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Flora 209 (2014) 547–555

Contents lists available at ScienceDirect

Flora

j o ur na l ho me page: www.elsev ier .com/ locate / f lora

tructures and functions of adventitious roots in species of the genushilodendron Schott (Araceae)�

itor Tenorio ∗, Cassia Mônica Sakuragui, Ricardo Cardoso Vieiraederal University of Rio de Janeiro, Institute of Biology, Department of Botany. Vegetable Morphology Laboratory. Av. Brigadeiro Trompowsky,idade Universitária, Ilha do Fundão, 21941590, Rio de Janeiro, RJ, Brazil

r t i c l e i n f o

rticle history:eceived 5 September 2013ccepted 20 July 2014dited by Dr. Rainer Lösch.vailable online 12 August 2014

eywords:oot anatomy

a b s t r a c t

We discuss here the anatomical variations of the arrangements and compositions of stele types observedin different roots types in four populations of the three species of Philodendron as probable adaptations totheir habitats. Terrestrial individuals of P. corcovadense have cylindrical steles while rupicolous individ-uals have lobate steles with dispersed internal cortical parenchyma. The Philodendron species sampledshowed polyarch structures. The crampon roots of P. oblongum and anchor roots of P. cordatum showmedullated protosteles, with the former species having a reduced pith with sclerified parenchyma cellswhile the latter has a wide pith and parenchyma cells with only slightly thickened walls. The feeder roots

oot dimorphismdaptationtele

of P. cordatum also show a medullated protostele—although a central vessel is present until approxi-mately 60 cm from the apex that later disappears, forming a parenchymatous pith. We conclude that thedifferent root types reflect adaptations of the subgenera Philodendron and Meconostigma to their differ-ent habits and habitats, such as in P. corcovadense, where the roots of rupicolous individuals have lobatesteles while the roots of the terrestrial plants have cylindrical steles.

© 2014 Elsevier GmbH. All rights reserved.

ntroduction

Philodendron is the second largest genus of the family Araceae,ith nearly 400 species (Govaerts and Frodin, 2002) although

uture investigations may expand this number to approximately00 species (Croat, 1997). It is a morphologically and ecologi-ally diverse neotropical genus occurring from northern Mexicoo southern Uruguay (Mayo et al., 1997), being very abundant inropical rain forest habitats (Sakuragui, 2001). One hundred andfty-six species of Philodendron have been described from BrazilSakuragui and Soares, 2010) that predominantly grow in humidropical forests, but are also found on rock outcrops and in swamps,iparian forests, and semiarid regions.

According to Benzing (1987), 20–25,000 species of vascular epi-hytes occur in the tropics, with 80% of all epiphyte species beingonocots. Epiphytes are largely encountered in tropical rain forests

Kress, 1989) and are partially responsible for the great diversities

ound in those complex terrestrial ecosystems (Gentry and Dodson,987). Rupicolous species develop in environments with high solar

� Part of the Master’s dissertation of the first author.∗ Corresponding author.

E-mail address: tenoriorosa@uol.com.br (V. Tenorio).

ttp://dx.doi.org/10.1016/j.flora.2014.08.001367-2530/© 2014 Elsevier GmbH. All rights reserved.

radiation and wide temperature variations and can be exposed tostrong winds and to water stress (Burke, 2002).

Philodendron taxa grow in a wide variety of habitats as terres-trial, epiphytic, rupicolous, or hemiepiphytic plants. Hemiepiphyticindividuals have epiphytic and terrestrial stages in their life cycles,with, prior to living as hemiepiphytes, germinating and initiallygrowing as epiphytes. After germination and initial developmenton host trees these plants later develop aerial roots that growtoward the ground. But Aroids are usually classified as secondaryhemiepiphytes, as they germinate as terrestrials and then developaerial roots that attach them to the host tree (Patino et al., 1999).According to Zotz (2013), however, the use of the term “sec-ondary hemiepiphyte” should be discontinued in favor of usingterm “nomadic vine” (for details see Zotz, 2013).

Due to the wide variety of adventitious root types in the Araceaefamily, a dimorphism related to their function as feeder roots(which absorb water and nutrients from the substrate) and anchorroots (which attach the plants to their hosts) can be observed.Feeder and anchor roots show morphological and physiologicaldifferences (French, 1997), and Mayo (1991) illustrated root dimor-phism in Meconostigma.

