranean
in form,structure,
betweenabove-ground
ferences
and orientation
and has numerous
Above groundthe rootis wide (mean diameter1.78 cm), unbranched,
poles (x = 42) arounda broadpith(meandiameter0.72 cm).Upon penetration
protoxylem
and provide
of the soil therootformsseveralbranchroots,some of whichgrowvertically
andproducenewshoots(rootsuckers).
whileothers
growhorizontally
anchorage
andabsorption,
rootand fromeach other.Horizontal
Bothsubterranean
fromtheabove-ground
typesdiffer
poles(9) aroundthenar(1.01 cm diameter)withthefewestprotoxylem
rootsare narrowest
values(root
rootshave intermediate
rowestpith(0.04 cm diameter).Verticalsubterranean
0.44 cm).Vesselsareon averagewider
diameter1.06 cm; 22 protoxylem
poles;pithdiameter
rootportions(36 to 84 ,umradius)thanin eitherabove-ground
in thexylemofthehorizontal
orverticalsubterranean
partsoftheroots(24 to 60,umand36 to 72,tminradius,respectively),
is performed
bywidervesselsizeclassesin horiand a greater
oftotalconduction
percentage
shootsproducedfromthehorizontal
portionsofthe
zontalsections.Fast-growing
vegetative
areas
rootsresultin largestandsofP. auritumand allowthespeciesto persistin successional
seeds.
ofitslight-dependent
forgermination
no longerfavorable
propagation,
roots,vegetative
aerial(proporstilt)roots,adventitious
Keywords:Piperauritum,
capacity.
roots,vesselconducting
rootsuckers,
dimorphic
Prop or stiltrootsare aerial,adventitiousroots
usuallyproduced on the lowerportionofa shoot
(Gill 1969; Gill andTomlinson 1975; Jenik1978).
These roots eventually make contact with the
substrateand provide anchoragein softor shallow soils; theymay also augmentthe plant's supply ofwaterand nutrients(Jenik1978). Unusual
in temperateplants (except forZea mays), prop
rootsoccur in diversegroupswithinboth monocots and dicotsin humid tropicalareas. Previous
studies of aerial roots have noted theirunusual
structureand form, particularlythe dramatic
changes in externaland internalstructureonce
I Thisstudy
inpartbya TinkerSummer
wasfunded
ofLatinAmerFieldResearchGrantfromtheInstitute
ofTexasand a Univerican Studiesat theUniversity
to N. Greig.
sityFellowshipfromthesameinstitution
2 We thanktheServiciode ParquesNacionalesde
Costa Rica and L. E. Gilbertforlogisticsupportat
forTropicalStudies
Corcovado,andtheOrganization
fromP. J.DeVries,
forthesameatLa Selva.Comments
J.B. Fisherand
reviewers
R. J.Marquis,andespecially
J.L. Seagoimprovedthemanuscript.
April30, 1990,andinfinal
Receivedforpublication
revisedformOctober15, 1990.
theyenterthe soil (e.g., Turner 1934; Kapil and
Rustagi 1966). However, few authors have attemptedto relateformand functionin theseroots.
An exceptionis the workof Gill and Tomlinson
(1969, 1971, 1977), which describesthe dimorphic natureof red mangrove(Rhizophora mangle) aerial roots and discusses these featuresin
relationto the functionalrole of these roots. In
this paper we presentobservationson the anatomy and formof the prop roots of the tropical
plant, Piper auritum,in relation to their functional roles.
Materials and Methods. STUDY ORGANISM
ANDFIELD SITES. P. auritumL. (Piperaceae) is
a small, weedy tree with a widespread distribution in theAmericantropics,rangingfromMexico to Colombia in highlydisturbedareas (roadsides, forestedges, and large treefallgaps) in
lowland rainforest(Burger1971, 1983). Its seeds
are bat-dispersed (Janzen 1978) and bird-dispersed (B. Loiselle personal communication;
Palmeirim et al. 1989), and theygerminateand
grow only in areas of high insolation (VazquezYanes and Smith 1982). A typicalheliophilespe-
176
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1991]
GREIG AND MAUSETH:
DIMORPHIC
cies, P. auritum is extremelyfastgrowing;it is
essentiallyan overgrownherb with a large core
of pith even in largewoody specimens. Secondarygrowthis initiatedwithinthe firstyearwhen
the plant is two to threemeterstall. Plants reach
sexual maturityin less than two yearsand probably live only about 5 years (N. Greig personal
observation).
