Germination of Forest Trees 21 
and not the cause of the factors. It is a most significant fact that 
the same habitat factors do not always produce the same effects 
upon vegetation even under apparently the same set of conditions. 
The effect of two habitat factors or groups of factors may be the 
same so far as the structure and behavior of the plant is con- 
cerned, yet upon inquiry into the causes concerned we might find 
in one case it was due to temperature and in the other to soil 
moisture. In a similar manner it 1s known that other factors 
besides light determine tolerance. In other words the study of 
the effect of habitat factors upon plants does not always lead us 
to safe assumptions as to what the underlying cause is. The only 
safe method in this kind of work is to measure the cause, thus 
employing a direct method instead of an indirect one. 
Methods and Apparatus Used in These Investigations 
The investigations herein described were carried on in the 
middle room of the west greenhouse of the botany department 
of the University of Nebraska. For the germination studies three 
series of cultures were used, namely, the light, soil-moisture, and 
soil-texture series. For the experiments and measurements in 
connection with the early development of roots and stem a fourth 
series was added, namely, the soil-depth series. In each series 
three degrees were used. In the light series open light, medium 
shade, and dense shade were used; in the soil-depth series shallow, 
medium deep, and very deep soil was used; in the soil-moisture- 
content series, dry soil, medium wet soil, and wet soil was em- 
ployed ; and in the soil-texture series loam, sand, and gravel were 
used. The values of each degree in each case will be given later. 
As the experiments progressed it was found that the amount of 
greenhouse space assigned to the work was not sufficient, so 
that the open light culture, the wet soil culture, and the loam cul- 
ture were combined into one since these were being run under 
identical conditions. (For arrangement of cultures see page 33.) 
The seeds for these experiments were obtained from any source 
it was possible to get them. Large orders were sent to almost 
all large commercial seed houses at one time or another. On the 
whole the response from these orders was very discouraging. At 
