Apr. 14,1923 
Physiological Requirements of Rocky Mountain Trees 147 
The seedlings have been developed with abundant sunlight, in the 
greenhouse, avoiding excessive temperatures as far as possible. To 
make avsatisfactory test each pan should develop at least 100 seedlings. 
When the final weight of a pan is secured, with the seedlings wilted, 
deductions are made for the known weight of paraffin applied, as also for 
the weight of the seed used, which is assumed to be the same as that of 
the wilted seedlings and the loose hulls. This weight could generally be 
ignored without affecting the result appreciably, for the moisture content 
is usually 60 or 80 gm., as against i to 5 gm. for the seed of any species 
except yellow pine. 
DIRECT COMPARISONS OF THE SPECIES 
Not until 1920 was it possible to conduct special tests with two or 
more species in the same soil. The most comprehensive test, and there¬ 
fore the least likely to be misleading, was conducted from April to Sep¬ 
tember, 1920. In this case the soils were not sterilized by oven-drying, 
and considerable damping off of the seedlings occurred, which may be 
confused with legitimate wilting in the early stages. The pans were 
watered and weighed daily, and the losses of seedlings were recorded, 
so that it is possible to consider the losses at any stage. Because of the 
damping off, and also to make these results more comparable with those 
in which but one wilting period was recorded—^that is, the time when 
practically the entire number collapsed—^it seems best to consider in all 
these more recent tests the mean wilting point for the last 25 per cent of 
the total number of seedlings observed. Not infrequently the weakest 
seedlings die with twice as much water available as is required to sustain 
the strongest. 
In this particular test the moisture equivalents of the five soils were 
determined first, under a force of 100 gravity; and, assuming that these 
quantities were indicative of the same degree of availability in each soil, 
the watering in each case was so gauged as to maintain this moisture 
equivalent. It may be remarked that this quantity was very favorable 
for germination and establishment. Later the water content of each 
pan was reduced to two-thirds of the moisture equivalent, finally to one- 
third, and from that point downward by 5 gm. stages. This is important 
because other tests indicate that the drought which a seedling will 
tolerate depends much on the moisture to which it has become accustomed. 
The soils in this case were not paraffined, but some water was given 
almost every day in order to ehminate, so far as possible, extreme drying- 
out of the surface. It is significant that, perhaps on this account, the 
wilting coefficients are relatively lower than usual. 
The results of this test, which have already been ^ven in the research 
manual (4, pt i) to illustrate the relation between wilting coefficient and 
moisture equivalent, are given in slightly different form in Table XIX. 
We shall not discuss the rather variable relations of these wilting 
co^cients to the physical measures of moisture-holding properties of 
the soils. Suffice it to say that other evidence points to the fact that the 
several wilting coefficients represent an osmotic constant in the different 
soils, while either the capillary moistmre or moisture equivalents fall 
considerably short of this. This relationship will be discussed in connec¬ 
tion with the fidd-moisture problem. The physical measures of soil 
moisture bear only a general relation to wilting coefficients and must 
be used with this understanding.® 
• The reader is urged to note the discussion of thb by Bates and Zon (4) where it is i^de pWn t^t a 
constant ratk> between wilting coefficient and moisture equivalent is impossible if a wide variety of sou 
types is considered. 
