ORIGINAL ARTICLE

Systematic relevance of seed coat anatomy in the Europeanheathers (Ericeae, Ericaceae)

Jaime Fagundez • Rocıo Juan • Inmaculada Fernandez •

Julio Pastor • Jesus Izco

Received: 27 February 2009 / Accepted: 25 October 2009

� Springer-Verlag 2009

Abstract The anatomy of the seed coat of the European

species of tribe Ericeae (Calluna, Daboecia and Erica) of

the Ericaceae family was studied, and the taxonomic

importance of their characters was analyzed. The seed coat

is mostly formed by a one-cell layer with thick, pitted inner

walls and thin outer walls that collapse at maturity over the

inner walls. The cell junctions are either raised with anti-

clinal walls up to four times the height of the periclinal

walls or are not raised with similar values for the height of

both the anticlinal and periclinal walls. Three main cell

junction types were found and described. The thickness of

the inner walls is variable, but there is a large overlap

among the results for different species. Calluna vulgaris is

the only species with no pits, and E. multiflora has a pitted

pattern on its inner walls, which is distinctive from the rest

of the species. Our main results agree with the external

seed morphology, and valuable new data were obtained for

certain groups such as the E. cinerea-E. terminalis or the

E. scoparia complex. The similarities that are found in seed

coat characters are not in accordance with the classical

taxonomic delimitation of infrageneric groups within

Erica.

Keywords Erica � Calluna � Daboecia � Seed coat �Seed anatomy � Taxonomy

Introduction

Since the thorough study of the Ericaceae performed by

Kron et al. (2002), the systematics of the family have been

clarified at different infrafamiliar levels. According to their

analysis Calluna Salisb., Daboecia D. Don and Erica L. are

the only genera that are integrated into the tribe Ericeae, and

consequently included in the subfamily Ericoideae. Before

this work, many different studies of the whole family

resulted in much discussion as to the systematics of these

genera (Stevens 1971; Anderberg 1993; Judd and Kron

1993). Daboecia has a septicidal capsule, a deciduous

corolla and differs from the other two genera in its leaf and

anther morphology. Calluna has a septifragal capsule, an

expanded petalloid calyx, sagittate leaves and a distinct pith

type. Both genera are monotypic, contrasting with the vast

Erica genus of around 860 species, which are characterized

by a loculicidal capsule, a persistent corolla and strongly

revolute leaves. Important differences are also found in the

seed morphology for the three genera, but many species of

Erica have not yet been studied.

Erica is a very large genus with a high level of mor-

phological variation and a wide ecological and geograph-

ical range (Oliver 2000; Stevens et al. 2004). The early

treatments of this genus, for example by Don (1834) or

Klotzsch (1838), considered several genera, whereas now it

is considered to be a unique genus. Bentham0s (1839)

arrangement included 19 genera, and Erica was divided

into four subgenera and several sections. The classic genera

Bruckenthalia Reichenb., Pentaptera Klotzsch, Philippia

Klotzsch, Ericinella Klotzsch, Blaeria L. and the ‘‘minor’’

J. Fagundez (&)

School of Biology, IE University, Campus de Santa Cruz la Real,

Cardenal Zuniga 12, 40003 Segovia, Spain

e-mail: jaime.fagundez@usc.es

R. Juan � I. Fernandez � J. Pastor

Departamento de Biologıa Vegetal y Ecologıa,

Universidad de Sevilla, 41012 Sevilla, Spain

J. Izco

Departamento de Botanica, Facultade de Farmacia,

Universidade de Santiago de Compostela,

15782 Santiago de Compostela, A Coruna, Spain

123

Plant Syst Evol

DOI 10.1007/s00606-009-0240-2

genera of species with indehiscent or partially dehiscent

fruits are now part of the large genus Erica according to

Oliver (1989, 2000).

Approximately 20 species of Erica are considered in the

Northern area of the genus, which includes Europe, the

Mediterranean coasts of Northern Africa and the Eastern-

most part of Asia (Webb and Rix 1972; Greuter et al.

