UNIT 5 HERBARIUM TECHNIQUES

Structure
5.1 Introduction
Objectives
5.2 Collection Process
5.3 Pressing of Plants
5.4 Keeping Wet Material
5.5 Drying of Specimen
5.6 Mounting of Specimen
5.7 Labeling of Specimen
5.8 Storing of Herbarium Sheets
5.9 HerbariumEthics
5.10 Summary
5.1 1 Terminal Questions
5.12 Answers

5.1 INTRODUCTION
Herbarium is a collection of dry, pressed and preserved specimens of plants
arranged systematically for the purpose of reference and identification. The
herbarium may also be regarded as a museum with a wide representation of
plant species from various geographical regions.

The modern herbarium is more or less a great filing system and can furnish
data in any field of plant research. In this unit you will study about various
techniques involved in the making of a herbarium because as a lab technician
you will have to collect, preserve and store plants.

Objectives
After studying this unit you will be able to learn about:
various equipment, which are required for plant collection,
pressing material to be used,
plant parts which are to be pressed,
keeping of wet plant material,
drying, mounting and labelling of plant specimens,
storing of the herbarium sheets.

We will describe in detail all the techniques used in preservation of plants for
their taxonomic studies.

5.2 COLLECTION PROCESS

As a plant collector you will need the following equipment:
i) a gardener's knife
ii) a plant press or vasculum
iii) blotting papers to dry plants
iv) trowel to dig and uproot the plants
v) collecting and mounting sheets
Introduction to vi) gum tape, labels, waterproof ink and pen
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vii) other equipment used in collection are collecting pick, secateur,
vasculum, hatchet, spade, etc.

Before coming to actual collection you must know about vasculum and plant .
press.

Vasculum: Vasculum is made up of a metal cylinder with a sliding door

usually worn on a strap over the collector's shoulder into which plant specimen
are placed (Fig. 5.1). Polythene bags and paper bags are also used for putting
fruits, seeds and small specimens after collection.

Plant Press: Plant press is an indispensable tool for pressing fresh plant
specimens to subsequently dry them and mount permanently. The plant press is
of two types as shown in figure 5.1.

The collection of plants can be done by thiee methods. The most common
method is to press the individual plant as soon as you see a particular plant.
This you can do by pressing in the field. You can press the plant in blotting
paper or newspaper in a field press.

At times specimens can be kept in vasculum. But before keeping plants in

vasculum you must line the vasculum with wet blotting paper or newsprint
to avoid wilting of the specimen collected. You should not overcrowd the
vasculum, it is always safer to fill the vasculum partly. You can keep plant
specimens in good condition for about twelve hours by keeping the
vasculum in a cool place. The specimens should be pressed immediately
after this period.

The second method is to collect plants in polythene bags or rucksack (a bag

strapped to shoulders) and pressed as soon as there is an opportunity for it. You
can adopt this method for the plants of tropical rain forests and chiefly in rainy
season when there is too much humidity.

Things to remember:

i) You should always collect specimen which is in flowering and fruiting

stage.
ii) Herbs are collected complete with their underground parts.
iii) At least five specimens of plants, preferably in different stages of
flowering and fruiting should be collected.
iv) You must visit the locality several times during the year.

You should select the blotting paper that absorbs a given volume ofwater in
the shortest period. You can also use newspapers. The standard size of a
folded blotter should be 30x45 cm. The blotting paper should be changed
after every 24,48 and 72 hours. You can reuse it after drying it. In case of
fleshy plant parts and during rainy season, remove blotters at short intervals.
 Herbarium Techniques

Hack saw

Vasculum

c.J Hatchet
Collecting pick

Knives

Plant press of
wooden planks

Knives

Perforated plant press

~lurnin'tumplant label A corrugate or ventilator

Fig. 5.1: Various equipment used in collection of plants.

Introduction to SAQ 1
Biology Laboratories
Write T for true and F for false for the follow~ngstatements.
I) Newspapers can be used in place of blotting papers.