Observations on the growthhabit of P. auritum were made in Corcovado National Park in
southwesternCosta Rica and at theOrganization
for Tropical Studies' field station La Selva, in
northeasternCosta Rica. Both areas are preserves of lowland tropical rainforest,receiving
fouror more metersof rain annually (see Hartshorn (1983) forsite descriptions).At both sites
P. auritumoccurs typicallyin earlysuccessional
stages of areas in regeneration,along with other
heliophilespecies such as Cecropia and Helicon-
ROOTS IN PIPER AURITUM
177
M
Hr
Vr
ia.
STuIDY. Field-collectedmaterial
ANATOMICAL
was cut into pieces and fixedin FAA. This materialincludedabove-groundportionsoftheprop
root, vertical and horizontal subterraneanportions of the same roots, and several small adventitiousshoots.Youngerstagesofadventitious
rootswereobtainedfromP. auritumplantsgrown
from seed in a greenhousein Austin and also
preservedin FAA. Sections were embedded in
paraffin,cut at 10 ,umon a rotarymicrotome,
and stained with safraninand fastgreen. Macerationsof above groundand subterraneanroot
tissuewere also prepared.Vessel diameterswere
measured with an ocular micrometer,then the
fourthpower of the radius, r4,was calculated as
an estimate of the conductingcapacity of each
vessel according to the Hagen-Poiseuille law
(Mauseth 1988; Zimmermann 1983). Vessel radius classes 6 ,umwide were used in presenting
the results.
Fig. 1. Piperauritumhabit.Ar = aerialroot;SVr
= subterranean
root;Rs
verticalroot;Hr = horizontal
= root sprout.
cence, a long, pendant spike (30 cm x 1 cm),
contains over ten thousand minute (0.14 mg)
seeds.
Almost all P. auritum individuals of over 3
cm DBH were foundto have adventitiousprop
roots.These arise on thelowermostthreeor four
nodes of the trunkand arch downward toward
thegroundas theyelongate(Fig. 1). Several roots
may arise at a singlenode, and largeplantshave
15-20 or moreproproots.Littlesecondarygrowth
occursuntiltherootreachestheground.Like the
aerial roots of red mangroveand of many other
tropical species (Gill 1969; Gill and Tomlinson
1975), theserootsdo not branchuntiltheycome
in contactwiththe soil, unless the growingtip is
injured.No root cap is present,but uninjuredP.
auritumroottipsare sometimescoated withclear
mucilagethatformsa largedropletover themerResults. MORPHOLOGY OF PLANTSAND RooTs.
MatureP. auritumplantsaverageabout 5 meters istem. Injuryto the above-groundroottips of P.
in height,witha mean DBH of approximately8 auritum is common, and usually resultsin tip
cm. The leaf-bearingbranches never become abortion, probably caused by fungalinfection;
woody and those lower on the trunkare shed the tips in such cases are black and appear to
quicklyas thetreegrowsin height,so thatmature have rotted. Several lateral roots may emerge
specimens have a long bare trunkwith an um- fromjust behind these aborted tips.
As it penetratesthe soil, the previouslyunbrella-likecrown of branches bearing the very
large,thinleaves (35 x 25 cm). Occasional plants branched prop root gives rise to threeor more
branch epicormicallyif growingat an angle to branchroots (Fig. 1). Some of thesegrowdownthe ground,producinga forkedtrunk,but most ward,are relativelyshort(<60 cm) and tapered,
plants have only one trunk.P. auritumflowers have numerousfinelateralrootlets,and probably
and fruitsthroughoutthe year; each infructes- function in absorption and anchorage. Other
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178
BULLETIN
>/
OF THE TORREY
A
branch roots reorientand grow horizontallyat
the soil surface,oftenbecoming completelyexposed or covered by a shallow layer of soil or
leaf litter.The ground under a group of P. auritumplants may be criss-crossedby a number
oftheselongsinuousroots.Some horizontalroots
may extendforat least 9 metersfromtheirpoint
of origin.These roots branch only occasionally
along theirlength,althoughtheymay be highly
branched terminally(Fig. 2).
