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 