2) You can keep as many plants as you like in the vasculurn.

3) Polythene bags or rucksack are used for plant collection in a
tropical rain forest or in rainy season. .
4) The standard size of a folded blotter should be 30 x 45 cm.

5.3 PRESSING OF PLANTS

Arranging plant specimens with pressing paper is most importtint step in the
preparation of herbarium and needs careful attention. Before pressing the plant
specimen the following points should be taken care of:
1. Underground parts like root, rhizome, etc., must be included in herbs.
2. Make it sure that specimens are in flowering or fruiting stage. Sterile
spechens are generally useless.
3. Ensure that specimens are free from insect infestation, fungal infection and
any pathological symptoms.
4. Fresh specimens are found all-over land, thus their parts can easily be
arranged within the pressing paper. The art of plant pressing comes from
experience. However, some suggestions are:
Pressing is most effective when the maximum surface of pressing
paper is covered with the plant specimen to be pressed.
As a general rule a single specimen should be pressed in a folded
pressing paper. In case of amlarge-foliaged specimen, it may be cut
into two or more pieces and each piece is arranged and pressed into
two or more folded pressing papers.
In case of small herbaceous plants, many specimens may be pressed
in one pressing paper.
Herbaceous specimens, longer than 40 cm. may be folded in toto into
a V-shape, N-shape or M-shape and then pressed. The points of folds
should be moderately macerated before bending brittle stems to avoid
breaking them.
Pruning of specimens is essential to avoid overlapping of plant parts.
Care should be taken to lcave the basal portion of pruned parts
(pedicel, petiole, etc) attached to the plant to indicate their natural
position.
A few leaves should be arranged with their lower surfaces facing
upward.
When the leaf is large and pinnate compound, it is necessary to excise
all except one leaf. If the remaining leaf is still too large, leaflets of
one side may be removed leaving the terminal leaflet, if present. Even
one complete leaf may be split lengthwise and half of a leaf pressed.
A large palmately compound leaf is usually split in half (lengthwise)
and one half is discarded while pressing.
Exceptionally tall herbs are pressed in three parts i.e., one from the
base, another from the middle and a third fiom the top indicating their
position.
 10) At times it is not possible to accommodate more than a leaf and an Herbarium Techniques
inflorescence within a herbarium sheet. In such cases, the arrangement
of leaves is noted down. Entire petiole and the portion of stem, from
which it is produced, may be included.
11) A few gamopetalous flowers should be pressed separately from the
specimens. Some of the flowers should be split open and then pressed
to expose their essential organs and the nature of the thalaillus.
12) Roots and underground parts should be washed thoroughly before
pressing to remove soil particles.
13) Fleshy and bulky organs should be sliced open to facilitate quick
drying. Paper padding or cotton batting around such organs is helpful
for drying.
14) To avoid decoloration and moulding, blotters should be changed
frequently depending upon the nature of the specimen and humidity.

Members of Iridaceae, Commelinaceae. etc. produce flowers with deliquescent

corolla or perianth (quiclcly becoming semiliquid). Such corolla or parianth
often stick to pressing paper or become so thin that they cannot be removed
easily. When such a specimen is pressed, a layer of absorbent cleaning tissue is
put over and beneath the flower before pressing. This tissue is peeled off after
the specimen becomes dry.

SAQ 2
Suppose you collect a plant 60 cm long. Show sviih the help of diagrams how
you will press this plant.

5.4 KEEPING WET MATERIAL

You may find it difficult to press the fleshy and non-fleshy plants during the
rainy season. Two techniques have been fouild useful in such cases.

