896 
Journal of Agricultural Research 
Vol. XXVIII, No. y 
moisture, as, for example, the juniper and western yellow pine types, a slower 
rate of growth and abnormally high osmotic pressures are concomitant. A low 
concentration of the soil solution would in turn favor selective absorption through 
the action of the osmotic gradient and still permit the maintenance of a relatively 
low sap concentration in the leaves of the plant desirable from the standpoint of 
stimulating rapid growth. 26 Forest trees of course make their best growth on 
the best sites, the sites from the best to the poorest being merely a relative 
classification of the growth capacity of different habitats as influenced by en¬ 
vironmental factors. The lowest sap concentrations for a given species are found 
on those sites on which the environmental conditions are most favorable for rapid 
growth. 
The intimate relation which exists between sap density and growth rate is 
emphasized in the present study. The growth rate of Douglas fir and white fir 
in Big Cottonwood Canyon is more rapid on the south aspect than on the opposite 
north aspect at an elevation of 7,500 feet where the aspen-fir type meets the 
chaparral type. The competition of the open stand of trees on the south slope 
is in this case nowhere intense, either with one another or with the shrubby species. 
The trees are rooted in a soil layer below that of the bulk of the shrubs; one in 
which the soil moisture supply is abundant, as evidenced by the presence of aspen ,. 
an indicator of an ample supply of subterranean moisture. On the north aspect, 
on the other hand, growth is retarded by competition for the soil moisture in the 
lower soil layers, not only between individuals in a denser stand of forest trees,, 
but also between the trees and the dense stand of deeply rooted shrubs. Higher 
sap concentrations are also encountered on this north aspect. The south aspect 
is a striking example of a low sap concentration of Douglas fir and white fir asso¬ 
ciated with a relatively high transpiration rate. This can be fully explained by 
the presence of an abundant suppty of subterranean moisture. The correlation 
of a low sap density with this rapid growth rate of well established sizeable trees 
should not be confused with the question of favorableness or unfavorableness of 
site from the standpoint of the establishment of reproduction. Practically no 
seedlings of either species are being established on the south aspect on account 
of the meagerness of the moisture in the upper foot of soil and the excessive 
evaporation, while on the north aspect with its favorable moisture conditions near 
the surface there is found an abundance of advance reproduction. 
A somewhat similar comparison can be drawn in the case of the spruce-fir type* 
A study of Table VII shows that the density of the cell sap increases as timber 
line is approached on ridge tops and south aspects above 10,000 feet in elevation. 
With this increase in sap density a falling off in rate of growth is also apparent in 
Engelmann spruce, alpine fir, and limber pine on these sites. 
INFLUENCE OF STRUCTURAL AND PHYSIOLOGICAL FACTORS 
The structure of most plants varies with the site and even with the fluctuating 
environmental factors of the same site. The leaf, because it is the seat of impor¬ 
tant physiological activities of the plant and because of its modification by 
external factors, has long been the field for numerous investigations. The major¬ 
ity of such studies have been confined to the morphology and histology of the 
leaf, independent of its relations to environmental factors. In contrast to these 
Clements (19), Hanson (49), and Sampson and Allen (112) took into account 
measured physical factors in relation to leaf structure. The importance of this 
relationship was repeatedly emphasized in the present study while collecting the 
26 According to Hoagland (66) marked absorption of nutrient materials may take place during all stages 
of growth when suitable concentrations of the various ions are maintained. Burd (14) has pointed out that 
for many plants high concentrations of the soil solution at certain stages of growth are probably undesirable.. 
