REIv.\TlON OF STIMULI TO CONE PRODUCTION 175 



The cause and the effect of stimulation are apparent, but the reason 

 why the cause produces the effect is probably not well known by plant 

 physiologists. Hartig assumes that an accumulation of food acts as a 

 stimulus to seed production. This hypothesis has not been proven to 

 be wrong. But this theory does not deny that other stimuli may also 

 cause seed production. 



SUMMARY 



Very little is as yet known regarding the factors which influence the 

 seed production of trees. Botanists have shown that the vegetative and 

 reproductive activity of plants may be controlled to a certain extent by 

 controlling the factors which influence the growth of the plant. 



If the vegetative activities of thrifty western hemlock trees are sud- 

 denly checked by injuries, the tree usually is stimulated to reproductive 

 activity. Factors which commonly stimulate the tree thus are injuries 

 by fire and insects, mechanical abrasions, wind-throwing, decapitation, 

 and girdling. 