Schultes (1947) has favoured the following technique. Specimens are pressed
between blotters for about 24 hours. Later these are taken out of the plant press
and dipped in a solution containing 2 parts of 40% formaldehyde and 3 parts of
water. After a few seconds of dipping, specimens are kept again in the pressing
papers. All the sheets containing specimens are piled one above the other. No
blotters and corrugates are put in between them. After applying pressure for a
few hours, the entire bundle is sealed inside cellophane, or a cloth bag with
several layers of thick rubber. Later, specimens are removed and dried at
convenience. 71
Introduction to The plant specimens may be kept in a wet condition for a month or so. If the
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drying process is delayed somehow, they may be taken out after a month and
dipped again in commercial formaldehyde-water solution. Rubber gloves
should be used when working with this solution to prevent cracking of skin on
hands. The specimens made by this method are of inferior quality as they lose
their natural colour and beauty.

Fosberg (1947) had proposed an improvement over this method. Schultes'

solution was replaced with one part of formaldehyde and two parts of 70%
alcohol. The solution was applied to specimens with a 5 cm flat brush instead
of dipping. The solution penetrated inside the specimens and there was found
to be a better control from decomposition.

Hodge (1947) observed that substitution of 40%-50% alcohol as preservative

was more satisfactory. Moore (1950) advocated the use of hydroxyquinoline
sulphate as a preservative.

It has been found that 15g mercuric chloride (HgCl*) mixed with one litre of
70% alcohol serves as a good preservative. The specimens are dipped in this
solution for a few seconds and then placed in pressing papers. The only
disadvantage in this method is that the solution leaves a blacltish mark on
pressing paper, which, of course, cannot be avoided. Specimens treated with
this solution are neither spoiled nor decomposed. Even these are well protected
from insects.

5.5 DRYING OF SPECIMEN

Drying of plant specimerzs witlzout artificial heat
It is almost followed universally and supposed to be most satisfactory. Plants
are pressed in pressing papers between blotters. Corrugates are not used in this
method. Plant press is locked for 24 hours. This is called the 'sweating period'.
After the sweating period is over, the press is unlocked, blotters are removed
and pressing papers are turned back. Now the specimells are rearranged as the
situation demands. At this time most of the plant parts become flaccid. They
lose their natural spring and turgidity and now it becomes easier to arrange the
branches. leaves, flower buds, flowers and their parts in the desired position.
Much attention is to be paid at this stage. A careful arrangement of plant parts
will result in nicely arranged specimens after drying when plant parts become
stiff and are fixed in a desired manner. After rearranging, folded pressing paper
is kept in between two fresh blotters. ,411 the specimens are provided with fresh
blotters in this manner and are locked again in the plant press for the next 24 or
36 hours. A third change foliows and specimens are locked up tightly for
another 48 or 72 hours. Most of the specimens dry in a week's time. Fleshy
and succulent illatcrials m?y require a much longer period to dry completely.

There are certain disadvdntages in this method of drying which are mentioned
here:
1. A much longer time i s needed for drying the specimens. It may be a week's
time or sometimes even more than that when plants are fleshy and
succulent.
2. Blotters are to be changed at least 3 or 4 times during the entire process and
wet ones are to be dried in the sun before using them again.
3. A large number of blotters are needed in comparison to drying the Herbarium Techniques
specimens with artificial heat.
4. Few specimens can be processed per collector as compared to drying with
artificial heat.
5. Specimens dried without artificial heat are liable to be infected by fungi
because much time is taken up in complete drying.
6. Larvae present within the flowers eat away some of the important parts.
7. The labour cost of drying per specimen is exceedingly high in comparison
to drying with artificial heat.

5.6 MOUNTING OF SPECIMEN

Dried specimens are mounted on herbarium sheets of standard size 29 x 42 cm.
The herbarium sheet must be of long lasting and durable paper and heavy
enough to support the specimen. A hundred percent rag paper has been found to
be the best but it is quite expensive. A paper of lower rag content is mostly used
since it is cheaper as well as easily available. It is not advisable to use paper of
much lower rag content because while lifting, it bends with the weight of the
mounted specimen and the specimen is liable to be broken (as it is brittle after
drying). Thick drawing sheets may also be used as herbarium sheets. Curators of
most herbaria use hundred percent rag paper for mounting the specimens.