Two subgenera were recognized in the revision of the genus byKrause (1913): Euphilodendron (=Philodendron) and Meconostigma.Subsequent analysis of anatomical characters led Mayo (1989) toelevate the section Pteromischum (subgenera Philodendron) to the

548 V. Tenorio et al. / Flora 209 (2014) 547–555

Table 1Specimens used in this study and the root types of each one.

Population A Population B Population C Population D

Species P. corcovadense Kunth P. corcovadense Kunth P. cordatum Kunth ex Schott P. oblongum KunthSubgenus Meconostigma Meconostigma Philodendron PteromischumHabit Terrestrial Rupicolous Nomadic vine EpiphyteBiome Atlantic forest Atlantic forest Atlantic forest Amazon, Atlantic forestRoot type Feeder Anchor and feeder Anchor and feeder Crampon

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Voucher RFA 37317 RFA 37490

ubgenus rank, so that three subgenera (Pteromischum with 75pecies, Meconostigma with 20 species, and Philodendron with over50 species) are now currently accepted (Mayo et al., 1997).

The root vascular plexus represents the connections betweendventitious roots and the vascular system of the stem (French andomlinson, 1984). The subgenera Philodendron and Meconostigmaave a vascular plexus formed by branched vascular bundles whilehe vascular plexus of Pteromischum species is composed of sim-le vascular bundles. This feature is related to the diversification ofhe habits and habitats of the subgenera, and a branched vascularlexus is a synapomorphy of the monophyletic clade compris-

ng the subgenera Philodendron and Meconostigma (Tenorio et al.,012)—and the presence of this branched vascular plexus possi-ly led to the occurrence of root dimorphism in these subgenera.ssuming that variations in Philodendron root types occurred due

o the vascular plexus, the objective of this work was to anatomi-ally describe the different root types and identify adaptations toheir habitats and their links to anatomical variations among theifferent root types.

aterials and methods

Three individuals from four populations each (representinghree different species) were examined in the anatomical analyses:opulation A (terrestrial) and population B (rupicolous) of Philo-endron corcovadense, population C (hemiepiphyte) of P. cordatum,nd population D (nomadic vines) of P. oblongum, all from areasf Atlantic Forest in Rio de Janeiro State, Brazil (Table 1). Voucherpecimens were deposited in the Herbarium of the Federal Univer-ity of Rio de Janeiro (RFA). The samples were fixed in FPA (50 mLf 95% ethyl alcohol; 5 mL propionic acid; 10 mL formaldehyde,nd 35 mL distilled water) (Ruzin, 1999) and stored in 70% ethanolJohansen, 1940). Anatomical studies were performed using light

icroscopy. Root samples were processed using the traditionalethod of PEG embedding (Burger and Richter, 1991). Transverse

ections (15–20 �m thick) were prepared using a rotary micro-ome, stained with safranin and astra blue dye (Bukatsch, 1972),nd mounted in Canada balsam.

Average values of the diameters of the root metaxylem andetaphloem were obtained from 30 measurements of each indi-

idual collected. Maximum, minimum, and average values areecorded here, as well as the standard deviations of each character.

esults

The adventitious roots of the species analyzed all emerge fromhe aerial nodal regions of the stems. Philodendron corcovadenseas represented by a population of terrestrial individuals with

dventitious roots—classified here as feeder roots—(Fig. 1A) andy another population of rupicolous individuals with adventitiousnchor–feeder roots which initially support the individuals on

ocks and then detach, growing toward the substratum and assum-ng the role of feeders (Fig. 1B). The roots of the hemiepiphyte. cordatum (Fig. 1C) analyzed were anchor roots (with a fasten-ng function) as well as feeder roots (whose function is to absorb

RFA 37309 RFA 37319

water and nutrients). The nomadic vine P. oblongum has roots of thecrampon type (Fig. 1D and E). The anchor roots of P. corcovadenseand P. cordatum were approximately 100 cm long and reddishbrown or light brown, becoming more orange tinted near the apex;the feeder roots were approximately 2 m long and showed a well-developed system of lateral roots. The crampon roots of P. oblongumwere approximately 5–8 cm long, with smaller diameters and abrownish coloration.