1986), including Erica spiculifolia, the only species

included in genus Bruckenthalia that was first considered

as part of Erica by Salisbury (1802) and was recovered by

Oliver (2000). The infrageneric classification of Bentham

(1839) has been modified for the Cape species by Guthrie

and Bolus (1905) and other authors. Hansen (1950) pro-

posed some changes to the infrageneric classification for

the Northern species. Recently, some attempts have been

made to use molecular analysis in order to determine the

phylogeny of the genus (McGuire and Kron 2005). The

researchers performed a preliminary phylogenetic analysis

for Erica, including Calluna and Daboecia as outgroups,

with puzzling results inconsistent with Bentham0s classifi-

cation. Moreover, different interpretations have been given

to closely related taxa, and some points regarding the

delimitation of the species are still unclear. For example,

the systematic position of Erica andevalensis Cabezudo

and Rivera has been discussed and subsequently placed as

a subspecies of Erica mackayana Bab. (McClintock

1989a).

Early studies in seed anatomy were developed by

Artopoeus (1903) and Peltrisot (1904). Both studies

focused on the whole family Ericaceae, and the seeds of

several genera were analyzed. Both Artopoeus and Peltrisot

included Calluna, Daboecia and several Erica species,

including Bruckenthalia, in their studies. Additional data

were obtained by Stevens (1971), again from general

studies on the family. Oliver (2000) used several seed coat

features for his systematic analysis on the Ericeae. The

testa thickness and occurrence of pits were coded, along

with surface cell characters. Szkudlarz (2006) performed

an analysis of seed characters in a set of 33 South African

species. More importance has been given to the seed sur-

face characters, such as the elongation of surface cells

(Stevens 1971; Anderberg 1993; Oliver 2000; Kron et al.

2002). In our studies on seed morphology of the Ericeae,

important diagnostic features were obtained for the

delimitation of the three genera in order to solve the

ongoing taxonomic problems regarding species delimita-

tion (Fagundez and Izco 2003a, b, 2004a, b, c, d, 2008,

2009a, b, c). However, difficulties were found when these

characters were used in a generic approach to the system-

atics of the Ericeae because of continuing variation, and

problems in coding characters arise.

In this paper, we studied the seed coat anatomy to reveal

the nature of the external features of seed surface that we

have previously studied. We also looked for new diagnostic

characters that could add information to enable the further

delimitation of the species. In a similar manner to what was

found with regards to seed morphology, the anatomy of the

seeds is only related to the systematic position in some

cases. Some closely related species have a different seed

coat type, while a similar pattern was found in others

considered as unrelated. This reveals once again that an

overall study of this genus is needed.

Materials and methods

Seeds of 29 samples were studied (Table 1), including 3

genera, 21 species and 4 subspecies. Nomenclature follows

the most accepted taxonomic treatment for the species

(Webb and Rix 1972; Stevens 1978; Greuter et al. 1986;

Bayer 1993 and others) except for the following taxa:

E. platycodon (Webb and Berth.) Rivas-Martınez et al.

subsp. platycodon, E. platycodon subsp. maderincola

(McClintock) Rivas-Martınez et al. and E. scoparia subsp.

azorica (Hochst.) D. A. Webb. Other treatments and sub-

specific taxa were considered and discussed in the text. Seeds

of Erica sicula Guss. that occur in Sicily were not studied.

Seeds were stored under low levels of humidity and

temperature at the Seedbank of the University of Santiago

de Compostela. A collection of Ericaceae seeds has been

collected and stored for 10 years, resulting in more than

400 samples available for conservation and research. Seeds

were collected in the field or obtained from different

seedbanks (Table 1). Samples were cleaned and dried

using silica gel and put into glass tubes with a certain

amount of silica gel with a color component to detect

possible humidification. These tubes were then stored in a

4�C chamber. Voucher specimens of field samples are

stored at the SANT herbarium.

Seeds sectioned for the light microscopy studies were

first fixed in FAA for at least 48 h, dehydrated in an eth-

anol–tertiary butanol (Johansen 1940 modif.; Juan et al.

1999) and embedded in paraffin (Paraplast plus). Sections

were cut at a thickness of 5 lm with a Micron HM-355

microtome. Three or four slides were mounted for each

sample. The slides were studied, photographed and drawn

with a digital camera adjusted to an Olympus BX-50

microscope. Image analysis was performed with Motic

Image Plus 2.0 ML software.

