Translucent Glands and Secretory Canals in

Annals of Botany 88: 637±644, 2001
doi:10.1006/anbo.2001.1514, available online at http://www.idealibrary.com on
Translucent Glands and Secretory Canals in Hypericum perforatum L. (Hypericaceae):
Morphological, Anatomical and Histochemical Studies During the Course of Ontogenesis
D A N I E L A C I C CA R E L L I , A N D R E A C E S A R E A N D R E U C C I and A N N A M A R I A PA G N I *
Department of Botanical Sciences, University of Pisa, Via Luca Ghini 5, 56126 Pisa, Italy
Received: 23 April 2001 Returned for revision: 5 June 2001 Accepted: 3 July 2001
Hypericum perforatum L., traditionally used in folk medicine as a therapeutic plant, is today being evaluated for its
antidepressant and antiretroviral activities. The species is characterized by the presence of dierent types of secretory
structure: translucent glands or cavities, black nodules and secretory canals. The aim of this work was to characterize
the translucent glands and secretory canals in both the ¯oral and vegetative parts, from morphological, anatomical
and histochemical points of view. Translucent glands consist of a sub-epidermal cavity delimited by two layers of
cells. There are three types of secretory canal: type A, with a narrow lumen, and types B and C, both with a wide
lumen, but with dierent patterns of development. Histochemical tests showed that all these structures contain
alkaloids and lipids but not pectic-like substances and proteins. Tests for resins, essential oils and tannins gave
# 2001 Annals of Botany Company
dierent responses in dierent parts of the plant.
Key words: Hypericum perforatum, St. John's wort, secretory structures, morphology, anatomy, histochemistry.
I N T RO D U C T I O N
Hypericum L. is characterized by the presence of dierent
types of secretory structure including translucent glands,
black nodules and secretory canals (Blenk, 1884; Green,
1884; Weill, 1903; Siersch, 1927; Metcalfe and Chalk, 1950;
Curtis and Lersten, 1990; Baroni Fornasiero et al., 1998,
2000; Bottega et al., 1999; Ciccarelli et al., 2001). Not all of
these structures are present in all species of the genus
(Blenk, 1884; Weill, 1903; Pereira Coutinho, 1950), and
their presence and/or frequency vary among plant organs
(Green, 1884; Weill, 1903; Siersch, 1927; Robson, 1981).
The secretory structures, which are sites of synthesis and/or
accumulation of biologically active substances, are important for discrimination among taxa (Robson, 1977, 1981;
Pignatti, 1982).
Hypericum perforatum L. has traditionally been used as
an external anti-in¯ammatory and healing remedy for the
treatment of swellings, wounds and burns. It is currently of
great interest due to new and important therapeutic
applications (Bombardelli and Morazzoni, 1995; Nahrstedt
and Butterweck, 1997; Erdelmeier, 1998).
The histo-anatomical knowledge of this plant is dated
and incomplete (Blenk, 1884; Green, 1884; Weill, 1903;
Siersch, 1927; Metcalfe and Chalk, 1950). More recently,
Curtis and Lersten (1990) provided data on the translucent
glands and black nodules of the leaves and petals of some
North American samples of H. perforatum, and Baroni
Fornasiero et al. (1998) studied black nodules on the leaves
of an Italian population. Ciccarelli et al. (2001) characterized the black nodules on both the ¯oral and vegetative
parts of Italian populations of H. perforatum from
morphological, anatomical and histochemical points of
view, but otherwise this ®eld of study has been neglected.
* For correspondence. Fax 00 39 (0)50 551345, e-mail pagniam@
dsb.unipi.it
0305-7364/01/100637+08 $35.00/00
The aim of this research was to complete the characterization of all secretory structures present in H. perforatum,
considering the translucent glands and secretory canals in
both the ¯oral and vegetative parts from morphological,
anatomical and histochemical points of view.
M AT E R I A L S A N D M E T H O D S
Materials
Samples of Hypericum perforatum subsp. perforatum were
collected in San Rossore Estate, Pisa, Italy, and grown in
the ®eld at the Botanical Gardens of the University of Pisa
(accession number 384/99). All the samples were diploids
