THE PLANT AS HEALER 153 
a matter of fact, some experiments are now in progress 
at Cambridge on the effect of different manures. 
_ In America Bailey (3) records the increase of gluco- 
sides in Phaseolus, Zea and Sorghum following upon the 
increase of nitrate manure. Tannin is also affected by 
manuring, none being found in Phaseolus, when chlorine 
is absent, while it is well known that the highest tannin 
yield is obtained from oaks grown on silicious soils. 
Bailey remarks also on the influence of climate on 
flavours—due to essential oils—as on other ingredients 
of plants. : 
An extreme instance of climatic control is furnished 
by the hemp plant, Cannabis sativa. The Indian grown 
plant alone is narcotic, with the result that it was at one 
time regarded as a separate species, but there is no doubt 
that the surprising difference between the supposed 
distinct species C. Indica and the European Cannabis 
sativa is climatic. The modern development in ecolo- 
gical aspects of field work bids fair to help considerably 
in the elucidation of problems of this kind, and the 
analysis of climatic and edaphic factors paves the way 
to that partial control of plant products to be desired 
in the service of man. But there is also another means 
to this end emerging as the result of modern experiments 
‘on heredity. Thus Bailey’s account of the breeding 
experiments with pumpkin and gourd, showing associa- 
tion of the size of the former and bitter principle of the 
latter, suggests that a similar but useful combination of 
robustness and medicinal content might be effected by 
the interbreeding of certain medicinal plants, and in 
fact some progress along these lines has been effected 
in connection with cinchona trees. - 
