Germination of Forest Trees 25 



This principle is fundamental in mathematics; a single algebraic 

 equation with two unknown quantities cannot be solved. In each 

 of the series used in these investigations it was the intention to 

 have only one variable habitat factor. In this way the study of 

 cause and effect was much more clearly brought out. 



The soil moisture determinations were made for four different 

 purposes : 



1. As a check upon daily watering in similar cultures. 



2. To show the effect of shading on soil moisture content. 



3. To show the minimum content in the soil moisture series. 



4. To compare soil moisture content in loam, sand, and gravel. 



The samples were taken in certain cultures and at intervals 

 varying with the purpose. The samples for the moisture content 

 series of cultures were taken once a week, all others once a month. 

 Each sample consisted of from 50-100 grams of soil, and was 

 taken at depths varying with the development of the plants in the 

 culture. In order to provide against error each sample consisted 

 of from two to five portions taken from spots several decimeters 

 apart, care being exercised that no soil was dug near holes where 

 previous samples had recently been obtained. The samples were 

 always dug between the rows of seedlings. The samples were 

 immediately weighed and dried at a temperature of 95-105" 

 Centigrade to constant weight (24 hours). The per cent, of 

 water was computed upon the dry weight of the soil. 



All the cultures except the dry soil and the medium wet soil 

 cultures were watered every evening. As was noted above, when 

 the amount of room for the germination tests became insufficient 

 the number of cultures was reduced from 12 to 10 by eliminating 

 two duplicate cultures. The only check moisture samples which 

 will be considered here are those that have to do with the three 

 cultures which were being operated under identical conditions. 

 Samples taken and recorded hereafter will take care of the other 

 cultures and series. During the time (three months) that these 

 duplicate cultures were run, one set of soil samples was taken as 

 a check to determine whether they were being watered equally. 

 These figures follow : 