(2n 16). The relevant vouchers are lodged in the Pisa
herbarium.
Light microscopy
All organs of the plant were studied: petals, sepals,
stamens, pistils, leaves, stems and roots. Sections (25 mm) of
fresh material were cut using a Leitz 1720 cryostat at ÿ14/
ÿ16 8C. Other sections (3 mm) were cut using a Leica 2055
microtome after the material had been ®xed in FAA (Sass,
1958) and embedded in LR White acrylic resin (Sigma,
Milan, Italy). All material was subjected to the following
histochemical tests: Toluidine Blue (O'Brien and McCully,
1981) as a generic dye for DNA, cytoplasm and some
components of the cell wall; PAS (O'Brien and McCully,
1981) for non-cellulosic polysaccharides; Alkanna tincture
(Faure, 1914) and Nile blue (Cain, 1947) for total lipids;
Sudan III with glacial acetic acid as a control (Johansen,
1940) and Nadi reagent (David and Carde, 1964) for
essential oils; Nadi reagent (David and Carde, 1964) for
resins; Wagner and Dittmar reagents (Furr and Mahlberg,
1981) and iodine iodide solution (Lugol) (Johansen, 1940)
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638
Ciccarelli et al.ÐTranslucent Glands and Secretory Canals of Hypericum perforatum L.
F I G . 1. Morphology of translucent glands and secretory canals of Hypericum perforatum: A, cross section of sepal with mature translucent gland
(toluidine blue, 450); B, cross section of sepal showing detail of the translucent gland: which is delimited by two layers of cells an internal layer
with ¯attened, thin-walled secretory cells and an external layer of thicker-walled parenchyma cells (toluidine blue, 900); C, cross section of root
showing type A canals generally delimited by four polygonal cells which are very thin-walled towards the lumen (toluidine blue, 350); D, cross
section of pistil showing type A canals with a variable lumen diameter and a variable number of secretory cells (PAS, 450); E, cross section of
petal showing a type B canal with the same structure as the translucent glands (toluidine blue, 350); F, cross section of petal; detail of type B
canal with an internal layer of ¯attened, thin-walled secretory cells and an external layer of thicker-walled parenchyma cells (toluidine blue,
1400); G, cross section of pistil showing a type C canal consisting of a wide cavity delimited by one or more layers of densely stained and thinwalled secretory cells, and one or two layers of ¯attened parenchyma cells (toluidine blue, 160); H, cross section of petal, type A canal (arrow)
associated with veins, especially in the phloem (Ph) (toluidine blue, 900).
for alkaloids; potassium bichromate (Faure, 1914) for
tannins; Dela®eld hematoxylin (Faure, 1914) and Ruthenium Red (Jensen, 1962) for pectic-like substances; antimony trichloride (Hardman and Sofowora, 1972) for
steroids; brilliant blue Coomassie R250 (Fisher, 1968) for
proteins; and concentrated sulfuric acid (Geissmann and
Grin, 1971) for sesquiterpene lactones.
R E S U LT S
Morphology of the secretory structures
The following structures were identi®ed:
(a) Translucent glands. Translucent glands, pale glands
or glandular pockets (Robson, 1981) are spherical or
oblong glands consisting of a sub-epidermal cavity delimited by two layers of cells. The internal layer consists of very
¯attened, thin-walled secretory cells. The external layer
consists of thicker-walled parenchymatous cells (Fig. 1A
and B).
(b)Three types of secretory canals. Type A: four
polygonal cells generally delimit the lumen of the canal,
which has a narrow diameter (Fig. 1C). These cells are very
thin-walled towards the canal lumen. In the pistil and root,
type A canals have a variable lumen diameter and/or a
variable number of secretory cells (Fig. 1C and D). Type B
canals have a wide lumen. In transverse section they have
the same structure as translucent glands; the B-type canals
look like elongated, pale glands (Fig. 1E and F). Type C
Ciccarelli et al.ÐTranslucent Glands and Secretory Canals of Hypericum perforatum L.
639
T A B L E 1. Localization of secretory structures in the ¯oral and vegetative parts of H. perforatum
Secretory structures
Sepals
Petals
Stamens
Pistil
Leaf
Stem
Root
Translucent glands
Type A canals
Type B canals
Type C canals

