156 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



In the bast, wood parenchyma, and medullary rays of 

 woody stems, fairly high pressures may be met with; for on 

 pressing out the sap or on centrifuging the stem after treat- 

 ment with liquid air, the values found are of consider- 

 able magnitude, even though diluted with the contents of 

 the tracheae of the wood. The latter does not approxi- 

 mate to pure water, as is commonly stated, but, owing to 

 the fact that it contains considerable quantities of sugars, 

 possesses an osmotic pressure of from 0-5 to 3-0 atmospheres 

 in the cases examined up to the present by Dixon and 

 Atkins (1915, 1). This will be explained in detail in 

 Chapter XI. 



FRUITS. 



The methods of cryoscopy and electrical conductivity 

 measurements together afford a ready means of studying the 

 series of changes which occurs during the ripening of fruits. 

 Isolated determinations carried out by the author (1910) 

 sufficiently illustrate the remarkably high pressures that are 

 attained in sugary fruits. Some of these are recorded below. 



That even these are under- estimates is proved by refer- 

 ence to Table XXX., p. 118, where it is shown that treat- 

 ment with liquid air to obtain the sap results in a much 

 greater depression of freezing-point being found, even in 

 such a soft tissue as the pulp of ripe grapes. Under M are 

 given the values calculated for the mean molecular weights 



s k 

 of the sap solutes, by employing the formula M = , x - , in 



which s denotes the weight of the solutes in a weight I of 

 solution, and k is a constant. The figures indicate that 

 part of the osmotic pressure must be due to the salts of 

 organic acids, and to the acids themselves, for the presence 

 of large quantities of glucose or fructose (mol. wt. 180) or 

 of sucrose (mol. wt. 342) would considerably raise the 

 average molecular weight. 



