Table Of ContentSECRETORY STRUCTURES
OF AROMATIC AND
MEDICINAL PLANTS
A
REVIEW AND
AnAS Of MICROGRAPHS
Authors: Katerina P. Svoboda
and Tomas G. Svoboda
Micrographs: Andrew D. Syred,
Microscopix photolibrary
Designed and Edited: Polly M.Syred
1\(''''to imu!:t',\ onfront cover:
Topto bottom: Wormwood.
Peppermint. Marjoram.Rosemary.
Greek Ore-gano. Lavender
Background: Eucalyptus leaf
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PREFACE
Aromatic and medicinal plants arc used in the food. flavour, perfumery. pharmaceutical and
healthcarc industries.Their usc is expected to increase every year due to a consumer-led trend
towards natural foods. food and cosmetic additives and complimentary medicine including
aromathcrapy. Many branches ofindustry are searching for alternative. cleaner antimicrobial,
antibiotic. antioxidant and crop-protection agents. The possibility of utilising volatile oils is
being investigated as it has been known since antiquity that they possess biological activities.
During the last decade many books have been written about herbs and spices. chemical
constituents foundinvariousplant familiesandtheiruses.
However,very few research articles deal with thedescription ofthe sites where these chemicals
are produced. either on the surface of the plants or internally. There is a distinct lack of
illustrations which would help the reader to visualise these anatomical structures. An early
acquaintancewithmicrographsofsecretorytissuesincreasestheappreciationoftherelationship
betweentheplantanditsbiochemicalproducts.
This book is a source of useful tenus and concepts for description of a variety of secretory
structures. It does not cover the subject exhaustively but presents numerous examples and
examines a fewindetail. The reader ismade aware oftheendlessvariability inanatomicalfonn
and will therefore be better prepared to interpret the structure ofother plant species and relate
them to those described inthisbook.An exhaustive list ofreferences collected over the last ten
years gives the reader guidance for scientific approach to this area. Use ofincreasingly more
advanced methods ofmicroscopicstudieshasresulted inremarkableinsight intoplant anatomy.
differentiationanddevelopment.
All photomicrographsareoriginal.prepared byMicroscopixfrom authenticplantsand material.
My botanical and phytochemical interest. directed towards research on aromatic plants.
unavoidably influenced the presentation of individual plant species. The problems of
classificationarenotresolved and theorderofplantpresentation isnotrelated totaxonomy.The
plantsarc presentedinalphabeticalsequenceoftheircommonnames,supporting interpretations
andconclusionsthathavebeenplacedinthetext.Thismethodoforganisationissimple,coherent
andquicklypermitsthereaderaccesstospecificplates.
This book aims to be useful to both students and scientists with an interest in aromatic and
medicinal plants and may be a valuable resource to all those involved in the research,
development.cultivation.marketingandusesofthesecrops.
Dr.K. P.Svoboda
Mev 2000
'Apricotsare ripe
With moonlight dripping
On each fruit'
Haiku byShibata Hakuyojo
3
PLATE I
Mobile distillation unitbeingUM-'J forchamomileoilextraction(Norfolk.Anglian Essential Oils Ltd.]
CONTENTS
ra~e no:
Preface 3
l ist ofplates _ 6
Introduction _ 7
Secretory structures in plants 9
Basil.. 15
Bog myrtle 16
Caraway _ 17
Catmint 18
Cedar 19
Chamomile. Roman 20
Citrus 21
Clove 22
Cumin 23
Dragonhead 24
Eucalyptus 26
Frankincense 27
Ginger 30
lIyssop 31
Juniper 32
Korean mint 33
l avender 34
l emon balm 35
l ovage 36
Marjoram 37
Myrrh 28
Nutmeg 38
Orchid. lady's slipper 39
Oregano 40
Oregano. Greek 41
Peppermint 42
Perilla .43
Plantain 44
Rosemary 46
Sage .47
Tansy .48
Wormwood 50
Bibliography 51
List of plant species used in arumatherapy 57
Glossary 58
Index ofspecies 60
,
LIST OF PLATES
Plate no: P..~e no:
Mobile distillation unit being used for chamomileoil extraction
(Norfolk. Anglian EssentialOils ltd.) .....
2 Collection ofvarious mint species.experimentalplotsat Auchincruivc X
3 Ecninacae angustitotia, experimentalplots at Auchincruivc 13
4 Takeda.Japanese pharmaceuticalcompany herb garden.Osaka 14
5&6 Basil 15
7& x Bogmyrtle 16
9& 10 Caraway 17
II & 12 Carmiru 18
13. 14 & 15 Cedar 19
16.17.IX&19 Chamomile 20
20.21& 22 Citrus 21
23,24.25 & 26 Clove 22
27, 2R. 29 & 30 Cumin 23
31& 32 Dragonhead 2...
