by the Method of Dark-ground I Humiliation . 629 
water to produce a new membrane at every surface ; permeability would 
thus be produced, and the cell-sap partially liberated, through the pressure 
set up by the state of tension of the protoplast ; this would consequently 
contract. It may be argued against this that the cell-sap contains enough 
electrolytes to maintain the vacuole wall, but it may be that both membranes 
together are required to maintain the cell in its normal state of turgor. The 
action of chemically pure distilled water has not been examined as yet by 
dark-ground methods. Hence practically all the argument just given is 
rather a surmise. There remains of course an explanation to be made of 
the action of non- electrolytes such as the sugars and so on in producing 
normal plasmolysis. 
The above short conclusion, although in part quite theoretical, may 
serve to indicate some of the general lines along which progress may be 
made by the study of the plant cell by such methods. Micro-chemistry in 
the plant cell has always necessarily lagged behind chemistry in the 
laboratory, and this must undoubtedly be the case in dealing with micro- 
colloid chemistry. 
It is, however, undoubtedly the case that the method of dark-ground 
illumination permits of a study of certain aspects of the structure of the 
cell, which is hardly possible by other methods. There is no idea that 
the method will ever displace the usual staining processes in cytological 
analysis, but, as has been partially indicated above, it should be extremely 
useful as a supplement to these, to examine the nature and value of fixation, 
the comparative modifications of the hydrosols, and the processes of the 
living cell. 
Summary. 
The method of dark-ground illumination, which has been only 
sparingly used in botanical work, has now been applied to a more detailed 
study of the plant cell, under various conditions. The method often reveals 
new structural features, and is useful in establishing the presence of minute 
particles which are difficult to see, or are unresolved in direct illumination. 
Some of the results indicate possible lines of work rather than completed 
results. 
The method is very restricted in application, on account of the 
difficulty of selecting suitable material for examination. An account of 
the methods used is given, and also descriptions of the structure of various 
types of material studied by the method, and used in further investigations. 
The protoplasm, as is generally recognized, is a colloid complex ; this 
seems to exist both in the hydrosol and hydrogel state, and to a certain 
extent these states appear to be spontaneously reversible. The gel in this 
state differs from that produced by the coagulation of the hydrosol, or by 
the action of a fixing agent on the hydrogel. 
U u 2 