During growth, the epidermis and exodermis of P. corcovadenseand P. cordatum are gradually discarded and substituted by a storiedcork layer through continuous periclinal divisions of the corticalparenchyma (Fig. 2A–C). The crampon roots of P. oblongum showeda uniseriate epidermis and exodermis, with the latter being scle-rified (Fig. 2D). Root hairs could be observed on P. oblongum. Theexodermis cells are uniform in all of the species investigated, withsimilar shapes and sizes.

The cortexes of all of the species can be divided into inter-nal, median, and external layers. The cortex of the mature rootof P. corcovadense shows loosely arranged cells (Fig. 2E) whilethe anchor and feeder roots of P. cordatum and the cramponroots of P. oblongum have compact cortexes with few intercellu-lar spaces (Fig. 2G). The external cortex P. oblongum is composedof approximately 13 cell layers with lignified walls (Fig. 2D).The roots of all of the species studied here show resiniferousducts distributed throughout the cortex, forming various num-bers of rings among the different species. Approximately tworings were observed in P. corcovadense, (Fig. 2E) while P. corda-tum has approximately 7 rings of ducts throughout the cortex(Fig. 2F); only 1–2 rings were observed in P. oblongum (Fig. 2H). Theresin ducts are of schizogenic origin, as their lumens are formedby the division and separation of cortical parenchymatous cells(Fig. 2H). P. corcovadense has 2–3 layers of thin-walled parenchy-matous cells surrounding the epithelial layer of the resin duct.The resiniferous ducts of P. cordatum and P. oblongum are sur-rounded by a sclerified sheath, with approximately 3–5 cell layerssurrounding the epithelial layer (Fig. 2F and G). The ducts of P.cordatum in the external cortex did not appear to have sclerifiedsheaths (Fig. 2C).

The endodermis layers of all examined species are uniseriateand have visible Casparian strips (Fig. 3A and B). P. corcovadenseretains the Casparian strips even in mature roots while the endo-dermis of mature roots of P. cordatum and P. oblongum attains stagesof greater differentiation—with depositions of lignin/suberin thatgive the cell walls an O-shaped appearances in microscopic crosssections. The anchor and feeder roots of P. cordatum show strongsclerification of the more internal cortical layers at later stages, withgreater differentiation (Fig. 3C–F).

The vascular cylinders of the investigated roots frequentlyshowed phloem strands surrounded by cells with highly lignifiedwalls. However, the cells that surround the phloem strands of P. cor-covadense, retain their thin primary walls (Fig. 3A). In P. cordatum

and P. oblongum such cells extend to regions of pericycle and endo-dermis which are external to the phloem strands (Figs. 3C and D).The phloem strands in P. corcovadense and P. cordatum roots appearshort or long in cross section, depending on their respective radial

V. Tenorio et al. / Flora 209 (2014) 547–555 549

Fig. 1. A–E. Species of Philodendron examined in the present study. A: P. corcovadense, terrestrial individual; B: P. corcovadense, rupicolous individual; C: P. cordatum,nomadic vine; D and E. P. oblongum, epiphyte. Note the crampon roots in E.

550 V. Tenorio et al. / Flora 209 (2014) 547–555

Fig. 2. A–H. Transversal sections of the adventitious roots. Storied cork formations in the roots of P. corcovadense (A and B) and P. cordatum (C). Note the collapsed epidermis,and pluriseriate and lignified exodermis (A) and cells in periclinal division giving origin to a stratified suber (A and B). C: Detail of the exodermis and periclinal divisions(arrow) in the feeder root of P. cordatum. D: Root of P. oblongum showing the epidermis dotted with trichomes, sclerified exodermis, and cortical cells with lignified cellwalls. E, F, G: Distribution of resiniferous ducts. Root of P. corcovadense (E). Note the distribution of the ducts (arrows) in 2 rings surrounding the cortex. Anchor root ofP. cordatum (F). Note the distribution of resiniferous ducts throughout the cortex (arrows). Root of P. oblongum (G). Note the resiniferous ducts (arrows) distributed in asingle ring. H: P. corcovadense, detail of a resiniferous duct, demonstrating its schizogenic origin. Ep (epidermis); Ex (exodermis); Sto (stratified suber); Ec (external cortex).Bars = 51 �m (A), 63 �m (B), 61 �m (C), 10 �m (D), 67 �m (E), 34 �m (F), 37 �m (G), 170 �m (H).