Handmade sections were prepared for seeds undergoing

scanning electron microscopy (SEM) observations. These

were mounted onto the stubs using double-sided adhesive

tape and coated with gold. The samples were studied

and photographs were obtained using a LEO-435VP

microscope (Leica Microsystems, Cambridge, UK) at the

University of Santiago de Compostela.

J. Fagundez et al.

123

The nomenclature was mostly obtained from Werker

(1997), using the most commonly accepted terms in the

field.

Results

General seed characters

The main results for the studied species are summarized in

Table 2. The sizes of the seeds from the studied species

range from 0.3 to 1.5 mm. Their shape varies from globose

to flat and elongated seeds. The external primary sculpture

is either alveolate, reticulate or has a slightly striated pat-

tern. The surface cells range from being isodiametric to

being elongated by three to four times, oriented on the

main axis of the seed. On the cross sections, an outer layer

that constitutes the seed coat is visible. The seed coat is

generally formed by a single cell layer at maturity. In some

cases, a layer of collapsed cells between the seed coat and

the endosperm can be observed (E. vagans, Fig. 2d,

E. cinerea, Fig. 2f); these correspond to the hypodermis.

The endosperm lies below the seed coat, and the embryo is

embedded within it. The two polar ends form two hausto-

ria. The micropylar haustorium is conspicuous in some

species (E. spiculifolia, Fig. 1f), while the chalazal

Table 1 Details of the material studied

Species BG-SANT Collector and date Locality

Calluna vulgaris (L.) Hull 120 Fagundez & Iglesias

11/1999

Spain, A Coruna, Carnota

Calluna vulgaris (L.) Hull 385 Pesu 9/2004 Finland, Varsinais-Suomi,

Vihti

Daboecia cantabrica (Hudson) Koch subsp. cantabrica 145 I. Agron. Lisboa Portugal

D. cantabrica subsp. azorica (Tutin & Warburg) McClintock 338 Fagundez 9/2004 Portugal, Acores, Pico

Erica andevalensis Cabezudo & Rivera 126 J. B. Cordoba Spain, Huelva, Andevalo

Erica arborea L. 125 J. B. Cordoba Spain

Erica australis L. 41 Fagundez 7/1999 Spain, A Coruna, Boente

Erica carnea L. 288 Izco 6/2003 Italy, Spezia, Cassale

Erica ciliaris L. 139 Fagundez & Aguilar

12/1999

Spain, Pontevedra, Oia

Erica cinerea L. 118 Fagundez & Iglesias

11/1999

Spain, A Coruna, Carnota

Erica cinerea L. 25 Fagundez 9/1998 Spain, A Coruna, Capelada

Erica erigena R. Ross 55 Fagundez 6/1999 Spain, A Coruna, Capelada

Erica lusitanica Rudolphi subsp. lusitanica 83 Fagundez & Reyes 8/1999 Spain, Huelva, Andevalo

E. lusitanica subsp. cantabrica Fagundez & Izco 322 Fagundez 5/2004 Spain, Guipuzkoa, Eibar

Erica mackayana Bab. 23 Fagundez 9/1998 Spain, A Coruna, Capelada

Erica maderensis (Benth.) Bornm. 354 Fagundez 9/2004 Portugal, Madeira, P. Areeiro

Erica manipuliflora Salisb. 401 Fagundez 11/2007 Croatia, Dalmatia, Ston

Erica multiflora L. 91 Izco 8/1999 Spain, Valencia, Dos Aguas

Erica platycodon (Webb & Berth.) Rivas-Martınez et al. subsp.

platycodon331 Unknown Spain, Canarias, Tenerife

E. platycodon subsp. maderincola (McClintock) Rivas-Martınez et al. 337 Fagundez 9/2004 Portugal, Madeira

Erica scoparia L. subsp. scoparia 85 Fagundez 8/1999 Portugal, Leirıa

Erica scoparia L. subsp. scoparia 267 I. Tapada de Ajuda 6/2002 Portugal, Algarve, Alportel

E. scoparia subsp. azorica (Hochst.) D. A. Webb 142 J. B. do Faial Portugal, Acores

Erica spiculifolia Salisb. 376 Bita-Nicolae 6/2005 Romania, Bucegi mts.

