Apr. X4,1923 
Physiological Requirements of Rocky Mountain Trees 135 
At first thought it does not appear probable that the density of the 
spruce sap, which is only one-half greater than that of yellow pine, is 
sufficient to reduce the relative water loss of spruce to one-third or one- 
fourth of that of the pine. It is not necessary to assume that the small 
water loss of spruce, as related to its photos5ntithetic activity and amount 
of growth, is entirdy the direct result of the physical properties of the 
dense sap of this species. To determine how the result may be brought 
about, all the following factors must be given consideration: 
1. Higher sap density means less evaporation per unit of available heat. 
2. Higher sap density means higher leaf temperatures before evapora¬ 
tion can take place at a given rate, with the possibility that in sunlight the 
leaf may become warmer than the air and therefore lose heat by radiation 
and conduction. 
a 
Q 8 
K) Ti 
Wo 
0 8 
er n 
>0, » 
;quit 
0 10 
emc 
BO M 
lit p 
>0 IZ 
»r ui 
00 )3 
it cfl 
DO H 
y-wi 
DO ti 
li^ht 
00 K 
accr 
DO n 
etior 
00 M 
1. CC 
00 18 
00 « 
00 
ttCi 
29 
oxn 
g^len 
ann 
28 
i atf 
la. r 
n 
••A— 
tJ 
^ / 
’ . 
—IT' 
/ 
. 
/ 
^^ 
u 
-r 
\ 
odM 
pOl€ 
.■fe 
pa « 
tl€C 
>nc 
7Ttl^ 
ZO 
18 
1 
imb 
rPfrv 
so 
la 
IT 
1^. 3.—Water requirements in 19x7 traxispiration tests compared with sap densities on nursery specimens. 
December 3. igij <«DUdline) and sap densitks in natural habitats. December 30.19x7 (broken line). 
■. j 
3. Higher sap density, by permitting higher leaf temperatures, should 
facilitate the photos5mthetic process, fiius relatively reducing the water 
requirement 
4. It must be equally true that greater photos5mthetic activity or 
capadty will tend toward higher sap density as well as increased growth. 
- When, is asked, '*Is the high sap density d 
spruce a direct cause of its low transpiration rate, or is the former merely 
a coincoitutaiit of greater photosyuthetic activity, and is this last the 
really important p^iologi^ characteristic?/' we are compelled to reply 
that the three things are so interdep^dent that all are equally causes and 
effects. It is left: almost beyond question, however, that of the species 
/we have considered the spruce represents the highest development and 
.that this devdopment is expressed in the highest growth rate (first 
columns of Table X) in the greatest current accumulation of soluble carbo¬ 
hydrates at the time of the^iPecem examination and in the most 