V. Tenorio et al. / Flora 209 (2014) 547–555 551

Fig. 3. A–F. Transversal sections of adventitious roots, showing the contacts between the cortex and central cylinder. A: P. corcovadense, pericycle, endodermis, and vascularelements. B, C: Anchor root of P. cordatum. Casparian strips (arrows) in polarized light (B). Note the internal cortex, endodermis, and sclerified pericycle (arrow) (C). D:F lem.

s ong ph(

eoDs

eeder root of P. cordatum. Note the vascular elements. Arrows indicate the protoxyeen under polarized light (E) and the vascular elements only (F). Ph (phloem); Lp (lendodermis). Bar = 46 �m (A), 23 �m (B), 10 �m (C and D), 27 �m (E), 34 �m (F).

xtensions (Fig. 3A, C, and D). The phloem strands of the feeder rootsf P. cordatum are longer than those of the anchor roots (Fig. 3C and) while the phloem strands of P. oblongum are consistently rather

hort (Fig. 3F).

E, F: Root of P. oblongum. Sclerification around the phloem strands (arrows) can beloem strands); Sp (short phloem strands); Mx (metaxylem); Px (protoxylem); End

All Philodendron species studied here show polyarch structuresin transverse sections. The crampon roots of P. oblongum and theanchor roots of P. cordatum have medullated protosteles, with theformer species having a reduced pith with sclerified parenchyma

552 V. Tenorio et al. / Flora 209 (2014) 547–555

Fig. 4. A–F. Transversal sections of adventitious roots showing the vascular cylinder. A: P. corcovadense feeder root. Note the cylindrical shape of the stele, with metaphloemdispersed throughout the xylematic mass (arrows). B: Anchor/feeder root of P. corcovadense. Lobate stele, although the vascularization pattern is similar to that of a feederroot. C, D: Crampon root of P. oblongum (C), and anchor root of P. cordatum (D). Note the medullated protostele. E, F: Feeder root of P. cordatum showing the development ofa vascular cylinder of the protostelic type. Note the metaxylem in the center of the stele (arrow), not occurring in the differentiated root (F). Note the differences in densitiesand diameters between the different root types of P. cordatum (D and F). Mx (metaxylem); Mf (metaphloem); Me (medulla). Bar = 10 �m (A, C and D), 20 �m (B and E), 460�m (F).

V. Tenorio et al. / Flora 209 (2014) 547–555 553

Table 2Mean values (minimum–maximum) ± standard deviation of metaxylem (MX) and metaphloem (MF) diameter (in �m) for each type of root of the studied Philodendronspecies.

Species Habit Root type MX MF

P. cordatum Nomadic vine Feeder 177(131.9–203.7) ± 21.6 42 (32.9–49.6) ± 5.4Anchor 106 (92–123.4) ± 11.2 27.5 (21.4–34.3) ± 4.1

P. oblongum Epiphyte Crampon 28 (24.2–31.4) ± 2.2 5.9 (4.3–7) ± 0.8P. corcovadense Terrestrial Feeder 100 (90.5–112.2) ± 6 36.5 (33.6–39.4) ± 2.5

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ells while the latter shows a wide pith and parenchyma cells withlightly thickened walls (Fig. 4A and B). The feeder roots of P. corda-um also possess medullated protosteles although a central vesselould be observed until approximately 60 cm from the apex. Thistructure would later disappear, giving way to the formation of aarrow pith (Fig. 4C and D). P. corcovadense showed a protostelicype in both populations, with elements of the metaphloem beingistributed throughout the xylem cross-sectional area (Fig. 4End F). The feeder roots of P. corcovadense have cylindrical stelesFig. 4E) while rupicolous P. corcovadense plants have lobate stelesFig. 4F), with the internal cortical parenchyma intruding into themFig. 4F). The anatomy of P. corcovadense generally has a somewhatntermediate character, with the distribution of vascular elementseing characteristic of feeder roots (with the metaphloem inserted

nto the xylem tissue), whereas the diameters of the vascular ele-ents themselves are more similar to those of anchor roots, i.e.,

maller than typical feeder roots (Table 2).

iscussion

In a study of the anatomy of the adventitious roots of theraceae family, French (1987a, 1987b) emphasized the occurrencef a sclerified hypodermis and resiniferous channels as well as theystematic implications of those structures. Vianna et al. (2001)tudied the anchor roots of Philodendron bipinnatifidum (whichave lobate steles) while Hinchee (1981) conducted studies on theifferent types of roots of Monstera deliciosa, classifying them aserial, aerial–subterranean, or lateral subterranean. These studiesemonstrated various important anatomical characteristics thatan be observed in Araceae roots.