Erica terminalis Salisb. 92 Izco 8/1999 Spain, Valencia

Erica tetralix L. 116 Fagundez & Iglesias

11/1999

Spain, A Coruna, Brandomil

Erica umbellata L. 81 Fagundez & Reyes 8/1999 Spain, Huelva, Tharsis

Erica umbellata L. 74 Fagundez 8/1998 Spain, Lugo, Quiroga

Erica vagans L. 131 Fagundez 12/1999 Spain, Lugo, Palas de Reis

Systematic relevance of seed coat anatomy in the European heathers (Ericeae, Ericaceae)

123

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b,

2004a,

b,

c,d

,2008

,2009a,

b,

c)

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the

text

and

Fig

.4

.S

eed

size

inm

m,

oth

erval

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inl

m

Note

s:*1

Daboec

iahas

are

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late

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tern

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ha

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illa

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(Fag

undez

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um

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(Fag

undez

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);*3

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ssure

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E.

mult

iflora

could

not

be

ascr

ibed

toth

ese

types

(see

Res

ult

s)

J. Fagundez et al.

123

haustorium is reduced and is hardly visible. The endosperm

is fleshy, formed by large cells with very thin walls filled

with starch, which in turn are surrounded by a cellulosic

membrane (Fig. 6d). The embryo is elongated on the main

axis of the seed to between 100 and 150 lm. It occupies a

central position on the surrounding endosperm. The two

cotyledons are short and thick, measuring between 30 and

50 lm (Fig. 6b).

Seed coat characters

The seed coat provides the most important diagnostic

characters regarding seed anatomy for these species. In

mature seeds, the testa cells are dead, and the thin outer

walls collapse over the thick inner walls. The outer walls

are very thin (E. multiflora, Fig. 2b) or moderately thin

(E. australis, Fig. 6d). Sometimes, the anticlinal (radial)

outer walls remain visible (E. erigena, Fig. 1e). In

Daboecia, the outer walls are not collapsed over the inner

walls. This gives rise to its distinctive seed coat surface,

which is papillate (Fagundez and Izco 2004d). The papil-

late pattern is the result of the sculpting of the outer peri-

clinal cell walls. Each papilla is formed by a single cell in

this outer layer. The raised rim that forms the outline of the

cell corresponds to the edge of this cell and its junction

with the neighboring cells (Figs. 6c, 5a).

The inner walls are thick but values are variable for the

species, from 2 to 20 lm. According to the thickness of

the periclinal (tangential) inner walls, four groups could

be assigned (Fig. 3). The thinnest walls are those of

E. maderensis and E. spiculifolia with values close to

2 lm. The second group of 3–10 lm is the case for most of

the species despite the differences due to overlapping

ranges (Table 2; Fig. 3). The third group of approximately

15 lm is formed by E. cinerea and E. vagans, and

Daboecia stands alone in the fourth group of ca. 20 lm.

Fig. 1 Light microscopy

photographs of: a Ericascoparia subsp. azorica;

b E. scoparia subsp. scoparia;

c E. platycodon subsp.

maderincola; d E. ciliaris;

e E. erigena; f E. spiculifolia.

a–e Seed coat and endosperm.

f Micropylar haustorium.

op Outer periclinal walls,

h micropylar haustorium.

Scale bar is 50 lm

Systematic relevance of seed coat anatomy in the European heathers (Ericeae, Ericaceae)

123

The case of Calluna is an exception: the inner walls are

thin, contrasting with the rest of the studied species. The

thickness of these walls could not be measured, and

interpretation was therefore difficult. The SEM pictures

revealed that the epidermal layer is thin and strongly

attached to the endosperm (Fig. 5b).