33 Detailofdragonheadtlowc showingdistributionofsecretoryglands 25
34 & 35 Eucalyptus 26
36.37.3S. 39 & 40 Frankincense and Myrrh 27 28
4
41 A young Somalian womangrading (1-6) freshly-collected resin for traders 29
...2 3 & ...... Ginger 30
...5 6 & 47 Hyssop 31
48.49 & 50 Juniper 32
51 & 52 Korean mint 33
53 & 54 lavender 34
55.56 & 57 Lemonbalm .35
58.59 & 60 Lovage 36
61 &62 Marjoram 37
63 &64 Nutmeg 3X
65.66&67 Orchid. lady's slipper 39
68 & 69 Oregano .40
70,7[ & 72 Oregano.Greek .4I
73 & 74 Peppermint .42
75 & 76 Perilla .43
77. 7X. 79 & so Plantain 44
81 AgastacheJi.)('"iClllllm at Auchincruive herb garden 5
82 & SJ Rosemary 6
84.85 & 86 Sage 47
ss
87 & Tansy .41\
89 Experimental plot oftansy grown for volatile oil with high thujonc content
(Auchincruivc) .49
90.91 & 92 \Von11\\'oOO 50
93 Street market with herbsand spices.Seoul. Korea 56
94 Statue ofa streetseller with various herbs.essential oils and perfumes
(PerfumeryMuseum.Grasse.South ofFrance} 61
INTRODUCTION
Plants and theirextracts have been used for centuries to relieve pain. aid healing and kill
bacteriaand insects.They have alsobeenemployed inperfumery. cosmeticsand religious
rites. Plant chemistry became an established discipline in universities at the end of the
nineteenthcenturyandsincethenmany newstructureshavebeendiscovered.Thenumber
ofnaturalproductsobtained fromplantshasnow reachedover 100000and everyyear new
chemical compounds arc being discovered. Although their functions in plants have not
been fullyestablished. itisknown that somesubstanceshavegrowth regulatoryproperties
whilst others are involved in pollination and seed dispersal. Many arc important as
antifungal and antihcrbivorc agents with further compounds being involved in the
symbiotic associations in plants. Detailed information on individual plant chemicals is
availableintheDictionary ofNaturalProductsand thePhytochemicalDictionary:
The complex nature of these chemicals, which arc usually produced in various types of
secretorystructures.isalso influencedandcontrolledbygeneticandecologicalfactorsand
significantly,bythemode ofextractionfromtheplants.Thetypeofsecretorystructureisan
important characteristic of a plant family. Detailed anatomical description of these
structures isrelevantto the market value ofthe plants. the verificationofauthenticity ofa
givenspeciesandforthedetectionofsubstitutionoradulteration. Italsoprovidesaguideto
the method ofprocessing.
Microscopical investigation of plant structures is an important part of the complex
biological research process which includes plant growth and development. genetics and
breeding. There is endless variability in fonn and structure; observing these with the
microscopeallows ustodiscoverNatureinoneofitsmostpowerful forms.
Plantmaterial received forthese investigationswas examined freshoraschemically-fixed
and stained sections for light microscopy (LM), chemically-fixed and critical point dried
(CPD) forscanning electron microscopy(SEM)or cryogenically fixed for cryo-SEM.No
single technique was ideal for all purposes since each had its ovvn limitation and risk of
introducing artifacts but by employing several methodsthese difficultieswere reduced to
giveaclearerunderstandingofthe structures. ForLM.thintransversesections(TIS)were
cut from wax-embedded material and stained to provide colour contrast. however, the
involved chemical processes invariably caused some damage to the sample. This was
alleviated by examining fresh. unstained sections either by differential interference
contrast(Ole)or,ifsufficientcontrastwasattainable.bright field illumination(BF)where
no chemical or staining processes were employed. SEM allowed high magnifications
togetherwithimproveddepthoffieldgivinganoverallview ofthestructures. SinceSEMis
incapable of recording colour. all images produced using this method were manually
enhancedusingcomputertechniques.
7
PLATE 2
Collectionof'variousmm• t....pecr•es.1>.:\P'-rimcntal plotsal Auchincruive
,
SECRETORY STRUCTURES II\' PLANTS
Plant chemicalscan beclassedasprimaryorsecondarymetabolites.dependingonwhetherornot
they have an essential role in plantmetabolism andarc universally present in all plants.Primary
metabolites include sugars. amino acids. nucleic acids and the chlorophylis. Secondary
metabolitesmakeupall the remaining plantchemicalsfromalkaloids tophenols.
Essential oilsand other secondary plant metabolites arc found in a wide range ofplant species
including annual, biennial or perennial herbaceous plants. evergreen or deciduous shrubs and
trees. The ecologicaland evolutionary role ofthese secondary metabolites has been associated
with defence against animals. healing ofplant organ wounds. protection from harmful insects.
resistance to microbial attacks and attraction of insects and animals for pollination. Several
species and varieties of plants. mostly those of commercial interest. were investigated
systematically and in depth. Recently. various studies concerned with secretory structures and
factorsinfluencing theirdevelopmenthavebeenundertakenbyresearchgroups inbiologicaland
pharmacologicaldepartments.
