56 EXPLOITATION OF PLANTS 
different types of soil might well serve as a guide to- 
wards the choice of nurse trees. Such would indicate 
the almost universal employment of birch, preferably 
Betula pubescens, on all non-calcareous soils, and the 
use of ash on those which are calcareous. The freedom 
with which these species colonise ground indicates 
that they might be utilised for natural regenera- 
tion, and later underplanted with shade-enduring 
species. 
One of the great decisions which a forester is often 
~ealled upon to make is the choice of species for a newly 
afforested area. In making this decision the soil con- 
ditions are of the greatest importance, but unless he 
has had the forethought to take soil samples for a year 
previous he may be quite in the dark as to the extremes 
of drought and moisture to which the area is subject. 
Moreover, in addition he should know the chemical 
constitution of the soil and its physical structure, each 
involving a laborious determination. Yet the ecologist 
could often tell at a glance, from the vegetation already 
present, what were the prevailing conditions and what 
trees would be most likely to succeed. In this con- 
nection I would strongly urge that more attention should 
be paid to the cryptogamic flora, which is often a very 
delicate index of the soil conditions. For example, 
the abundance of liverworts appears to bear a very 
close relationship to the relative proportion of coarse 
and fine particles, being very scanty both in individuals 
and species where clay and silt predominate, and 
becoming more numerous as the proportion of sand 
increases. 
Again, the proximity of the water table may often be 
