Measuring Transpiration Resistance of Leaves'
C. H. M. van Bavel, F. S. Nakayama, and W. L. Ehrler
United States Water Conservation Laboratory, Tempe, Arizona
Soil and Water Conservation Research Division, ARS, United States Department of Agriculture
nior did they verify their resuilts by (lirect experi-
Introduction
mentation.
T'he transpiratioln process can be described phvsically, following Raschke (10), in terms of a resistance
to diffuisive and turbulent vapor flux in the external
air, a similar diffusive resistance which results from
the internal leaf geometry, inclusive of the stomlata,
and1 parallel to the latter, a resistance to vapor diffusion1 throu-gh the cuticle. In contrast. the last 2
resistances do not exist in the evaporation froml an
open water sturface or a moist blotter paper. The
resistanice in these cases can be (lescribed using only
externial parameters.
applyinig the resistance concept, the internal
concenitration is generally computed from the
leaf temperatuire, assuminig that the state of the
leaf water is such that its relative vapor pressure is
substantially equal to one. This view is open to
somiie objections; see, for example. Banige (2) and
Heath (5). Nevertheless, it is applied by most plant
physiologists as it will be in this paper.
The leaf resistance miust be knowvn when computing leaf or canopy evaporation from measured
environmelntal paranieters, although it may not alvays be of siginificant magnitude compared to the
external vapor transport resistance. Thus, Bange
(2) developed a procedure for calculating the resistance from measurements of stomatal dimensions
and nuimbers, and substomatal cavity dimensions.
This procedure does not allow for the contributioni
of ctuticulai transpiration to the total water loss,
which still must be measured directly. Usinig an
experimelntal correction for cuticular loss, Bange's
work gave close agreement between measured and
computed transpiration from leaf disks of Zcbrinia
penidu/la, a hypostomatous plant.
Earlier, Penman and Schofield (9) suggested a
miethod similar to that of Bange for estimatilng the
stomatal resistance, the values to be applied in calcutlatinig caniopy evaporation. F-or the latter, they
prol)ose(l a combinatioln mletlho( (levised by Peinman
see Pe1toni and Tanner (8) for a critical discussioni
ill vhiclh the energy balanlce ail(l aerodynamics of the
vegetative sturface are simulltaneouisly taken into accouint. Penlman and Schofield did not give stuch
(letaile(l conisiderationi to substonlatal cavity dimenIn
vapor
sioIns anid
o
her geomiietrical (letails
' Received November 2, 1964.
as
did Bange,
Rasclhk-e (10) gave a metlhod for computing the
leaf resistance inidirectly fromii simlultaneous measurements of leaf transpiratioll leaf temperature. leaf
radia' ion balance, anid ambienlt temperature anid vapor presstire. Crigilnally. he expressed the resuilt by
means of a dimenlsiolnless factor a. the wetness factor.
being the ratio between the actual tralnspirationi rate
anid tlle calculated evaporation rate of a hypothetical
free water sturface having the same dimenisiols, temlperature. anid exposure as the leaf. Raschlke showe(d
in a later publication (11) that a was idenltical with
the ratio of the combined turbulent and laminar
vapor flov resistance in the air outsi(le the leaf (the
external resistance, R,,) to the sumil of RA anid the
leaf resistanice (RL)
a - RA/(RA + RL)I
RA canl be approximiiated from observing the evaporatioin from )proper:y exl)osed wet filter paper of leaf
dlimiiensiolls. RA then equals the vapor conicenitrationi
gra(lielnt fromi paper to bulk air (livi(led by the
evaporationi rate per unit area.
Raschke's method is sound, but obviously niot
simiple, nor always practical. Oni the other hand.
the approaches taken by Banige, and by Penmani
and Schofield, do not account for cuticular water
loss, anid require a precise knowledge of stomatal
apertuLre and internal leaf geometry. Whereas
Bange showeed agreement between theory and experiment. Peltoni and Tanner (8) report unsatisfactory results from tests of the Peniman-Schofield
procedure. These tests were niot stringent, and the
disagreenmenit may have originated in Penman an(d
Schofield's proposal to use a generalized value for the
stomatal resistance an(l apply it to a canopy as one
wvotld to a sinigle leaf.