There are several ways of mounting. Generally a paste or glue is used to fix the
specimen to mounting sheet. It is applied to the lower surface of a specimen
with a brush and the specimen is properly placed on a mounting sheet.
Sometimes the glue or paste is spread in a thin layer over a glass plate of size
36 x 5 1 cm. The specimen is lifted with the help of a forceps from pressing
paper and is placed over the glued surface of glass plate with the lower flat
surface in contact of the glue. It is then lifted and placed over the mounting
sheet. The pressing paper (of unprinted newsprint) is placed over it, gently
pressed, taken off and is rejected. It removes the excess of glue or paste which
comes outside the lower glued surface of the specimen. The glue is to be
reapplied on the glass plate after two or three specimens have been brought
into contact with glued-glass surface. It is better to scrap off the glue from the
glass plate before reapplying it as broken parts of dried specimen sometimes
get stuck to the paste. After the specimen has been mounted, white gummed
cloth strips (holland cloth) are also used to fix the specimen on the mounting
sheet. While using these strips, care should be taken that strips do not hide
important parts of the specimen.

SAQ 3
Write T for true and F for false for the following statements in the given
brackets.
1. It has been found that 100% HgC12 is a good preservative against
insects.
2. The locking of a plant in plant press for 24 hours is known as
sweating period.
3. Specimens dried without artificial heat are liable to be infected
by fungi because much time is taken in complete drying.
4. Larvae present within the flowers eat away some of the important
parts.
lntroduction to SAQ 4
Biology Laboratories
Fill in the blanks with appropriate figures or word(s).
1. The standard size of herbarium sheets on which dried specimens are
mounted is .........................
2. Curators of most of herbaria use ...................per cent rag paper for
mounting the specimen.
3. Specimens are mounted on sheet with the help of .................. or

4. One should not use paper of much ................rag content because while
lifting it will bend and specimen will be broken.
5. The ...................on the sheet provides some important data of about the
specimen collected.
6. The entries in labels must be in legible handwriting using ............
...............black ink.

5.7 LABELLING OF SPECIMEN

After mounting the specimen, herbarium sheet is given a final touch by pasting
a label at its lower right hand corner. The label is of 11 x 7 cm size but its size
may vary by 1.2 cm on either side. Some herbaria prefer to get the label printed
on sheets. The label provides some important data of specimen which cannot
be gathered by simply studying the herbarium material. A label must provide
the following information:

Flora o f . ...........................................No ....................................

Family ....................................................................................
Botanical name. ..............................................................................
Local name ................................................................................
Habit .......................................................................................
Flower colour ..........................................................................:...
Habitat.. .........................Locality. ................ Altitude.. ....................
Ecological notes ..........................................................................

..........................................................................
Date of collection ........................................................................
Collector ..................................................................................

It is advisable to type the data on the label. Entries on the label may be made in
legible handwriting using waterproof black ink.

5.8 STORING OF HERBARIUM SHEETS

After proper drying, the herbarium sheets are ready in all respects to be stored.
You can use wooden or steel almirahs which are generally dust proof. Now the
specimens are arranged according to the Bentharn & Hooker or Engler and
Prantl classification.

Each species is kept in a folder which is lighter in weight and smaller in

dimension than the genus cover. Many species (kept in separate species cover)
of a genus are kept together in a genus cover. The genus cover usually has the
name of genus written or printed at the lower left hand corner. When the cover
is restricted to specimens of a single species, the genus initial and the name of
 species are written at the lower right hand corner. It is customary to arrange the Herbarium Techniques
genera and species in alphabetical manner under each family. In large herbaria
the specimens are also segregated by the name of the continent to which these
are indigellous.

