]\Ir John Rattray on Evolution of Oxygen by Sea- Weeds. 245 



could be shown to occur in this connection among the 

 different species, and also consequently for the purpose of 

 arriving at some approximate data on the efficiency of the 

 several species in aiding the waves and other movements of 

 the sea in oxygenating the ocean waters. 



The method of work adopted was as follows : — A small 

 number of cylindrical glass jars of moderate dimensions 

 was procured, these selected measuring about 4| inches 

 in diameter and 9h inches in height. Their lower 

 extremities were left permanently open, while the upper 

 end of each, which was gradually attenuated to form a 

 neck, was closed by an india-rubber perforated stopper, 

 through which a collecting tube, graduated to cubic centi- 

 meters, was passed. The diameter of the neck of each jar 

 measured Ih inches, and the height of the collecting tube, 

 though not always constant, averaged about 7 inches, its 

 diameter being half an inch. The large size of the jars 

 was found to be essential, in order that weeds of consider- 

 able magnitude might be selected for experiment and kept 

 in their normal conditions. 



In all cases the specimens chosen were left attached to 

 the substratum upon which they grew, so that their rhizoids 

 as well as their more expanded thalli were invariably intact. 

 Where the attachment was, as sometimes happened (e.g., in 

 the case oi Fucus canaUculatus), upon rocks, the part of the; 

 rock upon wiiich any given plant was fixed was removed 

 at a distance of a few inches round tlie specimen by means 

 of a chisel, and no specimens were used in which the sand- 

 stone was fractured in the immediate vicinity of the point 

 of attachment of the plant. The exclusion of other Algfe 

 except members of the species in question was in most 

 instances attained by selecting from what might be called 

 exclimve QXQ'A?, ; that is, from places upon which one species 

 grew to the exclusion of all others of the same or of 

 different genera ; but in a few cases in wliich it was found 

 impossible to secure such conditions, and in which rounded 

 stones of small size, but bearing a good growth of a given 

 species were made use of, these stones were carefully 

 examined by a lens and rejected when a mixture of 

 organisms occurred. 



The specimen once selected, was now washed careful]}- 



TRAXS. BOX. SCO. VOL. XVI. R 