French and Tomlinson (1984) reported that the root vascu-ar plexus is a transitional connective region between the stemnd the root, and Tenorio et al. (2012) reported that the bun-les in this region can be branched or simple, as seen in crossection. The investigated species P. corcovadense and P. cordatumelong to the subgenera Meconostigma and Philodendron, respec-ively, P. oblongum belongs to Pteromischum. According to Tenoriot al. (2012), the subgenera Meconostigma and Philodendron haveranched vascular plexuses, and this character possibly may have

ed to a major diversification, both of their habits and of the habi-ats they could occupy. Thus the occurrence of different root typesn species of these subgenera can reflect their adaptations to differ-nt environmental conditions, and present results indicate that thepecies growing in different environments indeed show significantifferences in the compositions of their steles.

Such differences first of all exist between the diameters of theifferent root types. The feeder roots of Philodendron cordatumave the greatest average diameters of their vascular elements,hich may indicate that they are well adapted to the absorption

nd conduction of water and nutrients. The anchor roots of theame species, however, have vascular elements with considerably

maller diameters—which reflect the different functions of the twooot types. Similar differences were also observed in the differentoots of P. corcovadense plants that grow either terrestrially or onocky outcrops. According to Tomlinson and Fisher (2000), vessel

92.5 (82.3–103.5) ± 6 31.5 (26.6–38.7) ± 3.4

elements with wide diameters serve for water storage and its rapidtransportation. P. oblongum had the smallest average metaxylemand metaphloem diameters in the present study, reflecting theirprimarily crampon-like fastening function.

According to Carlquist (1975), vessel distributions in monocotaxes should be understood as different aspects of their conductioncharacteristics in different organs within the same plant. The feederroots of P. cordatum have larger vessel diameters and densities thanthe anchor roots, and the feeder roots have a vessel in the centerof the stele, which is absent in anchor roots. The anatomical differ-ences between the vessels of these two root types can be takenas a further proof of Carlquist’s statement (1975) that differentconductivity characteristics lead to differences in the distributionand structure of vessels in different root types, even in the sameindividual.

The shapes of the vascular cylinders in P. corcovadense differedamong the different roots types in the populations analyzed. Rootsof other Philodendron taxa belonging to the subgenus Meconostigmahave lobate steles, and this was observed in rupicolous individualsof P. corcovadense, too. However, the roots of these plants mustnot necessarily function as anchor roots simply because they havea stellar outline typical for this functional root type. The signifi-cance of this character still must be analyzed further, keeping inmind that it has only been found in Meconostigma species withinthe genus Philodendron and may be a primarily taxonomy-relatedtrait. Mayo (1991) reported that lobate steles are also present inthe genus Cercestis—a genus close to Philodendron, and both generaare included in the Homalomena clade, according to Cusimano et al.(2011).

In terms of the functional aspect of the lobate stele, further itmust be kept in mind that the adventitious roots of rupicolousindividuals of P. corcovadense first anchor the plant to the rocks,but detach later to become feeders although these roots prob-ably absorb rain water also in the anchor phase. However, thelargest quantities of water and nutrients are probably absorbedfrom the soil substrate—and it is quite plausible in this sense thatthe anatomy of these roots lies between anchor and feeder roottypes. The interlocking placement of cortical parenchyma and ste-lar tissue (leading to the lobate aspect of the roots in cross section)may increase the efficiency of water absorption and conduction byincreasing the lateral surface area of the stele while still maintain-ing an appropriate mechanical stiffness and flexibility.