Preservation of Specimens
In all the herbaria great attention is paid towards the preservation of herbarium
specimens because there is every likelihood of their being attacked by several
insects. Some of the most common insects which damage the specimens are
herbarium beetle, drugstore beetle and booklouse. All these insects complete
their life cycle within the dried specimens, eating away the plant parts,
herbarium sheets and badly damaging several specimens kept one above the
other. The herbarium beetle is the worst of all and completes its life history
within two or three months. To overcome insect-damage combined use of Repellents are
repellents and insecticides has proved to be most effective. chemicals which
prevent attack of
specimens by insects.
Two chief repellents are naphtha flakes (naphthalene compounds) and These chemicals
paradichlorobenzene (PDB). Merril(1948) has reported that two parts of simply drive them
naphthalene compound mixed with one part of paradichlorobenzene brings the back, when trying to
most effective result. Two to three ounces of this compound may be kept in the advance towards
herbarium specimens
pigeonhole of a herbarium almirah. The bags containing this mixture are but are neither lethal
refilled once a year. nor toxic to them.

Insecticides are fatal to the insects. These kill the insects either by coming into
contact or by being digested by them. Some of these are as follows:
1. Cyanide gas
2. Paradichlorobenzene (PDB)
3. Carbon disulphide gas
4. A mixture of 3 parts ethylene dichloride and 1 part carbon tetrachloride.
5. Dichloro diphenyl trichloroethane (DDT)
6. Mercuric chloride.

1. Cyanide gas is extremely poisonous and is a more effective ibsecticide.

The specimens are kept inside an air-tight chamber or room and are
fumigated with this gas. Cyanide gas is exclusively used at Kew (Ballard,
1938).

I 2. Paradichlorobenzene crystals vaporise at 43°C to 60°C. The specimens are

kept in a air-tight chamber for 3 to 4 days and paradichlorobenzene is
allowed to vaporise in that chamber killing all sorts of insects injurious to

II herbarium specimens. It is more commonly used as a repellent than as an

insecticide in most of the herbaria.
3. Carbon disulphide is a highly inflammable and volatile liquid. It is
converted into gas at 10°C to 15.5OC. This liquid should be kept at a
distance from the flame as it easily catches fire. Specimens are fumigated
with this gas in air-tight chambers taking all precautions that it does not
catch fire. Two to three'days of fumigation is sufficient to kill all the
insects and their eggs which are harmful to specimens.
4. A mixture of 3 parts (by volume) ethylene dichloride and 1 part of carbon
tetrachloride has been found to be a most effective insecticide. The mixture
is volatile at room temperature and does not catch fire. The specimens are
fumigated in the air-tight chamber.
/Ittoduction to 5. Dichloro diphenyl trichloroethane (DDT) is available in powder form.
lology Laboratories
Howard (1947) has suggested dusting of dried specimens after taking them
out from the plant press. It is used either in 100% or 25% to 50% strength
and has been found effective in killing herbarium insects. It is not a
permanent insecticide. Specimens need frequent dusting by DDT after an
interval of I or 2 years.
6. Mercuric chloride is a very strong poison for all sorts of herbarium insects.
A saturated solution is prepared in 95% of alcohol at room temperature.
One part of this stock solution is mixed with a part of alcohol and dried
specimens are dipped in it. After dipping, specimens are kept in blotters for
24 hours and then mounted on herbarium sheets. Some botanists advocate
the use of 2% solution of mercuric chloride in a mixture prepared by
adding 1 part of petroleum ether to 2 parts of 95% alcohol. Specimens are
dipped in it and dried for mounting.

While dipping specimens, penetration of liquid in fleshy and thick parts of

plants is not appreciable. The application of this liquid remains more or less
superficial and thus the insects and larvae, which have already entered deep in
the plant tissue, are not killed. Being a stomach poison this does not kill them
until they have already eaten some parts of the plants along with this poison. It
is not a permanent poison and loses its potency after a few years. The
application of mercuric chloride is not prescribed for already mounted
specimens because it dissolves some of the pigments of plant material and
leaves a black residue on herbarium sheets.