T'he stomatal aperture itself canl be miieasured by
variouis mlethods, sulch as visual observation of leaves
or their replicas, measturenlenlts wvith pressuire poromzeters. and in filtrationi techlmiqtues. Recent reviews
of suich teclii(Itues are founid in Heath (5) andl
Eckhardt (3). Regardless of technical feasibility.
nonie permits anl exact calculation of the leaf resistance factor mentioned above.
Until nowv, the only available field method for
measuring transpiration resistance directly was the
classical cobalt chloride method, described in many
textbooks. It is slow and subjective. and also likely
to influence both stomatal aperture and leaf tempera535
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Copyright © 1965 American Society of Plant Biologists. All rights reserved.
536
PLANT PHT YSIOLOGY
that is simple and rapid, and entails a minimum of
disturbance to the plant.
In this article we describe improvements in the
equipment and methods given by Wallihan. Further, we report on the calibration of the apparatus
and the interpretation of the data. Finally, we
show that the technique is physically sound and can
lead to quantitative prediction of the transpiration
rate of whole plants.
transpiration
discussed in some
detail by Milthorpe (6). A refinement of the desiccant technique, suitable for laboratory studies, was
proposed by Archer and La Mer (1) for measuring
evaporation resistance caused by monomolecular films
on open water surfaces. Their calibration and interpretation methods are quite similar to the ones
we will describe in the following section.
Recently, Wallihan (13), although concerned
with stomatal aperture rather than transpiration resistance, suggested a technique that can be the basis
for a direct method to measure leaf resistance, one
ture. Classified by Heath (5)
porometer, it has been recently
as a
Materials and Methods
The method evolved from Wallihan's report
NYeLON
NNUS
MX
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OWLQ?0 01K
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AMPENOL
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ELEMENlT
RANGE
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SCREW
2--4 PLAYTNAG
TUB
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20 BA.
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SYILEIB
1/4'
STlEEL
II
CLAMP
AnS
AND CUP
ALL DCIMISIN
MATRIAL
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INCHES
IN
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OLM
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FIG. 1. Construction drawing of resistance meter leaf cup. Connecting leads are omitted.
Downloaded from on June 17, 2017 - Published by www.plantphysiol.org
Copyright © 1965 American Society of Plant Biologists. All rights reserved.
SI1
A-A
x
0.0"
DEEP
TAP
ORILL
THrIu.
5 37
VAN BAVEL, ET Al,.-M\EA\SU'RING TRANSPIRATION RESISTANCE OF LEAVES
consists of exposing a small portion of the upper or
lower leaf surface to a hygroscopic surface, alwavs
in exactly the sanme spatial relationiship and for
only a short period, from 0.1 to 1.0 minute. The
hygroscopic surface is a lithium chloride-impregnated
resistor and, by calibration, the time rate of change
in its electrical resistance can be interpreted in terms
of the diffusion resistance, identified above as RL
and a known constant, typical of the device.
In effect, a portion of the leaf is al'owed to transpire under known and prescribed conditions. The
rate of transpiration wheni compared with the rate
of evaporation from a free water surface of identical
area under similar conditions, gives the value of RL.
A similar approach has been followed using leaf
chambers or plant enclosures. However, by using
a small leaf cup as proposed here, the plant and its
environment are not altered by the measuremenit,
and the particular leaf spot is affected only very
briefly. Also, the vapor transport in the cup appears to take place in a reproducible manner. Thus,
the many objections to an enclosure technique do
not apply. Further, as Wallihan alrea(ly demonstrated, the equipment can be quite simqple, inexpensive, and portable.
A construction drawing of the leaf resistance cup
is given in figure 1. The sampling area is 2.84 cm2
and the cup volume is 14.4 cm3, excluding the measuring element. The latter is connected to an AC
resistance meter with 30-gauge, stranded vinyl-insulated copper wire. An effective seal over veins
and other surface elements of a leaf is obtained with
low clamping pressure using sponge rubber gaskets.
The entire assembly weighs 30 g, and it can be
attached without further support to medium-size
leaves, such as cotton, citrus, corn, or cannia. Tf a
leaf lacks adequate strength the cup must be held by
hand.
