46 MASS. EXPERIMENT STATION BULLETIN 173. 





<5i -5^ 



11 



C ^ 



oq 













OO OOOCCi 

 00 C^ Tt< *C -^ -^ I 



J oo*c o o o o 

 . t-» iC t^ t~* c^ o <-> 



■rJ«tCOCCO(M.— '■— 't— QOCJOOfMO 



OOOOOOCMOOOOOOCO 

 O»0OM40»OC^iC00TfOO(MO 



O-^tM^-^WCOCgN-^M^N^HC^ 



C0C0COC2OG000Q0a0C:>OCb0)C» 