Secretion is a common feature of living cells and involves the discharge ofsubstances to the
exterior(exotropic secretion) or into special intercellular cavities (endotropic secretion).These
arc specialised cells and the secreted material may contain various salts. latex. waxes. fats.
flavonoids, sugars. gums. mucilages. essential oils and resins. It has been assumed that these
productsarc biosynthesisedinsituanddirectevidence forthebiosynthcticcapacityofgland cells
has becomeavailablerelatively recently withthe developmentofproceduresfor gland isolation.
These methods have yielded definitive proofof the presence of enzymes specifically within
glandcells.
Trichomes present on the leaf surface and other secretory tissues can beexamined using light.
scanning and transmission electron microscopy which enables detailed observation of major
stages in the development of secretory cells. including their membrane system and nuclei. the
overallsizeoftheglandandtheamountofmatcrialreleasedintothesubcuticularcavity.
Essential oils. with or without accompanying resins and gums. are most commonly found in
specialsecretory structures eitheron the surface oftheplantor within the planttissues.Thetype
ofstructureisfamilyorspecies specific.Thiscanbeusefulin identification ofplant materialand
verifying theauthenticityoftheplantsourceinthecaseofsuspectedaduIteration.
St:CH.ETOR,\·CI::U.S
Themost simplesecretorystructureisasinglesecretion-containingcellwhereitisonlytheactual
content that distinguishes it from adjacent non-secretory cells. However, it may also be larger
than the othercellsor havea thick cuticularized lining.This celltype is found in many different
plant tissues: in the leaf parenchyma of lemongrass tAndropogon spp.). bay (LaUrElS nohilis).
citronella (Cymbopogon nardus and C. winterianusv and patchouli(Pogostemonpatchouli). in
the seed coat of cardamom (Elellaria cardamonunn), in the rhizome of ginger (Ziflgiber
9
oj]icinale) [Plates 42-44] and turmeric (Curcuma longa), in the fruit wall of pepper (Piper
nignlm), capsicum and chillies (Capsicum annllm), in the pcrisperm and embryo of nutmeg
(Myristicafragrans) [Plates 63 & 64], inthe barkofcassia (Cassia angustifolia} andcinnamon
(Cinnamomumzeylanicum}andintherootofvalenan(Valeriana officinaliss,
OS.\IOPIIOR[S
Osmophores are areas of flower tissues with secretory cells differing structurally from the
adjacentcells(e.g.isodiametriecells inorchids).
SIT RI-:TORYC\VITIES
These cavities are more or less spherical structures that can be formed in two ways: the
parenchyma cells can separate one from another leaving intercellular spaces called lumina or
lacuna,or anactual cell candisintegrate leavingacavity withinthe tissue.These spaces are lined
with secretory cells or an epithelium that produces the essential oils. In high oil yielding plants
several layers of these secretory cells are fonned. The cavities continually enlarge and some
become filled withcellswiththin,convoluted wallswhichalsostoretheoilproduced from within
their plastids. Included in this group are fruits and leaves of plants in the Citrus family (c.
ourantifolia. C. aurantium. C. bergamia, C. sinensis. C. limom [Plates 20-22] as well as
Eucalyptus spp. [Plates 34 & 35] and buchu leaves. Citrus peel oils arc confined in oblate to
spherical-shapedoilcavities(glands),sometimescalledoilssacs,thatarclocatedirregularlyinthe
exocarpofthe fruit[Plate22].Thesecavitieshavenowallsandareembeddedatdifferentdepths in
the flavedo (the coloured outer portion or skin ofthe fruit).The glandsofgrapefruit liedeeply in
the flavedo and those ofmandarin are likely to be nearer the surface. Fruits and leaves of these
plants are covered by a thick cuticle which is waterproofand also the primary means ofwater
conservation. Being shiny and reflective it is capable of deflecting some of the excess solar
radiationintropicalandsubtropicalregions;italso reflectsultravioletlight.therebyprotectingthe
DJ Afromthemutageniceffectsofsunlight. Itisanexcellentprotectionagainstfungi andbacteria
since they have no enzymes capableofdigesting cutin.Secretory cavities arc also present inthe
flower budsofcloves(Syzygiumaromaticum) [Plates 23-26],thefruitwallsofpimcnto(Pimento
dioica), andintheelongatedcavities inthebarkofmyrrh(Commiplwramolmof),benzoin(Styrax
benzoimandfrankincense(Boswelttaspp.)[Plates36& 37].
SECRETORYDlJ(.. S
Ductsareelongatedcavities.Theycanoftenbranchtocreateanetworkextendingfromthe roots
through the stem to the leaves, flowers and fruits. They are composed ofan epithelium which
surrounds a central cavity. Several predisposed cells within the parenchyma undergo
asynchronousdivisionand indoingso theyexpand the initial space inthemiddle wherethecells
arcalladjacenttoformacavity.Someofthesecellsformingthewallofthecavitywillchangeinto
secretory epithelialcells.The oils are biosynthesised within their leucoplasts and move via the
endoplasmicreticulumintothecavity.Thesecavitiesthen becomejoinedtoformducts.Theycan
be found in all of the Umbelliferae family including anise tPimpinella anisum}; fennel
tFoeniculum vulgares. dill tAnethum graveolenss. coriander (Coriandrum sativunu, cumin
10