The above-ground portion of the prop roots
and
undergoesconsiderablesecondarythickening
may be 2-3 cm in diameterand verywoody in
large P. auritum specimens. The underground
portionof these roots also undergoessecondary
thickening.Verticalportionsbranchabundantly
and taper rapidly. The horizontal portion may
become nearly 2 cm thick in some specimens,
but remainsmore flexiblethanits above-ground
or verticalsubterraneancounterparts.
Adventitiousshoots (root~suckers)arise at irregularintervalsfromthe horizontalroots (Fig.
1). Normally,buds are produced on roots at or
near the soil surface,but occasionally we found
root suckers growingas deep as 15 cm below
ground.These shoots grow quicklyand eventually produce theirown adventitiousroots. They
appear to be produced randomly; the oldest
shoots are not necessarilythose on portions of
the root closest to the parentplant (Fig. 2). Almost all juvenile P. auritumplants foundat the
studysitecould be tracedto horizontallygrowing
subterraneanportions of prop roots fromlarge
plants in the vicinity.Very fewwild plants were
foundthatwereunequivocallyderivedfromseed,
[VOL. 118
CLUB
I= 1cm
~~=1m
of
extensions
Fig.2. Formofspreadin horizontal
Filleddotsrepresent
theproprootsin Piperauritum.
size of sprout.Open dot
rootsprouts,size indicating
Large
scarwherea sproutgrewformerly.
represents
theparentsourceplant(dots
stippleddot represents
root
are notto scalewithroots).Onlyone horizontal
out from
is followed;theremaybe severalradiating
theparentplant.
BOTANICAL
B
hr
Fig. 3. A. Seedling,and B. rootsproutof Piper
Hr= horizontal
auritum.
rootcomingfrom
adultplant.
and these were found scatteredin recentlydisturbed,sunnyareas-a gardenclearingand large
treefallgaps in the forest,forexample-usually
withno largeP. auritumindividualsin evidence.
Seedlings can be distinguishedfromroot-derived plantletsmorphologically,at least in early
stages (Fig. 3). Seedlingshave small, ephemeral
cotyledons,have extremelyshortinternodes(2
to 5 mm), and produce theirfirstleaves when
only a few millimeterstall. All leaves, independent of size, have both petiole and blade. A taprootis present.Adventitiousshoots,on theother
hand, do not have cotyledons,and have much
thickerstemsand longerinternodes(5 to 50 mm).
Being buds of the parentroot,theyinitiallyhave
no roots, and never produce a tap root; all of
theirroots are adventitious.The firstfewleaves
of adventitiousshoots are oftenscale-like,consistingof petiole only or petiole with a veryreduced blade.
ANATOMY OF THE ROOTS. Table 1 shows several structuraldifferences
betweenabove-ground
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GREIG AND MAUSETH:
1991]
DIMORPHIC
179
ROOTS IN PIPER AURITUM
Table 1. Root diameter(cm), pithdiameter(cm), and numberofprotoxylempoles in threeroottypes(x ? SE;
rangein parentheses).
Root position
Above ground
Subterraneanvertical
Horizontal
Rootdiameter
1.78 ? 0.13
(1.10-2.58)
1.06 ? 0.29
(0.61-2.19)
1.01 ? 0.14
(0.59-1.64)
vertical, subterraneanvertical, and horizontal
portionsof the prop root system.Mean root diameteris greatestin above-groundportions.Pith
is also broadestin above-groundportions,but is
wide in subterraneanvertical portions as well.
Narrowingof the pith in subterraneanvertical
branches is proportionalto root width; i.e., in
above-ground and below-groundroot sections
the proportionof pith to root diameter is approximatelythe same (0.40). Pith is narrow in
horizontalportions,and the proportionof pith
to total diameteris significantly
less (0.04) than
in the othertwo portions(ANOVA, F = 75.976,
P < 0.0001; data were arcsinesquare roottransformedto meet the assumptions of analysis of
variance). The photograph(Fig. 4) of a bisected
above-ground and horizontalroot clearlyillusin theirstructure.
tratesthe dramaticdifferences
among
Numberofprotoxylempoles also differs
the threeroot regions(Table 1). Above-ground
the roots have an average of 42, verticalsubterranean brancheshave 22, while horizontalportions have only 9 protoxylempoles. Again, the
ratio of protoxylempoles to total root diameter
is approximatelyequal in bothabove-groundand
subterraneanverticalroot sections(;24 per cm
diameter),while the ratio is much less in horizontal portions(-9 per cm diameter)(ANOVA,
F = 24.171, P < 0.0001; data were log transformedto meet the assumptions of analysis of
variance).
