W. J. V. OSTERHOUT AND A. R. C. HAAS 7 



the surface so as to eliminate ripples, or by using in place of the tub a 

 tank with a vertical glass wall against which the tube is held in such a 

 way that the light falls upon the tube after passing through the glass 

 wall. 



The temperature of the bath was kept constant within 1°C. There 

 was no need of more accurate control of temperature under the condi- 

 tions of the experiment. A thermometer was inserted into the tube 

 in many of the experiments.^^ On exposure to sunlight the tempera- 

 ture of the tube rose and remained slightly above that of the bath, 

 but under the conditions of the experiment the difference was almost 

 constant. Since the tissues used were so extremely thin, it is safe to 

 assume that the temperature of the tissues was practically the same 

 as that of the liquid in the tube. 



The tube containing Ulva was allowed to remain in sunlight, 

 with occasional shaking to stir its contents, until a standard shade of 

 pink was produced which matched that of a selected buffer solution 

 (for purposes of matching a clear space was left in the tube above or 

 below the frond) . When the color of the tube was seen to be closely 

 approaching that of the selected buffer solution, the tube was removed 

 for an instant from the bath, shaken, and placed beside the selected 

 buffer solution under the "Daylight" lamp. If the tint was not as 

 deep as that of the buffer, the tube was returned to the bath. As 

 soon as the desired tint was reached the time was noted, the sea water 

 was emptied from the tube, and a fresh sample of sea water (containing 

 indicator) was poured in. The tube was again exposed to sunlight and 

 the time required to produce the same tint was noted. This proce- 

 dure was repeated as often as necessary. 



The experiments were carried out on cloudless days during the 

 month of August. In case clouds interfered with the sunlight at 

 any time during the course of the experiment the whole experi- 

 ment was rejected. At first it was feared that the increase in the 

 intensity of the sunlight during the morning and its decline during the 

 afternoon might affect the results. In order to ascertain whether this 

 was the case the experiments were started at various times during the 



^^ The glass of the thermometer did not give off sufficient alkali to affect the 

 results. 



