86 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



From its appearance, the sap might be judged to represent 

 the composition of the vacuoles correctly, as it consists so 

 largely of cell debris. However, numerous experiments 

 have proved that a progressive concentration of the sap 

 is effected by pressure, so that the first liquid expressed 

 is more dilute than the last, whilst what remains in the 

 tissues is even still more concentrated. Accordingly, it is 

 necessary to render the protoplasmic membranes perme- 

 able before applying pressure. To this end the employ- 

 ment of narcotics such as chloroform and toluene was tried, 

 as was also exposure to a high temperature for a short 

 time. All of these methods were found open to objection. 

 Finally it was ascertained that the object in view could 

 most efficiently and accurately be attained by immersing 

 the tissues in liquid air. As an illustration of the study of 

 the equilibrium between the cell and its surroundings, the 

 work of Dixon and Atkins (1913, 3) on yeast is quoted 

 below : 



THE RELATIONSHIP OF THE YEAST CELL TO ITS MEDIUM. 



1. CRYOSCOPIC RESEARCHES. In view of the rapid 

 metabolism of the yeast cell as regards carbohydrates, a 

 study of the osmotic equilibrium between it and the solu- 

 tion which it ferments seemed to be of interest. 



It has recently been demonstrated by Paine (1911) that 

 alcohol penetrates the yeast cell readily, a state of equilib- 

 rium being soon reached in which the ratio of alcohol in 

 the cell to that outside is a constant, deviating only slightly 

 from 0-85. Salts, on the other hand, penetrate to a small 

 extent, the ratio of the internal and external concentra- 

 tions being no more than O'l to 0-25, except in the case 

 of poisonous substances. Indeed, it is an open question 

 how much of this apparent absorption is really due to 

 adsorption on the surface. 



