DARK-GROUND ILLUMINATION 349 



postulate a similar organisation and structure for cells of all 

 types. 



Spirogyra is undoubtedly one of the best and most easily 

 obtained objects, and this part of the subject can hardly be 

 introduced better than by a short description of the general 

 appearances presented by the cells under this type of illumina- 

 tion. A species of rather large diameter with a fairly loose 

 spiral chloroplast is most suitable, but any species of not too 

 small diameter will suffice. 



As is well known, the protoplast forms a layer lining the 

 wall of the cell ; in this layer is the chloroplast, while the nucleus 

 is suspended in the central vacuole by cytoplasmic threads. Under 

 dark-ground illumination the protoplasmic layer is seen to 

 contain large numbers of small particles, manifested of course as 

 bright points of light, and in the living cell exhibiting a constant 

 oscillating motion, generally about a small orbit. As is well 

 known, the protoplast in direct illumination appears as practic- 

 ally homogeneous. These particles (which are probably to be 

 classed as sub-microns x ) can be brought into focus above, that 

 is outside the chloroplast, so that without doubt they are actually 

 in the protoplasm. So also these particles can be seen in the 

 cytoplasmic threads which suspend the nucleus, where they also 

 show this oscillatory movement. 2 More careful study, and the 

 examination of plasmolysed cells, 3 reveal the presence of smaller 

 particles in the protoplasm, which are undoubtedly completely 

 ultramicroscopic. 



On focussing below the upper part of the chloroplast, that 

 is to say in the vacuole, particles of much larger size can usually 

 be observed also in oscillation. These particles can often be seen 

 on careful examination of the cell in transmitted light, and they 

 are obviously of quite another order of magnitude. Gaidukov 

 thinks that they are particles of some colloid nature in the cell 

 sap. Such particles seem to occur quite frequently in the sap 

 vacuoles of plant cells, and on account of this they may be 

 referred to as " sap particles " or " sap inclusions." 



The chloroplast shows little detailed structure, giving rather 



1 The terms are generally thus applied : Microns are small particles visible 

 with direct illumination in the microscope. Sub-microns are ultramicroscopic, but 

 may be made visible by methods of dark-ground illumination. Amicrons are 

 below the limits of observation. 



2 Price, loc. cit. 



3 Price, from unpublished work. 



