by the Method of Dark-ground Illumination. 615 
varies inversely as this. It appears also that particles of apparently the 
same size in the protoplasm move more slowly than those in the cell-sap, 
probably on account of the greater viscosity of the continuous phase of the 
protoplasm. 
§ 6. The Nucleus. 
Only in certain favourable cases has it been possible to study the 
structure of the nucleus by this method, which from the point of view of 
cytological questions connected with the nucleus is certainly disappointing. 
There is, however, still room for much work in this direction with really 
suitable material. The selection of this is even more difficult than of 
material for general observation, but in a few cases moderately clear 
analysis of the resting nucleus has been possible. 
The nucleus has been frequently observed in Spirogyra , Elodea cana- 
densis , and E. densa , and some other cases. A species of Spirogyra with 
a loose spiral and narrow type of chloroplast furnishes the best material, but 
in no case as yet, with this or any other material, has any success been 
obtained in observing the processes of division by this method. Attempts 
have been made to observe the nucleus in pollen mother-cells, but generally 
in the few cases so far tested the cytoplasm is gather dense. Darling (T2) 
has shown that it is possible to observe stages of nuclear division in the 
pollen mother-cells of Acer. Root hairs have also been tried, especially 
those of Trianaea and Mustard. These would not be expected to show 
division stages, but generally nuclei are very difficult to see at all well with 
the method. The nuclei of most Fungi are extremely small, and as yet no 
very clear observations have been obtained, using fungus hyphae. 
The appearance presented by the nuclei of Spirogyra and Elodea will 
be described in some detail. 
The nucleus of Spirogyra (cf. Gaidukov) is suspended at the middle of 
the cell vacuole by cytoplasmic strands, as already described, which merge 
into a thin layer of cytoplasm surrounding the nucleus. As already 
mentioned, microsomes are present in this thin layer of cytoplasm on the 
outside (PL XLII, Figs. 1 and 2). There seems to be no very definite limiting 
layer to the nucleus here, but the cytoplasmic covering is very thin (cf. 
Lawson, Tl) : no movement of particles can be detected inside this layer, 
and the inner portion is nearly clear. The nucleolus, which in some cases 
was first mistaken for the nucleus, appears as a slightly milky looking 
body, sharply marked off apparently by a definite membrane from the 
general nuclear plasma. This boundary has a bright appearance under 
dark-ground illumination, indicating that a really definite membrane is 
present here. The nucleolus seems to contain a gel of very fine structure. 
This is probably to be regarded as chromatin material, and the appearance 