The inner anticlinal walls either are raised to a height of

up to 50 lm or are almost equal to the periclinal walls

(Table 2). The highest ratio values for anticlinal walls

compared to periclinal walls were found in E. erigena

(4.85) and E. spiculifolia (4.36). Three testa cell junction

types are present (Fig. 4): (a) Cell boundaries are continued

by the neighboring cells and no prominent anticlinal walls

are present. (b) Cell boundaries are raised and the inner

anticlinal walls are gradually narrowed, giving the

boundaries a triangular aspect. (c) Anticlinal walls are

conspicuous on both neighboring cells (double anticlinal

walls) and are rounded at the confluence. Type A junctions

are clearly found in E. australis or E. arborea (Fig. 6d, h),

and type C is present in E. cinerea and both species of

section Callicodon: E. carnea and E. erigena (Figs. 1e, 2f).

Type B is the most common, although the height of the

anticlinal walls is variable. It is found in several species

such as E. vagans or E. lusitanica (Figs. 2c, 5c). Type B

juctions are also found in Daboecia. Close to the cell

limits, the outer layer is collapsed over the thickened inner

walls, which are responsible for the hardness of the testa

(Fig. 6c, 5a).

The outer walls collapse over the inner walls, but at the

junction of neighboring cells the outer walls may raise over

the inner walls. This was observed mostly in species with

Type C junctions, as in E. erigena (Fig. 1e).

The inner periclinal walls have no pits in Calluna, but

are always pitted in the rest of the species. E. multiflora has

a lower density of pits, and most of these are incomplete as

they do not reach both ends of the walls (Fig. 2b). The

Fig. 2 Light microscopy

photographs of: a Ericamanipuliflora; b E. multiflora;

c E. vagans; d E. terminalis;

e E. maderensis; f E. cinerea.

a–e Seed coat and endosperm.

Scale bar is 50 lm

J. Fagundez et al.

123

density of the pits is high in the rest of the species of Erica

(for example, Fig. 6g) and Daboecia (Fig. 6c).

Discussion

Seed anatomy in Ericaceae

The seeds of the Ericaceae are derived from anatropous,

unitegmic, tenuinucellate ovules (Peltrisot 1904; Neto-

litzky 1926; Stevens 1971; Corner 1976; Stevens et al.

2004). Seed sizes range from 0.2 to 3.5 mm (Szkudlarz

2001; Stevens et al. 2004), their shape is variable from

globose to strongly flattened seeds, and they are winged in

genera such as Rhododendron or Ledum, in which wings

develop at both of the polar ends and continue the seed

coat. A caruncle is only rarely present, such as in Erica

australis (Oliver 2000; Fagundez and Izco 2004a). The

endosperm is cellular, according to Stevens et al. (2004),

and generally develops two haustoria at the micropylar and

chalazal ends (Peltrisot 1904; Stevens 1971). In our

material, these haustoria are reduced. The micropylar

haustorium is visible in some species like E. spiculifolia

(Fig. 1f) or E. manipuliflora, but the chalazal haustorium is

very narrow as found by Palser and Murty (1967) in 54

Erica species. Ganapathy and Palser (1964) consider

Daboecia to be an exception within the Phyllodoceae

because both haustoria are similar in size. In our study, the

embryo is axial and straight with two short cotyledons, and

this is the most common type in the family (Peltrisot 1904;

Szkudlarz 2001). Rarely, the embryo is formed by a few

undifferentiated cells as in Monotropae (Olson 1980;

Stevens et al. 2004). The seed coat is commonly a single-

cell layer (Szkudlarz 1999, 2001, 2002, 2006, 2008;

Stevens et al. 2004), although two to three cell layers have

been recorded in different genera such as Andromeda or

Oxycoccus (Szkudlarz 2001). Ganapathy and Palser (1964)

considered that Daboecia has a hypodermal layer of cells

in its seed coat, but results from Stevens (1971), Oliver

(2000) and our own work show only a single layer. Sub-

epidermal layers of tanniferous cells, which are strongly

compressed or disappear at maturity, have been reported

(Peltrisot 1904; Stevens 1971; Brisson and Peterson 1976).