The vascular tissuesin the above-groundportion of the prop root systemare located farfrom
the center of the organ, and form a cylinder
around thewide pith.Upon formationof a cambium in this section,ray parenchymais formed
opposite protoxylempoles, and axial secondary
xylem and phloem are deposited opposite protophloem poles. Subsequent growthin this portion of the root resultsin a stem-likeconfiguration,a cylindricalstelearound a pithcore (Fig.
4, upper). However, thereare no scatteredvascularbundlesin thepithoftheproproot,as there
are in the stems of P. auritum,and in stems of
Pithdiameter
0.72 ? 0.06
(0.33-1.04)
0.44 ? 0.15
(0.13-1.03)
0.04 ? 0.01
(0.02-0.07)
Protoxylem
poles
N
42 ? 3.2
(18-52)
22 ? 2.3
(16-29)
12
9 ? 0.6
(7-13)
5
8
otherPiperaceae (Metcalfeand Chalk 1950). The
of the vascular tissuesis similarin
configuration
subterranean vertical portions. In horizontal
branchesof the root,however,protoxylempoles
occur much closerto the centerof the organ;the
broad sectionsof secondaryxylem,separatedby
broad rays,radiate out fromthe center(Fig. 4,
lower).
Rays are wide in all three sections, but are
widest in horizontalroots. In above-groundand
subterraneanvertical root sections, ray parenchymahas thick,lignifiedwalls; rayparenchyma
is not lignifiedin horizontalroots.
Vessels in thesecondaryxylemofall threeroot
sections have scalariformpitting,and simple
perforationplates. In the above-ground root,
vessels are few and narrow,and are embedded
in a dense matrix of fibers(Fig. 5). Secondary
xylem in below-groundverticalroots is similar
(Fig. 6). Horizontal roots have abundant, very
wide vessels, clearly visible to the naked eye.
Fibers in horizontal roots are correspondingly
reduced in numberand have thinnerwalls than
in the otherroot categories(Fig. 7).
Figure 8 shows the distributionof vessels in
different
size classes (vessel radius)forthreesample roots. Several roots were examined; those
illustratedhere are representative.In thisparticular above-grouyd root, vessels range from 24
to 60 ,umin radius. The broadestvessels are not
especiallynumerous,makingup onlyabout 15%
of all the vessels present,but because of their
verygreatr4value (Fig. 9), theyaccountforabout
35% ofthetotalconductingcapacity.(The fourth
power of the radius of conductingcells, r4,can
be used to compare relativeconductingcapacities.) The mostnumericallyabundantsize classes
were comprisedof vessels withradii of 48 or 54
,um;about 43% of all vessels of thisroot fallinto
this size range.Despite theirnarrowerdiameter,
which resultsin a lower r4 value for each cell,
thereare so many cells in these classes thatthey
account forover 50% of total conductingcapacity. Vessels of 36 ,umradius and less have such
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p = pith,ph = phloem,r = ray,v = vessel.-4. Photograph
oftherootinPiperauritum.
Figs.4-7. Structure
Notethebroadpith
root(lower)ofPiperauritum.
proproot(upper)and horizontal
ofa bisectedabove-ground
- 5.
root.Arrowsindicateraysin thisfigure.
in theabove-ground
root,and thewideraysin thehorizontal
of a
portionof a proprootof Piperauritum(x 58). - 6. Cross-section
of theabove-ground
Cross-section
portion
(subterranean)
verticalrootofPiperauritum(x 58). - 7. Crosssectionofthehorizontal
subterranean
oftherootsystemofPiperauritum(x 58). Notethelargediameterofmanyvessels.Notealso thatthefibers
in theabove-ground
in thispartoftheroothavethinner
verticalroots.
and subterranean
wallsthando fibers
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All use subject to JSTOR Terms and Conditions
1991]
GREIG AND MAUSETH:
DIMORPHIC
ROOTS
IN PIPER AURITUM
181
30
U
above-ground
M subt. vertical
0
horizontal
24
30
n 20
0)
0)
C.
