Dixon and Atkins — Changes in Osmotic Pressure, Sfc. 221 



was not feasible to determine the weight of the dissolved substance. Attempts 

 were made to remove the undissolved matter by centrifuging, but the speeds 

 then accessible — viz., 2000-4000 revolutions per minute — were insufficient to 

 clear the sap. Subsequently it was found possible to clarify the sap in a 

 centrifuge giving 9000 revolutions per minute. This machine was obtained 

 too late to be utilized in the present research. 



From the foregoing table it will be seen that the osmotic pressure of the 

 sap of the buds rose from February 19th up to March 12th. It seems rational 

 to attribute this rise to the transport of dissolved substances into the buds, 

 and to the solution of previously undissolved bodies in them. 



On March 19th, while the older leaves of the buds were still cohering, the 

 pressure of the sap of the buds taken as a wliole was 11"49 atm. (iA = 0"9o5°). 

 As the buds opened and the leaves expanded it was possible to obtain the 

 freezing-point of the sap of the older leaves apart from that of those still 

 cohering in the bud. On April 6th the older leaves had an osmotic pressure 

 of 9'97 atm. (A = 0"829°), while in the younger leaves still cohering together 

 in the buds the osmotic pressure was 11-60 atm. (A = 0-965^). On April 22nd 

 the older leaves, which were now over 2 cm. long, had a pressure of lO'OS atm. 

 (A = 0'838°), while those from the same buds which were less than 2 cm. in 

 length had a pressure of 11-03 atm. (A = 0-917°). 



The rapid increase in size during the beginning of April was associated 

 with a dilution of the sap, indicating that during this expansion the absorption 

 of water predominated over the accumulation of dissolved substances. At the 

 end of April assimilation and transpiration more than counterbalanced the 

 absorption of water, and the osmotic pressure rose to lO'OS atm. 



From that on, as the leaves grew and matured, the pressure continued to 

 rise, until in June the pressure attained 13-56 atm. (A = 1-128°). Here this 

 series of observations had to be brought to a close. 



Observations on mature leaves of Syringa in the preceding year make it 

 probable that the pressure continues to rise in the leaves during the summer. 

 These observations made on leaves from plants grown in a similar position 

 indicate an average osmotic pressure for the month of September, 1909, of 

 15-45 atm. (A = 1-310°). Such a rise also is to be expected owing to the 

 accumulation of electrolytes during continued transpiration and to the 

 increased efficiency of the leaf in the production of carbohydrates. Towards 

 the end of tlie season it seems probable that the pressure due to the former class 

 of dissolved substances amounts to almost 11-5 atm. — a pressure which was 

 twice found in Syringa leaves after they had been screened from the light for 

 several days, and in wliich it is improbable that any assimilation products 

 contributed to the osmotic pressure. 



