226
Plants and Plant Communities
Example 14.1. Find which is cooler in a hot, dry environment, a wide
leaf or a narrow leaf, if both have access to an unlimited water supply.
Assume leaf widths of 3 mm and 3 cm; T, = 38" C, e, = 1.1 kPa,
u = 1.5 d s , and Rabs = 750 w/m2. Pressure is 100 H a . Also assume
the leaves are hypostomatous with an adaxial conductunce of zero and an
abaxial conductance of 0.5 rnol m-2 s-'.
Solution. From Table A.3, it can be seen that e,(T,) = 6.63 kPa, A =
0.359 kPa, B = 532 w/m2, and g, = 0.23 rnol md2 s-' . Using these
values the following can be computed
s = 0.359 kPa C-'/lo0 kPa = 0.00359 C-'
we also have (Table A.1), c, = 29.3 J mol-' C-', and (Ch. 3) y =
6.66 x
C-' . For the leaf specific calculations we have the following
table.
10 cm wide leaf
d = 0.7 x 0.1 = 0.07m
g ~ ,
= 1.4 x 0 . 1 3 5 f i = 0.875 rnol m-2 s-I
= 0.95 rnol m-2 s-I
g ~ ~
= 0.875 + 0.23 = 1.105 rnol mF2 s-I
- 0Sx0.5x0.953 - 0 164 mol m-2 s-I
- 0.5+0.953 - '
y* = 6.66 x
'.'05 = 0.00449 C-'
,00448.'~~
234
5. 53
TL = 38 + . 00449+. 00359 ( 1. 105x29. 3 100~0.00449 )
= 35.2" C
3 mrn wide leaf
d = 0.7 x 0.003 = 0.0021 m
= 5.OSmol rn-' s-I
= 5.50 rnol rnd2 s-I
g ~ r
= 5.05 + 0.23 = 5.73 m0l m-2 s-'
- 0.5~0.5~5.50 = 0.229 mol m-2 s-l
,- 0.5+5.50
y* = 6.66 x
= 0.0167 C-I
0 1 6 7 ~ ' ~ ~ ~
( 234
5. 53
TL = 38 + - 5.73x29.3 100~0.0167 )
= 36.4" C
While the temperatures of the two leaves do not differ by much, it
is significant and interesting that the large leaf is cooler than the small
one. The greater rate of convective energy exchange associated with the
smaller leaf causes the leaf to be nearer to air temperature than the larger