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J. E. WEAVER 



weeds. Of these 30 per cent were dominants with an average 

 height of 2.64 m. while 70 per cent were suppressed and averaged 

 only 1.23 m. in height. The dominants had a diameter of 1.5 

 cm., the suppressed group were only 0.6 cm. in average diameter. 

 In October another quadrat where morie plants had matured 

 was included and the number, height and diameter of the 

 individuals were recorded. These data are shown in the table. 



Assuming that the quadrat with the 192 survivors had as many 

 plants originally as the average of the other three, ^re., 10,500, 

 then the average mortality in the four areas was 98.8%, that is 

 only 1.2% of the plants reached maturity! 



This is certainly a striking lesson in competition. An exami- 

 nation of the table reveals a close relation between the number 

 of individuals and their height and diameter, the plants always 

 being larger (with the exceptions shown in parentheses) where 

 the individuals are fewer. Competition is essentially a decrease 

 of the amount of light, water or solutes available for each indi- 

 vidual. By affecting these factors it affects the response of the 

 plant. That water was an unportant factor was shown by the 

 plants growing in a shallow drainage ditch. They were just as 

 high as those on the adjacent area. Viewed from the tops of 

 the plants one would not suspect a depression was present. On 

 the other hand light was shown to play an important role for in 

 quadrats containing an average of 16,000 individuals of Polyg- 

 onum aviculare seedlings on April 27, 2,700 or an average of 

 17% reached maturity in October. Among these grass-Uke 

 plants competition for light does not occur in any such degree 

 as with the broadleaved A7nbrosia. 



This experiment was repeated by the class of 1916-17 with 

 similar results. The mortality among Ambrosia averaged 95%. 