The sensor is an Aminco-Dunmore2 element
(#4-4817) with an effective range from 14 to 27 %
relative humidity. \Vhen used with an appropriate
AC resistance meter (see Appendix), the relation
between meter current and relative humidity is nearly
linear from 0.10 to 0.80 full-scale. Before the cup
is clamped onto the leaf, the air in the chamber is
dried out with a stream of dry air to a reading of
about 0.10 full-scale. After clamping the ctup on,
a stop watch is started when the reading attains a
value of 0.20 full-scale and stopped when the value
is 0.60 full-scale, thus giving a transit time /t.
An appropriate analysis of the water vapor diffusioIn in the cup shows that the following relation
should applv:
11
RL = (SAt - Lo)/D
where RL, is the resistance of the leaf in min cm -1,
S is an instrument constanit in cm min-' which can1
2 Trade names are included for the benefit of the
reader and do not infer any endorsement or preferential
treatment of the product listed by the United States
Department of Agriculture.
.3
F
0.2
0.
0.c
0
1
3
2
DIFFUSION
PATH
LENGTH,
4
5
6
Cm
FIG. 2. Experimentally found relation between diffusion path length (L) and transit time (At) at 42.00.
be designated as the sensitivity, L. is a diffusion
length or shape factor, characteristic for the cup, in
cm, and D is the molecular diffusivitv of water
vapor in air in cm) minl'.
Relation (II) is obtained by assuming that the
temperature throughout the system is constant, that
steady-state evaporation occurs, and that the relative humidity sensed by the element does not change
appreciably with respect to unity. Experiments on
the time rate of change of the element resistance,
after exposure of the cup to a wet blotter, demonsLrated the adequacy of the second assumption. \Vith
respect to the last assumption, a change in meter
reading from 0.2 to 0.6 full-scale corresponds to a
change in relative humidity of about 0.04, nearly
independently of temperature. In fact, therefore,
the condition assumed to be constant changes bv 4
parts in 80, a change that may be ignored.
The effective validity of equation II was tested
by measuring At at various temperatures and at
varying values of a dummy RL, obtained by interposing cylinders of the same internal diameter as
the cup, but varying lengths L, between the cup and
a wet blotter. The temperature of the blotter was
measured with a fine thermocouple and was never
more than 10 below that of the environment. CoInsidering, in this case, that the equiva!ent R1, equals
L/D, equation II can be written as
III
L = SAt - Lo.
By plotting L vs At, a straight line should be obtained, as exemplified in figure 2. Thus, not only
is the validity of (II) established but, by measuring
S and L. at different temperatures, the absolute
calibration of the instrument is obtained. This
is necessary, since in equation II both S and D are
temperature dependent, although L. is not.
\Vith our instrumentation, the average value for
Lo was 0.72 cm. The dependence of S upon temperature is shown in figure 3. Values of D at
1000 mb pressure, taken from standard tables, are
also plotted. Thus, from a measurement of At,
and using the appropriate S and D from figure 3, RJJ
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Copyright © 1965 American Society of Plant Biologists. All rights reserved.
5 38
PLANT' 1'11lYSIOI.OG\.N
inigs coli(l ibe tAken
ill) to
p)er
1
lilnte h)
a
siii¶le
op)erator.
Figyure 4 gives
AE
example of (lata ob)taiine(l
an
controlle(I environment o1
co'toi lplanlt (
u,m burbudciisc L1., var. IUma S-2 ) grown in
a
a
ill
(Goss vp-
nltitrient
Stomatal opiening \w ind(ulce(d by turnintg
oni fluorescenit lights in ani otherwise conistalit enisolution.
as
Assutiniig that stomiates were fullv close(d
in the (lark, cuticular resistance is approximately 2(
times that of the combined ( parallel) stonatal aiid
vironment.
4.
30
cuiticuilar resistance in the lighit. 'I'lTe timie rate o
of leaf resistalice can be (locunlente(l (quite
1-
change
imlilie(l by the4
laIta, appeiars quite ralii(l in this
Figure-l
also demonstrates tihat ttiringvo off the lights gave
ireciselv. Stomiatal movement,
as
case.
rapi(llv increasing stomatall
reslponse of
resistanice. follow e(l by aii el)hemeral (lecrease in
the dark. The latter behavior has been ol)serve(
a reverse
TEMPERATURE,
1o.
