398 



HARPER— ORGANIZATION, REPRODUCTION 



the limits of error by any method of measurement I have been able 



to use. If the ordinary semicircular protractor is used and newly 



placed for each reading the results show fluctuations of from i°-3° 



between the angles. If, however, the circular protractor with lines 



marking the angles of 120° is carefully placed so that the three 



angles can, as it were, be simultaneously read, it is at once apparent 



how closely the angles approximate 120°, and that the deviations in 



most cases are so slight as to be practically indistinguishable from 



appearances due to inequalities in the thickness and density of the 



cell walls, middle lamellae, intercellular substances, etc., as shown in 



the photographs. 



TABLE I. 



Angles of Intersection of the Cell Walls in a Selected 

 Colony of P. asperum. 



Higher magnifications making the wall lines bounding the angles 

 longer do not essentially help the situation. I have enlarged to a 

 diameter of about 8 cm. the photograph of P. asperum (Fig. 6) and 

 carefully and repeatedly measured the angles at each point of con- 

 tact for the whole colony. The results are given in Table I. The 

 order in which the angles were read is indicated by the lettering 

 a, b, c, at the points g'^ and d. I have not been able in these meas- 

 urements on P. asperum to distinguish between the angles adjacent 