An other way of killing insects has been suggested by O'Neill(1938). It has

been found that a temperature of 60°C to 77OC is enough to destroy all the
insects, their eggs and larvae in 4 to 5 hours. This much of heating neither
burns nor makes the specimens more brittle. Steel herbarium almirahs are
fitted with thermostaticaliy controlled electric heating units which keep a
constant temperature of 60°C to 77OC within the compartments.

5.9 HERBARIUM ETHICS

Though ethics is not central to this topic we would like to mention a few points
in this regard:
Every herbarium should provide information on working days and hours, filing
arrangements, loan procedures, availability of microscopes, use of library
collections, etc. A visitor should be aware of these regulations beforehand.

Even otherwise, one should always practice the following rules while working
in a herbarium:
1. Specimens are fragile; the sheets on which they are mounted should be
kept flat.
2. Folders and sheets should be lifted one at a time.
3. While carrying the specimens from the cabinet to the working table, a
supporting sheet should be kept below.
4. Heavy books or elbows should not be placed on the sheets.
5. Any broken part should be kept in a small packet in the same folder.
6. Materials for dissection should be taken only sparingly and with
permission.
7. Damaged specimens should be kept aside for repair.
 Herbarium Technic
5.10 SUMMARY
In Illis unit you have studied that:
A plant collector needs equipment such as a gardener's knife, plant press or
vasculum, blotting papers or newspapers: trowel, mounting sheets. gum or
glue tapes, labels, waterproof ink and pen, collecting pick, secateur,
hatchet spade etc.
Blotting papers are a very important part of plant drying. They should be
changed every 24,48 and 72 hours.
Before pressing the plants some important points should be taken care of
during selection of plant specimens.
The drying of specimens is of utmost importance and is done without
artificial heat for natural and better drying.
Labelling is a very important task for the collector because it provides a lot
of information about the plant specimens.
Storing is one of the most important tasks to be performed because without
proper storing all the hard labor involved in collecting, pressing, drying,
mounting, Iabzlling is lost forever.
Herbarium ethics should be followed for the preservation and conservation
of the specimens.

1. Write down important things one must remember while collecting plants
for making a herbarium.

2 , Wr!.,. ally ten points that you think should be kept in mind before pressing
t l : ~~~~lsnts,

3. Describe the Schultes method (1947) of keeping wet material.

.........................................................................................

4. Describe the disadvantages of drying plant material without artificial heat.

.........................................................................................
Biology Laboratories

5. Take a plant of your choice and fill in the information given below.
.........................................................................................

Flora o f . . ....................................No ....................................

Family.. ................................................................................
Botanical name.. .....................................................................
Local name ...........................................................................
Habit .................................................................................
Flower colour ........................................................................
Habitat. ....................Locality. .................Altitude.. ....................
Ecological notes ....................................................................

..........................................................................
Date of collection.. ...................................................................
Collector ..............................................................................
6. List various insecticides used as preservatives for plant specimens and
describe any one in detail.
.........................................................................................

7. Write down the important points which should be kept in mind before
storing dried herbarium sheets.
.........................................................................................

78
 Herbarium Techniques
5.12 ANSWERS

Self-assessment Questions
1) 1 - T , 2-F, 3 - T , 4 - T
2) You can fold a plant in given the shapes.

3) 1 -F, 2 - T , 3 - T , 4 - T ,
4) 1) 29 x 42 cm, 2) loo%, 3) paste or glue, 4) lower, 5) label, 6) water proof

Terminal Questions
1. Refer to section 5.2.
2. Refer to section 5.3.
3. Refer to section 5.4.
4. See section 5.5. Drying of plant specimens.
5. The answer will vary from person to person. YGUcan get the help of your
counsellor for this question.
6. See section 5.8. Storing of Herbarium.
7. The points which should be remembered:
i) Specimens should be stored either in steel or in wooden almiral~s
which must be dustproof.
ii) Specimens should be arranged according to the Bentham & Hooker or
Engler or Prantl systems of classification.