132 College of Forestry 
whatever cause, represent an approach toward conditions 
which exclude vegetation, the heath shrubs which persist be- 
yond or dominate over most other species must be regarded as 
having noteworthy tolerance and resistance. As bearing 
on this problem of soil adaptation, the work of Covyille* on 
blueberry, trailing arbutus, ete., is noteworthy. Especially 
are you recommended to read the bulletin on blueberry eul- 
ture. Coville found that in the case of blueberry and other 
heaths, the rootlets instead of possessing the normal absorb- 
ing cells called root hairs, are inhabited by a fungus —one 
of the mycorhizal fungi which appear to exist in a symbiotie 
relation with the plant whose roots they inhabit. He found 
further that such fungi exist only in an acid medium and 
accounted for the presence of blueberry, ete., on peat soils 
and on sand by reason of the acidity of both these substrata 
(1. c. page 85). It is his view furthermore that the acidity 
of peat soils is the main factor in inhibiting the activity of 
nitrifying bacteria and therefore of the non-available condi- 
tion of the nitrogen. He thinks that the mycorhiza of the 
heath shrub roots play a role in transforming the non-avail- 
able to available nitrogen which the heath plants then absorb. 
THE DEVELOPMENT OF VEGETATION UPON 
A SUBSTRATUM SUBJECT TO PREVAILING 
WATER DEFICIT. 
This case carries us just to the opposite extreme of the 
water relation from what we have been considering. Con- 
cretely the problem is for vegetation to establish itself upon 
a substratum of solid rock, boulders, gravel, sand, ete., in a 
climate like that of New York State where rainfall, humid- 
ity, fogs and dew make possible a luxuriant vegetation upon 
a substratum eapable of maintaining a relatively constant 
water supply. Such constancy of moisture supply is of 
course impossible upon bare rock and sand if these lie above 
1 Coville, Frederick V. Experiments in blueberry culture. Bull. 
U. S. Dept. Agr., Bur. Pl. Ind., No. 193:1911. 
