PLANTS AND WATER 131 
even in outline, the actual relations of cause 
and effect. We have as yet no certain or 
definite knowledge of the physical machinery 
of heredity. We do not know why one plant 
reacts in this, another in that manner towards 
an apparently identical set of external con- 
ditions. But we have reasons for believing 
that the difference lies somehow and some- 
where in the obscurities of individual or racial 
character which in turn are dependent on 
differences in physical and chemical consti- 
tution. But as yet we can do little more than 
guess wherein the nature of these differences 
may lie. 
CHAPTER XII 
RELATION OF PLANTS TO WATER 
WE have already become acquainted with 
the manner in which the ordinary land plants 
absorb the water they require. Now water 
plays so significant a part in connection with 
all the principal functions of living things 