±
±
±
±
±
±

±
±
±

±
±

±
±
, Present; ±, absent.
canals consist of a wide cavity delimited by one or more
layers of densely stained and thin-walled secretory cells.
These structures are delimited by one or two layers of
¯attened parenchymatous cells (Fig. 1G). Type C canals
dier from type B canals in the pattern of dierentiation
during ontogenesis.
Localization of the secretory structures
The translucent glands or glandular pockets were present
within the lamina of the leaf, close to the lower surface.
They were distributed in spaces delimited by veins, but not
in association with them. In the sepals and petals the
translucent glands were oblong and distributed predominantly along the margins.
Type A canals were present in all ¯oral (with the
exception of stamens) and vegetative parts of the plant.
Generally, they were located at the level of the veins
(Fig. 1H). In the stem and root, they could also be found in
the cortical parenchyma (Fig. 1C).
Type B canals were numerous and alternated with the
veins in the sepals and petals. They looked like pale laminar
streaks. In the stem, these structures could be found in the
parenchyma below the epidermis.
Type C canals were located in the ovary and style. There
were some glandular streaks, called `vittae' by Robson
(1981), along the midrib of each carpel, and numerous
vesicles in a `herringbone' pattern where the carpels joined
each other (Fig. 3E). Although they were frequently visible
in the ¯ower, they tended to enlarge and become prominent
in the fruit. There were no anatomical dierences between
the vittae and the vesicles.
Localization of the translucent glands and secretory
canals is shown in Table 1.
Ontogenesis of the secretory structures
Glandular pockets and type B canals showed a typical
schizogenous development and had the same ontogenesis.
At the earliest stages of ontogenesis, a type B canal
appeared as a cluster of cells in contact with the lower
(abaxial) epidermis (Fig. 2A). Each of these cells was
usually larger than any of its neighbours and all had thin
walls. At the next stage, the lumen of the canal became
evident and undierentiated secretory cells (Fig. 2B)
surrounded it. As the structure grew, these cells proliferated
(Fig. 2C) and two layers of cells delimited the internal
cavity. The internal layer consisted of secretory cells with a
dense cytoplasm and large vacuoles, and the outer layer was
made up of ¯attened parenchymatous cells forming a
sheath (Fig. 2C). As the cavity expanded, the cells of the
internal layer became ¯attened and the cellular lumen was
occupied by a large vacuole that pushed the cytoplasm close
to the wall. These cells had thin walls, whereas the cells of
the sheath had thick walls (Fig. 2D). At maturity, the
secretory cells appeared empty, and the sheath was
composed of some modi®ed epidermal cells, which had a
narrower cellular lumen and a less cutinized external wall
than the neighbouring epidermal cells (Figs 1E and F). In
the sepal, the mature structure occupied the entire thickness
of the mesophyll and came into contact with the two
epidermes (abaxial and adaxial) (Fig. 2E).
At the earliest stages of ontogenesis, a type C canal
appeared as a cluster of dividing cells in contact with the
external epidermis of the pistil (Fig. 2F). Each of these cells
was usually smaller than any of its neighbours and all had
thin walls. As the pistil grew, these cells proliferated
(Fig. 2G) and the lumen of the canal became evident,
generally in an eccentric position, and surrounded by
undierentiated secretory cells. During cavity expansion,
there were many irregularly arranged secretory cells with a
dense cytoplasm, and a sheath was formed by one or two
layers of ¯attened parenchymatous cells (Fig. 2H). The
intermediate stages of development showed an irregularlyenlarging cavity containing rows of cells, some undergoing
lysis (Fig. 2I). At maturity, the canal consisted of a wide
lumen surrounded by one or more irregular layers of
secretory cells (Fig. 1G), which became very ¯at and
degenerate (Fig. 2L). Residual cells which had lysed during
the formation of the lumen were also visible. As in type B
canals, the sheath was composed of some modi®ed
epidermal cells. A fully dierentiated type C canal occupied
almost the entire thickness of the carpel, but most of the
secretory cells appeared empty and inactive.
As the early stages of ontogenesis of type A canals were
very dicult to observe because of the presence of so few
cells in these structures, only the mature structure is
described here (Fig. 1C).
Histochemistry
Results of the histochemical analyses are shown in
Table 2. The secretory canals and translucent glands
stained positively for the presence of alkaloids, lipids,
resins and essential oils (Table 2, Fig. 3A±I). However,
there were dierences in the composition of the secretion
depending on the organ involved. Unlike type B canals,
type A canals did not show essential oils (with the exception
640
Ciccarelli et al.ÐTranslucent Glands and Secretory Canals of Hypericum perforatum L.
Ciccarelli et al.ÐTranslucent Glands and Secretory Canals of Hypericum perforatum L.
641
T A B L E 2. Histochemistry of secretory structures in the ¯oral and vegetative parts of H. perforatum
Petals
Alkaloids
Pectic-like substances
Lipids
Resins
Essential oils
Proteins
Tannins
Sepals
Pistil
Leaf
Stem
Root
Type A
canals
Type B
canals*
Type A
canals
Type B
canals*
Type A
canals
Type C
canals
Type A
canals
Translucent
glands
Type A
canals
Type B
canals
Type A
canals