Teniperature
3.
and(I
dog
hand(lbook
iiietcr
resistaIice
of
( D),
(letermine(l, the
of the water vapor
sensitivity (S )
former havinig been experimeiitallv
froni
in otir work.
rel)eate(llv
(lelpeildeiice
cliffusivity
tlle
latter
a
values.
mlore rigorotis test of the adequacy of the
resistaniice measurement lies in its ability to plre(lict
absolute values of tranisliiration. A.gain., miieastirements Nvere imia(le oii cottol lplant grown in niutrienit
soltitioni ani(l in a constant env ironment. Transpir,atioli. leaf temperature, and resistanices of tipper iidl
lower leaf epi(lerimiis were iieastire(1 sinutiltaileouslv
a
can
lie calctilated froiii equatioii 11. At pressu res
significantly (lifferenit from
100)) mb, the vatlue of
I) is to
nimiltiplied by
Ip 1) heiiig the amibient
lbe
pressure
in
000))
nib.
with the leaf resistance iiieter- showed
Experience
it
couild
of plants.
tes,
and
(leriiiis,
\vere
tioiis
oii
1
lbe
uised conveniently
on v ariouis
and(I
(litirnal openiig
The
as
v. ere
Results and Discussion
that
The valties,
shown in fi-tire
all)proxiniatelv constant with tinile. except
the resistailce of the upp)er epidlermis. w hich flticttilate(l. Aeasurem,ents of eval)oration rate andl teniiperature of wet blotter papers (5 X 1i) cmii rectan,-les),
adle in the samile environment, -ave .a valtue of R.
miiii cm'1 1. TransI irl'tion
= (0.23
oImptite(l
on1 the same planit.
uililper
(lifferences between
cotil(l lie established
tvpes
closilg (of stomaand
lower epi-
reprodutcilblv.
al15
Rea(ingsi
substantially i(lentical lietween (lifferent posion any 1 leaf anid
lietw-eeli (lifferent leaves
lilailt. provided expostire was the sanei. Read-
c
was
f roni
X
RA
1(:-
X
+ 41,
Ae
where
k.
is the
water
loss
-Cll-111}
c
mI
K
A
-~~
per
(iilt
1
miiii
2
IV
suirface
leaf
area
lower epidermis. Ae is the clifferenice iin vapor l)resstire betw een the air and
s.aLttirated atmosphere at leaf teml)erattire in mb. an(l RA
aniid R, are the air and leaf resistances, in miin cmi-'.
Th'lle conversion factor 7.15 X 1() 7
ciii ,mb-,
taken at 30))
re(luire(l because of the uiselhere of
vapor lressuire valties rather than vatlpor con1ceiitratioiis.
Iromi the average 1\, valutie for the lower epi(ler-miis of 0(.02))0 mii ci
tIle traisliiratioill \x ls Calilate(d as 3.4 X I() 4 Cm' 2 Ilill- I.
F'or tile Ijj)per_
el)i(leriiiis, wvith .an average R1 of 0.148 mm1ill cill ', the
of either uppel-
or
a
.60
TEMPERATURE
30.0
0.3
CARBON
.50
C
15.I mb
PRESSURE
8uc
vol.
|
DARK
ppm
LIGHT
20
is
DARK
klux,
.40
c
.30
49
transpiratioi
the leaf as
\was
().(4
I()
'
miiil
'
lI>0r
xxhbole ( 2 si(les) tile traliispilr tiollrate
\ould thlein le obtaiined by adldition as 4.08 X Y-(1
.20
C
mill
a
',
wher -eas
the
averawe
measured
va
tie
i
from
\ater-loss (leteriniaiitioni of the eintire lal'nit
was 3.7.5 1(X
ise
X
Iiil
agreement is
1
coilsi(lere(l
60
TIME,
FI(,.
of
a
4.
Trainspiration
cotton leaf,
(larkefling.
as
Tinile
errors
.
coiisi(lerinig
of
lowxer epidermis
sud(leni illumiinationi
arbitrary.
and
This
tthe
involvedl in imieasuirinig R,
temilperature., slhort-timie
min
resistance
influenced by
referenice is
scale
80
adequate
as
transpiration
experimental
as leaf
andl, perhaps
well
iin particular., RA.
Accepting the im1east1re(I valuies for R1
oiie comptutes the following valuies for
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Copyright © 1965 American Society of Plant Biologists. All rights reserved.
as realistic.
