5 68 
Notes. 
appears not to have been noticed hitherto. It may persist or not, 
thus explaining the occurrence of cell-chains or of isolated cells in 
different races of Yeast. (4) Carbohydrates are stored up in Yeast 
in the form of glycogen, which accumulates or disappears from the 
vacuoles very rapidly, according to conditions of nutrition and growth. 
The colour given by a known quantity of iodine-solution to a known 
amount of Yeast-culture shows these variations most sharply. The 
change of tint by heat after iodine-action, and the destruction of the 
intracellular glycogen by saliva, also give very clear results. 
L. ERRERA, Brussels. 
OSMOTIC OPTIMUM AID MEASUREMENTS. — Recent 
researches made by Dr. F. Van Rysselberghe in the Botanical 
Institute of Brussels have shown that vegetable cells generally answer 
an osmotic stimulus by an appropriate osmotic reaction , and that the 
relation between stimulus and reaction follows, within wide limits, 
the ‘law of Weber/ Hence results the possibility of predicting the 
existence and value of an osmotic optimum. 
Let n be the normal osmotic pressure in a given cell ; 
x the osmotic pressure of an external solution applied as 
stimulus ; 
R the reaction, i. e. the change in the osmotic pressure of the 
cell in response to this stimulus. Then one has, according 
to Webers law : 
X 
R = c log — ( c and ^ being constants). 
The total value of the osmotic pressure in the cell is of course 
R + n, and its excess over the pressure of the surrounding solu- 
tion is, 
y = R + n — x, 
X 
or y — c log — +n — x. 
It is easy to find by differentiation that this excess has a maximum 
value when x — c log e ( e being the basis of the Napierian logarithms 
= 2,7182818 . . .). 
Experiments made with Tradescantia , Symphoricarpus ) Allium , 
Elodea , Spirogyra , agree most satisfactorily with these theoretical 
results. 
Additional interest arises from the fact that these values of x really 