±
±
±

±
+

±
±
±

±
+

±
±

±
±
±

±
±

±
±
±

±
±
+
, Positive; +, weak positive; ±, negative.
* The results of histochemical tests for the translucent glands gave the same response as those for type B canals.
of type A canals in the pistil). The translucent glands of the
leaf stained negatively for resins, but positively for essential
oils. The test for tannins gave a positive response in the type
B canals of the sepals and petals (Fig. 3L), and in the type
A canals of the root and stem.
DISCUSSION
H. perforatum is characterized by the presence of dierent
types of internal secretory cavity: translucent glands and
three dierent types of secretory canals with dierent
shapes, ontogenesis and localization. The frequency and
diversity of these secretory structures is evidence of the
intense secretory activity of the species.
Distribution of the translucent glands and secretory
canals varied among dierent organs and they were not
always present at the same time. Spheroidal translucent
glands are typically found in the leaf, making this organ
look perforated. In the sepals and petals there are oblong
translucent glands, though fewer than in the leaf. Canals of
diering length (type B canals) are present in the stem,
sepals and petals. They dier from the translucent glands in
their length. Both glands and type B canals are located in
the parenchyma of dierent organs, far from the veins.
According to Curtis and Lersten (1990), translucent glands
and type B canals have a schizogenous origin.
The canals located in the ovary and subsequently in the
fruit (type C) are particularly signi®cant. Robson (1981)
showed that the ovary walls of Hypericum include
resin-containing canals or glands, which tend to enlarge
and become prominent in the fruit. These structures have a
characteristic distribution, which varies depending on the
species, and they have not been fully characterized from the
morphological and anatomical points of views (but note the
data on H. elodes L.; Bottega et al., 1999). The ontogenesis
of type C canals described here is similar to that described
by Curtis and Lersten (1990) for cavities in the leaves of
Hypericum balearicum L., and is a combination
of schizogenous and lysigenous development. In the
Asteraceae, where secretory reservoirs of dierent shapes
and lengths occur, dierent developmental pathways of the
lumen have also been reported in various species: schizogeny, as in the spheroidal cavities of Conyza canadiensis (L.)
Cronq. (Lersten and Curtis, 1987); and lysigeny, as in the
secretory cavities of Porophyllum lanceolatum DC. (Monteiro et al., 1995). In H. perforatum, there seem to be two
dierent and simultaneous patterns of development (schizogeny and schizolysigeny). Further studies, especially from
an ultrastructural point of view, could provide more
information on lumen development and the cytological
features of the secretory cells of these canals.
Type A canals have not previously been described for
H. perforatum, except for some dated and incomplete
studies (Green, 1884; Weill, 1903; Metcalfe and Chalk,
1950). They have been studied in H. elodes (Bottega et al.,
1999), where they showed the same localization as that
observed here in H. perforatum. Type A canals are
associated with veins, especially in the phloem, and are
also present in other families of angiosperms (Metcalfe and
Chalk, 1950).
Because of the presence of so many types of secretory
tissue, it appears that all of the epigeal and hypogeal organs
F I G . 2. Ontogenesis of the secretory canals of Hypericum perforatum: A, cross section of small ¯ower bud with unopened type B canal appearing
as a cluster of cells in contact with lower (abaxial) epidermis (toluidine blue, 900); B, cross section of ¯ower bud with the lumen of a type B canal
