1898 ] RAMALINA RETICULATA 417 
This explanation of the mechanics of hole formation pre- 
supposes ,the peculiar structure and composition of the thallus 
asa whole and the peculiar structure and mode of growth of 
the apex. Why the apex is consistently narrower and circin- 
ately inrolled, why the gonidia are distributed as they are, 
these are questions not answered by this mechanical explana- 
tion. They remain for further physical and physiological 
examination. 
LELAND STANFORD JUNIOR UNIVERSITY. 