In mature seeds, the outer walls collapse over the inner

walls, which are commonly thick (Brisson and Peterson

1976; Szkudlarz 2001). The inner periclinal walls and

sometimes the anticlinal walls of the testa cells are thick

and present plasmodesmata (pits) in many different genera

(Peltrisot 1904; Stevens 1971; Takahashi 1993; Szkudlarz

2001; Stevens et al. 2004). In our results, pits are present in

every species except Calluna, but differences are found

between species as regards the density of these pits. The

outer walls that collapse over the inner walls are sometimes

indented, as a result of the thin outer wall covering the

pitted inner wall. The seed coat is loosely attached to the

Inner periclinal wall thickness

DaboeciaE. vagans

E. cinereaE. australis

E. carneaE. erigena

E. multifloraE. platycodon

E. terminalisE. maderincola

E. manipulifloraE. andevalensis

E. tetralixE. lusitanica

E. mackayanaE. ciliaris

E. arboreaE. umbellata

E. scopariaE. azorica

E. spiculifoliaE. maderensis

0 2 4 6 8 10 12 14 16 18 20 22

Fig. 3 Mean ± SD of the

thickness of the inner periclinal

seed coat walls. Values in lm.

Dashed lines show limits for

non-overlapping groups (see

text)

Fig. 4 Seed coat cell junction types (see text for description). Arrowspoints to cellular limits

Systematic relevance of seed coat anatomy in the European heathers (Ericeae, Ericaceae)

123

endosperm membrane, and the fixation process was diffi-

cult in many samples.

Systematics of Ericeae: the treatment of Daboecia

and Calluna

The three genera today considered as tribe Ericeae (Kron

et al. 2002), Daboecia, Calluna and Erica, have histori-

cally undergone different systematic treatments. Daboecia

is a single species, a puzzling genus that has been related to

the Phyllodoceae (Ganapathy and Palser 1964) but has

mostly been included in the Rhododendroideae (Stevens

1971; Anderberg 1993; Judd and Kron 1993). Daboecia

has a septicidal capsule and a deciduous corolla. These

characters are not found in the other members of the tribe,

but other molecular and morphological characters support

its inclusion in the Ericeae (Kron et al. 2002). The anatomy

of the seeds of Daboecia has been studied by Peltrisot

(1904), Ganapathy and Palser (1964), Stevens (1971) and

Oliver (2000). The fact that its distinctive papillate surface

is only found in this genus and in some South African

Erica species (Oliver 2000; Stevens et al. 2004) within the

Ericaceae supports the relationship of these two genera.

However, anatomical studies of those Erica species should

be performed to confirm that these seed types are homol-

ogous. Calluna is another ambiguous genus, with its own

tribe according to Stevens (1971) or Oliver (1989). It not

only has significantly individual characteristics with

regards to its leaf and calyx morphology, phylotaxis and

type of inflorescence, but also has individual anatomical

characteristics as recorded by Watson (1964) with regards

to its stomata and pith structure, which would relate this

species to Cassiope, now placed in the subfamily Cassio-

poideae, sister to the Ericoideae (Kron et al. 2002). In

Calluna seed development, the micropylar haustorium

intensively develops and breaks through the tegument,

leaving a pore at the polar hilum end (Peltrisot 1904;

Szkudlarz 2001; Fagundez and Izco 2004c). This feature is

unique within the family, and again its systematic signifi-

cance is not clear. The absence of pits on the inner walls

has not been found in the other species studied here, but

Oliver (2000) reports this feature in several African Erica

species. Pits are present on Erica and Daboecia, but also in

many different genera from other subfamilies within Er-

icaceae (Peltrisot 1904; Stevens 1971; Szkudlarz 2001).

This character was not included in the family delimitation

study by Kron et al. (2002). The interpretation of the seed

coat elements in this species is uncertain, and therefore no

relationships could be asserted. However, the differences

on seed anatomy compared with the other members of the

Ericeae studied here are clear.