The adventitious roots of the Philodendron species investigatedhere all originate from the stem in nodal regions, but differ in theirfunctions—either tapping the soil to absorb water and nutrientsor providing support to the rupicolous plants respectively thosethat are growing as nomadic vines. French (1997) denominatedthe roots of epiphyte and hemiepiphyte (nomadic vine) plantswith support functions as anchor roots, and considered as feederroots all those that are extending to the soil to absorb water andnutrients. However, Philodendron taxa, and the Araceae in general,

are somewhat peculiar in this sense, since they can initiate theirlife cycles as terrestrial plants, developing only later anchor roots(crampon or not) that are bound to support structures (usually treetrunks), and growing upward into the canopy region toward better

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54 V. Tenorio et al. / Fl

ight conditions. Ball et al. (1991) underlined that epiphytismwhere access to light is not a primary problem) can considerablyiminish access to nutrients and water. The root structures of theseifferent life forms (including one genus investigated in the presenttudy) do, in fact, represent different strategies coping with short-ge of different resources. The dimorphism of the adventitiousoots of Philodendron cordatum—with greater feeder root diametersnd densities as compared to anchor roots—shows a way, how thisomadic vine can maintain itself both on the phorophyte and theoil, getting sufficient access to both, the soil and the atmosphericesources. No strict dimorphism was observed in the two P.orcovadense populations analyzed here, but rather anatomicalifferences between the terrestrial and rupicolous populations.he anchor roots of the crampon type in P. oblongum, finally, areompletely adapted to the epiphytic habitat of that species.

Root hairs were observed in all species although in the epiphyte. oblongum they are only present on the mature roots. The presencef a pluristratified epidermis (velamen) composed of dead cellsan be observed on aerial roots of epiphytic plants in the familiesromeliaceae and Araceae (Pita and Menezes, 2002), described, e.g.,

n detail in Philodendron appendiculatum (Sakuragui, 1998), and areery well known from many members of the Orchidaceae (Bonat al., 2004). The velamen serves to reduce water loss, providesechanical protection, and increases water absorpt. In the epi-

hyte P. oblongum, no distinct velamen was observed, but its rootairs and the sclerified exodermis apparently contribute preven-ing retarding desiccation of the atmosphere-exposed roots.

Hose et al. (2001) described the exodermis as a hypodermalayer with Casparian strips, and Cutter (1987) and Dickison (2000)efined it as a type of hypodermis typical of certain roots that mayr may not be suberized or lignified, or even contain Caspariantrips. Beck (2005) characterized cell strata with lignified walls justelow the exodermis as a hypodermis that can be found on theoots of many herbaceous dicotyledonous and monocotyledonouslants, as well as in certain palm trees. French (1987b) described aclerified hypodermis to occur in eight genera of Araceae (includinghilodendron) and distinguished it from the exodermis, mainly onhe basis of morphological characteristics—including cell size andhe thickness of the cell wall. In present study we called all subepi-ermal layers as exodermis tissues. Where proximal lignified cell

ayers, below the exodermis, were present (such as in P. oblongum)hey were treated as part of the cortex, considering the exodermiss a special type of root hypodermis.

The resiniferous ducts observed in the studied species are ofchizogenic origin, as it was described also by Vianna et al. (2001)or the anchor roots of Philodendron bipinnatifidum. The origin ofuch ducts was described by Beck (2005) and Evert (2006) as aeparation of cells from each other, resulting in a central space linedy an epithelium of secretory cells.

In the species studied here the cells of endodermis and pericyclehat are located immediately adjacent to the protoxylem poles,id not show any cell wall thickening, unlike those adjacent tohe phloem strands. However, the unthickened cell walls of thendodermis possess well-developed Casparian strips, so that theassage of water and solutes will be channeled primarily throughhe endodermis symplasm. According to Peterson and Enstone1996), endodermal cells that contain only Casparian strips (stage) are radially aligned with the protoxylem poles, and cell matura-ion in stages II (deposition of a suberin lamella) and III (additionf a lignified and cellulosic wall) is generally asynchronous, occur-ing first in those cells arranged radially to the phloem. In all of theoots analyzed, with the exception of P. corcovadense, the endo-

ermis reaches the differentiation stage III showing “O”-shapedhickenings. Enstone et al. (2003) state that development of stages IInd III in the endodermis provides protection against environmen-al stressors, such as desiccation, and confers mechanical support

9 (2014) 547–555

to the root. Sufficient mechanical and physiological robustness isgiven, therefore, for all types of the investigated Philodendron roots,albeit structurally different according to their respective function.

Acknowledgment

The authors would like to thank Ricardo Sousa Couto, for assis-tance in collection, Elaine Santiago Brilhante de Albuquerque, forrevising an earlier version of the manuscript, and Coordenadoria deAperfeic oamento do Pessoal de Nível Superior (CAPES), for financialsupport.

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