Measurement of Rate of Shrinkage of Turgid Tissues . I. 289 
Section IV. Characteristics of the Material used. 
The leaves of onions and the scapes of dandelions were used through- 
out the experiments, since these are readily obtainable, of suitable diameter, 
fairly uniform in structure, and not too rigid. In order to ensure as far as 
possible the supply of comparable material, a number of scapes and leaves 
were marked and kept under observation. An examination was also made 
of their structure to see what tissues were mainly concerned in the plasmo- 
lytic shrinking. It was clear that in both cases the cavity of the interior is 
well lined with thin-walled living cells which would be freely exposed to the 
action of any solution flowing through it. 
A. Onion Leaves. 
Onion leaves were used from plants in their second season of growth, 
i. e. from full-grown bulbs which had been planted some weeks previously. 
The leaves grow most actively soon after they have burst through the 
Fig. 2. Diagrammatic representation of cross 
section of middle region of leaf of onion, pal. 
palisade tissue ; cp. colourless parenchyma ; v.b, 
vascular bundle. x 7. 
Fig. 3. Transverse section of middle region of 
onion leaf, drawn with Zeiss D.D. and 5*5 objective 
(Beck). 
sheath which at first surrounds them. At this time the growth involves 
the entire length of the leaf, but after four or five days it is practically 
restricted to the basal region. This basal region continues to grow slowly 
for a week or more, according to the age of the plant. When all growth in 
length has .ceased, a leaf generally falls over, making a sharp bend in 
the middle or near the base, and though such leaves may live for many days 
they were generally more or less flaccid and were avoided for experimental 
purposes. 
The onion leaf is a hollow cylindrical structure for the greater part of 
its length, but the central cavity is filled with a delicate parenchymatous 