Systematics of Erica

Since Bentham0s classification (Bentham 1839) and the

changes that were proposed by Hansen (1950), no

taxonomical study has included the Northern (European

and rest of the Mediterranean) species of the Erica

Fig. 5 Scanning electorn

microscope (SEM) photographs

of seed sections: a Daboeciacantabrica subsp. azorica;

b Calluna vulgaris; c Ericalusitanica subsp. cantabrica;

d E. cinerea

J. Fagundez et al.

123

genus. However, the results of the anatomy of the seeds

may help to solve several taxonomic problems regarding

the Northern heathers. Bentham established four sub-

genera, with most of the European species included in

subgenus Euerica and one in subgenus Ectasis (E. car-

nea). Today, two species are considered to be in this last

group: E. carnea and E. erigena. Their specific status

has been at least partially solved by seed morphology

(Fagundez and Izco 2003a; Szkudlarz 2008). Seed anat-

omy also reveals differences with the rest of the Euro-

pean species, including the feature of the highly raised

anticlinal walls with Type C junctions and outer raised

anticlinal walls. Our results agree with those of Szkudlarz

(2008).

Fig. 6 Light microscopy

photographs of: a Erica vagans;

b Erica platycodon subsp.

maderincola; c Daboeciacantabrica; d Erica australis;

e E. mackayana; f E.andevalensis; g E. lusitanica;

h E. arborea. a Whole seed

section; b embryo; c–h seed

coat and endosperm. eEndosperm, s seed coat, ccotyledons, o outer walls, i inner

walls, m endosperm membrane,

a anticlinal walls, p periclinal

walls. Scale bars are: a 100 lm,

b 30 lm, c–h 50 lm

Systematic relevance of seed coat anatomy in the European heathers (Ericeae, Ericaceae)

123

In another group of related species, the systematic

position of the three species of section Gypsocallis has

been discussed on several occasions (Webb and Rix 1972;

McClintock 1989a; Fagundez and Izco 2009c). Seed mor-

phology added several important features for the correct

interpretation of these taxa. In a previous paper (Fagundez

and Izco 2009c), we studied the seed morphology of this

set of species. E. multiflora has seeds of approximately

1 mm, flattened, yellowish and with a striate major orna-

mentation. We also reported the thin outer periclinal walls

that were detected in the SEM surface view, and it can now

be observed in the anatomical study of the seed coat

(Fig. 2b). The distinctive feature of the low density of pits

in the inner walls, most of them incomplete, confirms the

exclusive seed type of this species within the Northern

nucleus. E. vagans and E. manipuliflora, the two other

species of this section, are closely related, although several

features such as anther morphology, sepal fusion or the

presence or absence of lignotuber confirm the specific level

of these two taxa (Fagundez and Izco 2009c). The seeds of

E. vagans are 0.5 mm long, globose and have a strongly

reticulate primary ornamentation. E. manipuliflora has

larger, ellipsoid seeds, but the seeds of the populations

from the Dalmatian coasts, recognized by McClintock

(1989a, 1991) as subsp. anthura, are closer to the E. vagans

type (Fagundez and Izco 2009c). The anatomy of the seed

coat discriminates between both species, with thicker inner

walls and Type C cell junctions for E. vagans (Table 2),

while E. manipuliflora has Type B cell junctions and

thinner inner walls. This feature can be used as a new

diagnostic character that discriminates E. vagans seeds

from E. manipuliflora seeds of the anthura group.

In the E. scoparia group (section Chlorocodon), differ-

ent treatments have been proposed for the Macaronesian

taxa (from Acores, Madeira and Canarias Atlantic Islands)

either as a species or subspecies within E. scoparia. In our

studies in seed morphology, the continental species was

mostly discriminated by their fused surface cell boundaries,

while in the Macaronesian these were observed to be

channeled (Fagundez and Izco 2003b). Later, we found that

some of the Azorean populations had partially fused walls.

The testa thickness of the continental and Azorean seeds

are of ca. 4 lm, and the Canary and Madeiran populations

have values of 8–9 lm (Fig. 1a, b, c, Table 2; Fig. 3).

This supports a new alternative classification in which

E. platycodon would be raised to a specific level with two

subspecies: platycodon for plants from the Canaries and

maderincola for those from Madeira and Porto Santo.

E. scoparia would remain as two subspecies: scoparia for

the continental, Western Mediterranean species and azo-

rica for the plants from Acores. This interpretation is, in

our opinion, also supported by flower size, with larger

flowers in E. platycodon as stated by McClintock (1989b),

and leaves that are larger and with less revolute margins in

E. platycodon. Moreover, E. scoparia and E. azorica have

very weak diagnostic characters as pointed out by Webb

and Rix (1972).