0)
36
42
48
54
60
66
72
78
84
Vessel radius size class (~tm)
vessel size classes
Fig. 8. Distributionof vessels (shown as percentof total vessels per root) among different
(vessel radius) in representativeabove-ground,subterraneanvertical,and horizontalportionsof the root system
in Piper auritum.
a low r4 value and are so scarce that they contributeless than 7% of total conductingcapacity
of the above-groundportion of prop root.
Vessels in thesubterraneanverticalrootofFigs.
8 and 9 are slightlylarger,rangingfrom30 to 72
,umin radius. The vessel size classes from48 to
60 ,m radius are both most abundant and accountforabout 70% oftotalconductingcapacity.
Vessels in the horizontalrootsare much wider
than those in the verticalportions,eitherabove-
40 -
U above-ground
a.
subt.vertical
02 horizontal
3
30 -
0
C
0
,20
10
C
C)
C)
0)
24
30
36
42
48
54
60
66
72
78
84
Vessel radius size class (jim)
size classes in representative
Fig. 9. Percent of total conductingcapacity provided by vessels of different
above-ground,subterraneanvertical,and horizontalportionsof the root systemin Piper auritum.
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All use subject to JSTOR Terms and Conditions
182
BULLETIN
OF THE TORREY
BOTANICAL
CLUB
[VOL. 118
from36 to the formeroccurs in primaryforestand in small
withradiiranging
or below-ground,
84 ,m; over 10% of all vesselshave a radius gaps, whileP. auritumis confinedto large,sunny
greaterthan78 ,m (Fig. 8). Comparedto the disturbances.
Janzen(1978) suggestedthat"large even-aged
otherroottypes,vesselsin thishorizontalroot
fallintoa broaderrangeof size classes,and a stands" of P. auritumare due to bat dispersalof
vesselsizeclassescon- large numbers of seeds to concentratedareas.
largernumberofdifferent
ca- Evidence fromour fieldstudyindicatesthatvery
to theroot'sconducting
tributesignificantly
fewP. auritumindividuals establishfromseed,
pacity(Fig. 9).
abundantin thehori- but plant numbers are increased via vegetative
Phloemis particularly
havea mul- propagation within an area once a plant is eszontalroot(Fig.7). Allrootportions
cortexoutsidethe tablished.Since P. auritumgrowsextremelyraptiseriateband of persistant
roots. idly (up to 3 m a year; N. Greig unpublished
stele;thecortexis broadestin horizontal
and sim- data), what appears to be an even-aged stand
thecortexis unremarkable
Otherwise
Smallgroupsofsclerified probably consists of one or a few older plants
ilarinall rootportions.
offspring.
cells,abouttwiceas longas wide,forma punc- and theirvegetatively-produced
The large dropletof mucilage over the merituatedringin theoutercortexin all threeroot
stemin youngaerial rootsofP. auritumpresumtypes.
both ably protectsthe growingtip and keeps it moist
Mucilagecanalsareabundantthroughout
shootand rootsystemin P. auritum.In roots, (cf.Mollenhauer 1967). A similarmucilagecoatmucilagecanalsoccurinthepith,inphloemrays, ing has been found on the tips of other aerial
and occasionallyin xylemrays.A singlelarge roots,forexample Hedyosmumarborescens(Gill
mucilagecanal may occupythe entire,narrow 1969), and on the adventitiousrootsof such epiphytesas Cattleya orchids (Mollenhauer 1967)
branchesoftheproproot.
pithin horizontal
and Monstera (N. Greig personal observation).
The anatomy of the adventitiousaerial roots
Discussion. Aerialrootsystemsdescribedto
con- of P. auritumis unusual when compared to that
serveas anchoring,
date in the literature
mangle(GillandTom- of otherdicot roots. Most dicot roots have two
and,Rhizophora
ducting,
linson1977)and Prestoeamontana(Arecaceae) to several protoxylempoles and no pith. The
organs,but above-groundportions of the aerial roots of P.
