24 Richard H. Boerker 
case of certain Pacific coast species four months was apparently 
necessary. Three months ordinarily is plenty time enough; 
usually if a seed in the forest fails to germinate in that time, it 
usually does not germinate at all, especially in the west where 
the dry period sets in after the spring is over. 
Ten of the species mentioned in the foregoing tables failed to 
germinate. These species were Larix europea, Acer saccharum 
(both), Liriodendron tulipifera, Taxodium distichum, Liqui- 
damber styraciflua, Fraxinus americana, all of which were supplied 
by commercial seedmen. If the data regarding the collection of 
these seeds is bona-fide, their failure to germinate must be ex- 
plained by the fact that they had not completed their resting 
period. In the case of Jumniperis virgimana, Gleditschia tria- 
canthos, and Pinus monticola, whose place and date of collection 
is known absolutely there can be very little doubt as to why they 
failed to germinate. 
The soil used in all cultures (except the sand and gravel) was a 
garden loam of excellent quality with a mixture of about 25 per 
cent. of white sand. The mixture was prepared in the green- 
house. ‘This made a very good soil for experimentation purposes. 
The sand used was common white, quartz sand with but a very 
small per cent. of hornblende and magnetite. The gravel was the 
kind used by the large construction companies around Lincoln 
for concrete work. Mechanical analyses of representative 
samples of these soils are given elsewhere. 
All seeds were planted in rows at a depth which was 2% times 
the shortest diameter of the seed as near as this was determinable 
by the unaided eye. The rows averaged 3 inches apart and about 
24 inches in length. In general 200 seeds were used of each 
species when the seeds were of medium size or smaller; for some 
of the western pines only 100 were used because of their large 
size. 
The Control of Habitat Factors 
As has been pointed out, the only safe way to study the effect 
of the factors of the habitat upon the life of the plant is to 
measure one variable factor while all the rest are kept constant. 