E. maderensis is another case of a Macaronesian

species that has undergone different treatments and is

now recognized as a separate species instead of the long

held view that it was a variety or subspecies of

E. cinerea (McClintock 1989a; Fagundez and Izco

2009a). In our morphological seed studies, we found that

the seeds of E. cinerea had an exclusive set of features

within the group, mostly with respect to its ‘‘inflated’’

anticlinal walls, while E. maderensis had seeds that are

almost identical to those of E. terminalis. Anatomical

studies confirm that E. cinerea has a distinct testa type,

which is very thick, with Type C cell junctions and very

sunken cells, the thick anticlinal walls giving the inflated

aspect (Figs. 2f, 5d). E. terminalis has moderate values

for testa thickness, but surprisingly E. maderensis has the

lowest values of all the species for testa thickness with

approximately 2 lm (Fig. 2d, e; Table 2; Fig. 3). This

feature confirms the differences between it and E. cine-

rea, but also with E. terminalis, with which we found

strong similarities in their seed morphology (Fagundez

and Izco 2009a). The systematic position of this species

remains unresolved.

The systematics of Erica, and particularly the Northern

species, is far from being concluded. An infrageneric

classification was proposed by Bentham (1839) with some

modifications made for the Northern species by Hansen

(1950), but a phylogenetic, natural arrangement of this

genus is still lacking (Oliver 2000). Reasons for this are

predominantly due to the complexity of the variation

between many features of this family, such as pollen types

and anther appendages. Palser and Murty (1967, p. 304)

concluded that classifications of Erica ‘‘…must be artifi-

cial; that is, they are based on the obvious gross charac-

teristics and serve more for the construction of keys to be

used in identification of species than for the recognition of

closely related species.’’ They claim that the use of ana-

tomical features might clarify the natural relationships

between the species. In our studies, we found that some

classical groups have different seed coat types. As an

example, E. arborea and E. lusitanica are grouped in the

Arsace section, but seeds of E. arborea have many features

discriminating them from E. lusitanica (Fagundez and Izco

2009b), such as size, shape or primary ornamentation.

These differences can also be found in the seed coat

anatomy (Fig. 6g, h). E. lusitanica has similar seeds to the

E. tetralix type, a group of species that seems to be unre-

lated to E. lusitanica. This shows that our results do not fit

with the classical proposal of Bentham, which uses sub-

genera and sections. Neither do those obtained by

J. Fagundez et al.

123

Szkudlarz (2006) from seed characters of a set of South

African species.

A molecular study was also performed in this genus

(McGuire and Kron 2005), but the resulting cladogram

grouped species into even more artificial groups. Oliver

(2000) found a high homoplasy in his phylogenetic

approach from a wide set of characters, and he recognized

that it is difficult to extract any hypothesis on how to

construct new taxonomic groups within the genus. Palser

and Murty (1967, p. 306) suggest that ‘‘…one possibility is

that the majority of species can interbreed so that at least

certain characteristics become scattered through the entire

species population.’’ Some sort of low hybridization could

also explain the resulting phylogenetic trees of McGuire

and Kron (2005). The position of several species such as

E. umbellata, E. australis or E. spiculifolia (previously

considered as Bruckenthalia spiculifolia) is still uncertain,

and extensive studies into different fields of morphology,

anatomy and molecular analysis are needed. In a limited

manner due to the restricted species number and charac-

teristics, this study presents new features that may make

useful contributions to a natural rearrangement of the

genus.

Acknowledgments The head of the Animal Physiology Department

at the University of Santiago de Compostela, Joaquın Espinosa,

supported this experimental study. Help was also obtained from the

following researchers in different aspects of the histological tech-

niques: Rodrigo Carbajal, Raquel Iglesias and Pablo Farinha from

Santiago de Compostela University, and Carmen Acedo from Leon

University. This study was partially supported by the Spanish

government, project CGL2006-06890, Ministerio de Ciencia y

Tecnologıa, Spain.

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