becoming evident (toluidine blue, 730); C, cross section of young petal showing a type B canal with enlarged central space and two layers of cells
which delimit the internal cavity: the inner layer consisting of secretory cells with a dense cytoplasm, and the outer layer made up of ¯attened
parenchyma cells forming a sheath (toluidine blue, 730); D, cross section of petal with an almost mature type B canal showing the ¯attened cells
of the internal layer with thin walls, and the thick-walled cells of the sheath (toluidine blue, 550); E, cross section of sepal, where a mature type B
canal occupies the entire thickness of the mesophyll and is in contact with the two epidermes (toluidine blue, 400); F, cross section of small
¯ower bud, where an early type C canal appears as a cluster of dividing cells in contact with the external epidermis of the pistil (toluidine blue,
680); G, cross section of ¯ower bud showing a young type C canal with a narrow lumen (arrow), in an eccentric position (toluidine blue, 500);
H, cross section of young pistil, showing an opened type C canal with many irregularly arranged secretory cells with dense cytoplasm, and a sheath
(toluidine blue, 500); I, cross section of pistil showing an intermediate stage of opening of a type C canal with an irregularly-enlarging cavity
containing rows of cells, some undergoing lysis (arrow) (toluidine blue, 500); L, cross section of mature pistil showing a mature type C canal
with irregular layers of secretory cells which are very ¯at and degenerate (arrows) (toluidine blue, 250).
642
Ciccarelli et al.ÐTranslucent Glands and Secretory Canals of Hypericum perforatum L.
Ciccarelli et al.ÐTranslucent Glands and Secretory Canals of Hypericum perforatum L.
of H. perforatum are involved in the production of
secondary metabolites. Histochemical tests revealed localization of alkaloids, lipids, resins, essential oils and tannins;
this is in accordance with recent chemical studies (Dionis'ev
and Chernomaz, 1938; Zolotnitskaya, 1954; Mathis and
Ourisson, 1964a, b, c, d; BerghoÈfer, 1987; Roth, 1990;
Brantner et al., 1994; Bombardelli and Morazzoni, 1995;
Nahrsted and Butterweck, 1997). These secondary metabolites are important in interactions of the plant with its
abiotic and biotic environment (Harborne, 1993). The
function of type A canals, which are associated with veins,
could be to transport photosynthates and protect the
phloem (Williams, 1954). Glandular pockets and type B
and C canals produce biologically active metabolites
(alkaloids, essential oils, tannins, etc.), which may protect
the plant against herbivores and parasites (Harborne,
1993). In particular, the type C canals of the ovary, which
enlarge in the fruit, could play a defensive role in seeds.
Other chemical studies have identi®ed additional secondary metabolites, such as naphthodianthrones, phloroglucinols and ¯avonoids, which are of interest from a
pharmacological point of view (Bombardelli and
Morazzoni, 1995; Nahrsted and Butterweck, 1997). The
naphthodianthrones hypericin and pseudohypericin are
characteristic of the black nodules and they are not present
in either glands or canals (Mathis and Ourisson, 1963;
Bombardelli and Morazzoni, 1995; Nahrsted and Butterweck, 1997). It has not been possible to locate phloroglucinols as there is currently no reliable histochemical test for
these compounds.
As a consequence of the early appearance of secretory
structures in the ontogeny of tissues of H. perforatum, their
distribution, and their numerous secondary metabolites, it
is likely that the translucent glands and secretory canals
play an active functional role from the ®rst primordium of
the various organs until the fruit matures.
AC K N OW L E D GE M E N T S
This research was carried out with ®nancial support from
M.U.R.S.T. Italy. We thank Mr Antonio Masini for his
helpful technical advice.
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