Rasclhke's a-
539
V'AN BAVEL ET AL.-,\IEASURl,NG TRANSI'IRATION RESISTANCE' OF LEAVES
4.o0
Transpiration
.c
E
O
_:
4N
a: 1E
z 'O
Z*
30
r
29
,-4
c
YIn
3.0
Temperature
28 o r.
-4
C
27
.251-
TEMPERATURE
VAPOR PRESSURE
AIR MOVEMENT
CARBON DIOXIDE
LIGHT
30.0
C
15.1 mb
0.3 m Sec7I
340 vol. ppm
20
klux
F
.20
1\
Al
*Upperml
epidermis
\
.15
z
procedure, a value of Rb, for the lower epidermis was
found as 0.0041 min cmn-. The generalized value
of 1.56 X 103 cMn1 that Penman and Schofield
have proposed for single stoma resistance would
result in RI, = 0.0062 min cim-1 for the low er epi(lerimiis (1 .5000 stomates/cm112). On the other hand,
Bange's calculationis for fully open stomates of Zcbriia(i gave a value for RI, of 0.018 mnm cm -1, agaill
for the lower epidermis. This an(l our corresponlding figure for R, of 0.020 minl cm'-1, suggests that
the Penman-Schofield procedure considerably underestimates stomatal resistalnce, conceivably because the
resistan1ce of the substomiatal cavities is nlot accounlted
for.
The physical signlificanice of the mleasure(d REI,
values depends uponl their magniitude relative to the
associated RA values as is suggested by equation IN'.
Oine should bear in minid, how ever, that as R, increases Ae will also increase and vice versa. A recent examlple of the application of equation IN' is
fotund in Slatver and Bierhuizen (12), where additioiial discussioni may be founid. In this reference
alues for RT, for cottoln were obtained in a leaf
chamber. At 20 kilolux a value was found of 0.022
nun
4-.
0J
cmi-' for cotton, virtuallv identical to
our meas-
ureilients.
In conlsidering the evaporation fromli a vegetative
prolblenl, in micrometeorology, an equation similar to equationi IX' can be wvritten. Theni,
RA or its analogue is determinie(d by windspeed, roughmust refer to the
ness, and(l air stability. and RK
canopy as a wlhole. As of now, it is niot clear how
RX, for a leaf is related to RL, for a canopy, though
obvxiously the latter value must be smaller. For a
(letaile(l (liscussioni referelnce is mla(le to Mfonteith (7).
.10-
44
canopy, as a
w
.05
F
Lower
0*-0
0000
00
000
* epidermis
I
20
0
40
80
60
TIME, min
FIG. 5.
Transpirationi, leaf temiperature, anid leaf
sistance of
upper and lower epidermis
in a controlled, steady elnviroinmenit.
of
a
cottoin
re-
0.13, and for the lower
for the upper epidermis
= 0.53. Thus, the leaf as a whole
epidermis
evaporated 0.33 that of a hypothetical wet blotter at
the samiie temlperature and exposure to air miiovemenit.
However, in reality, a simiiilarly exposed blotter would
lhav e a lower temperature and the ratio would be
greater than 0.33. Therefore, the disparity between
leaf transpiration and blotter evaporation is less
thani stuggested by the value of a. Raschke (10)
cites values for a that are generally lower than
0.53, tlhou-gh he states that its value may lie between
0.004 and 0.6, the latter value applying to wide open
stomates. Direct observation showed that the cota
=
a
ton
stonnates
had
a
width
of about
,
in
our
test,
length being albout 15 /u. A rep)ort on simutltallevariation of RI, and transpiratioln in cottonl is
giveni by Ehrler, et al. (4).
Frouii the stomatal (lillieisionis an(l nluiml)ers o0i
our cottoni leaves, following Penmani and Schofield's
the
otis
Summary
planit
A method, suitable for fiekl use, is given whereby
the resistanice offered by the leaf anatomy to diffusive
loss of water can be measured directly, rapidly, and
w-itlh simple, portable equipment.
A small cup, containiing a senisitive humidity
senlsor, is claml)el onlto the leaf and the time rate
of chanige of the sensor indicatioln is mleasured over
a short period, often less than 1 minute. Appropriate calibration permits the conversion of the time
rate of change to the diffusion resistance, inclusive
of a temperature correction. Construction details
of suitable, portable equipment are supplied.
The diffusion resistance, thus measured, can be
used to compute the absolute transpiration rate of
leaves and plants, and to characterize the movements
anid
responses
of
stomates
with
direct
reference
to
the transl)iration process.
