189 
The Osmotic Strength of Cell Sap. 
Plate IV.— The Root (Figs. 7 & 8). The Leaf Blade (Fig. 9). 
Fig. 7. Transverse section of stele from root similar to that shown in Fig. 3. 
The drawing is bounded by endodermis and pericycle ; d.px., double pro- 
toxylem groups, of which there are three ; s.px., single protoxylcm group, 
of which there are also three, alternating with the three double groups ; 
ph., phloem groups, composed almost entirely of sieve tubes. The centre 
is occupied by the large xylem body consisting of tracheides and very 
plentiful parenchyma, (x 170). 
Fig. 8. Transverse section of rootlet as it passes through the cortex of the 
parent root; o.c., outer cortex; m.c. middle cortex; i.c., inner cortex. 
Compare the relations of the three cortical zones in much older root 
(Fig. 3). Note that the middle cortex here consists of cells with living 
contents, (x 100). 
Fig. 9. Transverse section of the lamina of the leaf ; hyp., hypodermis ; p.m., 
palisade mesophyll ; s.m., spongy, mesophyll ; st., stoma, (x 170). 
THE OSMOTIC STRENGTH OF CELL SAP IN PLANTS 
GROWING UNDER DIFFERENT CONDITIONS. 
By Eric Drabble and Hilda Lake. 
C OMPARATIVELY few observations on the osmotic strength 
of cell sap in plants growing under different physical 
conditions seem to have been made. The most familiar is 
probably Mr. Francis Darwin’s determination for the Dandelion, 
given in his “ Practical Physiology of Plants.” The method 
employed by him consisted in cutting the inflorescence-stalk 
longitudinally and placing strips of the stalk in pure water. This 
resulted in curvature of the strips in an outward direction— i.e., 
the cells occupying the inner face of the strip expanded more than 
those occupying the outer face, owing largely to the mechanical 
arrangement of the tissues in these regions. The curled strips 
were then placed in solutions of potassium nitrate of known 
strengths, and the solution in which the strip retained the same 
degree of curvature was taken as isotonic with the cell sap. This 
method gives very accurate determinations of the strength of the sap, 
but is only applicable in relatively few cases. A method involving ob¬ 
servation of the plasmolysis of the cells can however be very generally 
applied. By this means a number of plants have been examined 
with regard to the strength of their cell sap. A series of solutions 
of sodium chloride was made of the following concentrations in 
gram molecules—TO, Tl, T2, etc., up to *30. Portions of the 
plant the strength of whose cell sap was to be determined were 
placed in pure water. Strips of the epidermis of the leaf were 
placed in a drop of the solution on a slide, and gently covered with 
