INTERCELLULAR RELATIONS OF PROTOPLASTS. 
63 
contains a considerable, and often very large, percentage of 
water. Under the influence, as it is usually stated, of this 
aqueous content the protoplasm is kept in an expanded state, 
closely applied to the inner side of the cell-wall, and pressing 
upon it. This pressure, under the name of “ The Mechanical 
Theory of Growth,” has been looked upon, from the teaching 
of Sachs, as the great cause of the increase in size of the 
cell.* Under the influence of various water withdrawing 
media the water can in part, though never altogether, be 
withdrawn from the protoplasm, the latter then contracting 
to one side of the cell. This phenomenon, known to all 
observers for at least the last thirty years, had been the 
origin of the name “ primordial utricle.” The outer layer of 
protoplasm, in contact with the cell-wall, Pringsheimf had, 
so early as 1854, shown to be differentiated from the inner 
portion, and had given to it a separate name. In its con¬ 
tracted state Pringslieim had shown that sometimes at least 
the protoplasm remains attached to the cell-wall by proto¬ 
plasmic threads,J and had figured them. Nageli § had 
observed the same phenomenon as the result of the con¬ 
traction of the protoplasm under the influence of sugar 
solution, and had figured it especially well in the case of 
cells in the petals of Dentana dujitata (Taf. ii., fig. 5) and of 
Spirogyra alpina( Taf. iii., fig. 5), in which the threads go only 
to the end walls, while in other cases (Taf. iii., fig. 12) the 
threads are branched. It is to Hugo de Vries, however, that we 
owe the most extended researclies|| into the phenomena of 
protoplasmic contraction under the influence of salt solutions, 
to which contraction he gave the name of Plasmolysis. Using 
as water-withdrawing (or plasmolytic) solution sugar, solutions 
of various salts, especially of saltpetre and common salt, he 
carefully described the effects not only on the protoplasm 
but also on the cell-wall. Of all salts he found solutions of 
common table salt to be the best. Varying in rate with the 
strength of the solution, de Vries found that when under the 
influence of the salt the watery cell sap was withdrawn, the 
protoplasm contracted away from the cell wall (this latter 
also shrinking) into a rounded lump, which he always 
describes and figures as lying free in the cell-cavity, or only 
* I shall have occasion later to discuss the sufficiency of this cause.. 
f Pringslieim, “ Bau und Bildung der Pflanzenzelle,” 1854, p. 4. 
I Pringslieim, l.c., Taf. iii. 
§ Nageli, “ Pflanzenphysiologisclie Untersuchungen,” von Nageli 
und Cramer, 1855. 
Ii Be Vries, “ Unters. iiher die median. Ursachen d. Zellstreckung,” 
1877. - _ 