The leaf resistance, as measure(d directly. can
also be used in combiniation with a knowledge of
atiiiosphleric tranlsp)rt processes to predict evaporationl froil)plant caniopies.
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Copyright © 1965 American Society of Plant Biologists. All rights reserved.
54050PLANT PH1 YSIOLOGY
Acknowledgment
The
authlors
lani McIllroy,
for
express
a
thianks to
critical
their
reading of the
colleague,
-Mr.
mianiuscript.
1. ARCHER, R. J. AND V. K. LAMER. 1954. The effect
of monolavers on the rate o.' evaporation of
Water. Ainn. 'New York .Acad. Sci. 58: 807-29.
2. BAN(iE, G. G. J. 1956. On the (quantitative explanation of stomatal transl)iration. Acta Botan.
Neerl. 2: 255-97.
3. EKIJARDT, 1F. E. 1960. Eco-l)hvsiological nlea-
4.
5.
6.
7.
lFio;.
WNirinig diagrami
portahle, AC resistance
meter to he used vx itlh resistance meter leaf cul) shown
in figure 1.
of
Literature Cited
suiring techniques applied to research on water
relationis of plants in arid and( semi-arid regions.
I n: Plant-xx ater relationiships in arid and semiari(l conditions. U-NESCO .Ari(d /oiie Res. Ser.
15: 139-71.
1h,TiRLER, Ws. L, F. S. NAKAYAMA., ANI) C. H. M.
VAN BAVEL. 1965. Cyclic changes in water halatice and tranispiration of cotton leaves. In press.
HEATH, 0. V. S. 1959. The water relations of
stomatal cells and the mechaniismiis of stomatal
movement. In: Plant Phvsiol. F. C. Steward. el.
.Academic Press, Nex- York. 2: 193-250.
'MILTHORPE, F. L. 1955. The signiificanice of tlle
measurenmeint made by the cobalt clhloride paper
method. J. Exptl. Botany 6: 17-19.
MIONTEITH, J. L. 1963. Gas exchainge in plant
communities. In: Environmllental Control of
Plant Growtth. .\cademic Press, Ne\w York. 95-
111.
8. PELTON, \W. L. A.\NL C. B. TANNER. 1960. Potenitial evapotranspirationi estimiates bv the approximlate eniergy halanice metlho(d of Penmiiianl. J. Geop)llys. Res. 65: 3391-3413.
9. PFN-MAN, H. L. AND IK. 1K. SCHIOFIELD. 1951.
Somiie plhysical aspects of assimilationn and( transp)iration. Svmp). Soc. Exptl. Biology 5: 115 -29.
IsACHKE, 1K. 1956. tTber die physikalischen Bezii
hunigeni zxvischen \V'drmeiiberganigszahll, Strahlulngsaustausch, Temperatur un(lI Transpiration cilnes
Blattes. Planta 48: 200-38.
11. RASCHKE, K. 1958. -Cber dleni Eiinflusis der )iffusionswiderstainde aui die Transpirationi un(l die
Temperatur eines Blattes. Flora 146: 546-78.
12. SiLATvER, R. 0. \NI) J. F. BIERI11IZEN. 1964.
Tranispirationi fromii cottoni leaves under a ranige
of enviroiinlenital conditionis in relation to internial
anid external diffusive resistances. Australian J.
Biol. Sci. 17: 115-30.
13. \\VALILIHAN, E. F. 1964. Modification anld use of
an electric hygrometer for estimating relative
stomiatal apertures. Plant Physiol. 39: 86-90.
10.
Appendix
Thle changing resistanice of the humiditv senisor can
measured usiIng commn1tlercially available equipm.ent;
however, the xu-eight and bulk of this equil)ment is excessive for fiekl wxork.
Figure 6 provides a x-iring (liagramii of a lightweight
(800 g), comp)act (8 X 10 X 16 cm) meter, that permits
use of factory-stupplie(d calibratioins for the sensors. The
meter consists of a Lnijuuctioil trigger circuit and a
flip-f lop circuit, providing a pure and stable square wave
voltage of amplitude 22 and 250 cps frequency. Currenit
consumptioin is 30 milliamps and maximum current
he
v
tlhrouiglh tlle
seilsor is
10
microamps.
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