(Frangiand Ponce 1985),as aerating
likethoseofRhizophoramangle,how(Gill & Tomlinson auritum,
have no rolein regeneration
the prop rootsystemin P. ever,have numerousprotoxylempoles in a ring
1969). In contrast,
or- surroundinga large pith,and thus more closely
auritumservesas a propagating/regenerating
and conduc- resemblethe adventitiousroots of some monoanchorage
ganas wellas providing
fromrootsis well-documentedcots, which are also polyarchand contain pith
tion.Propagation
1975), (Fahn 1982; Mauseth 1988). However, thereis
taxa(Peterson
ina widerangeoftemperate
roots. no secondarygrowthin the rootsofthesemonoto specialized
notusuallyattributed
though
on
the
hori- cots.Below groundtheparallelsbetweentheroots
shoots
produced
The adventitious
zontalrootsof P. auritum,perhapsbecauseof ofP. auritumand those ofRhizophoraor monoprovidedtothemfromadultplantsvia cots are less striking.In R. mangle the subternutrients
root,can growand survivein ranean portions of aerial roots, like the abovethe connecting
couldnot,e.g.,sitesin suc- ground portions, have numerous protoxylem
areaswhereseedlings
cessionwherelightlevelsare lowerthanin new poles and abundant pith. The aerenchymatous
disturbances.
Vegetativepropagationis wide- tissue in these white and spongyorgans has an
Piper(Gartner1989; obvious role in gas exchangein the waterlogged
spreadamongneotropical
but mostspeciesre- substratewhereR. mangleoccurs(Gill and TomN. Greigin preparation),
fromfallenstemsorleavesorfromad- linson 1977). In P. auritum,the horizontalroots
generate
ventitiousshootsproducedat the base of the have fewerprotoxylempoles and less pith. The
Of37 Pip- very wide vessels and abundant phloem may
trunk
(N. Greigpersonalobservation).
transportofwaterand nutrients
er speciesat CorcovadoNationalPark,P. auri- provide efficient
tumis theonlyspeciesto propagatefromroots. to adventitious shoots formed along the horiC. DC. shares zontal roots.
AtLa Selva,P. pseudobumbratum
theroot-cloning
habit,butnoneofthe48 other The diameterof a vessel is relatednot onlyto
Piperspeciesat thissitewas foundto propagate the ease of water movementthroughit but also
andP. au- to the ease of cavitation:wider vessels can cavin thismanner.P. pseudobumbratum
but itate and form an embolism, or "air bubble,"
formand stature,
ritum
aresimilaringrowth
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All use subject to JSTOR Terms and Conditions
1991]
GREIG AND MAUSETH:
DIMORPHIC
ROOTS IN PIPER AURITUM
183
moreeasilythancana narrowvessel.Xylemthat CARLQuIsT,S. 1975. Ecological strategiesof xylem
evolution.Universityof Califomia Press,Berkeley.
has a wide rangeof vesseldiametersconducts
N. R., C. B. FIELD AND H. A. MooNEY.
almostexclusively
through
thebroadestvessels CHLARELLO,
1987. Midday wiltingin a tropical pioneer tree.
whenever
wateris freely
availableinthesoil,but
Func. Ecol. 1: 3-11.
in timesof waterstress,the wide vesselsmay FAHN,A. 1982. Plantanatomy.PergamonPress,Inc.,
New York.
tensionis shifted
cavitateand thetranspiration
to waterin narrower
vessels,causingthelatter FRANGI, J. L. AND M. M. PONCE. 1985. The root
systemof Prestoea montana and its ecological sigto conductmorerapidly.If all vesselswereof
nificance.Principes 29: 13-19.
all mightfailsimultaneouslyGARTNER,B. L. 1989. Breakageand regrowthofPiper
thesamediameter,
Biotropica21: 303speciesin rainforestunderstory.
(Zimmermann
1983).
307.
Becauseofrootpressure,
cavitationis usually
A. M. 1969. The ecology of an elfinforestin
nota problemforsubterranean
roots.Theirwa- GILL,
Puerto Rico, 6. Aerial roots. J. Arnold Arbor. 50:
teris underpressure,
nottension;onlyata height
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