348 SCIENCE PROGRESS 



greater portion of the light is scattered by the lower cell layer. 

 This at once limits the field of choice ; sections with torn 

 cell walls and escaping contents are generally useless, the 

 scattering effect of these preventing any good observation of the 

 contents of unbroken cells. Unicellular organisms, filamentous 

 Algae, leaves of some water-plants, leaves of some Bryophytes, 

 fungal hyphae, and plant hairs give most of the categories 

 from which selection can be made. There are still other 

 desiderata for good observations to be possible. The diameter 

 of the cell or the filament must not be very small, as if this is 

 the case the diffraction effects produced by the walls greatly 

 interfere with observation of the cell contents. The walls 

 should be free from markings and generally optically homo- 

 geneous, the slightest heterogeneity again preventing satisfactory 

 study of the cell contents. If possible also chromatophores 

 should not be too conspicuous as the}'' also tend to scatter light, 

 though not very strongly in most cases, but their images mask 

 those of some of the smaller particles. 



The scarcity of good material is undoubtedly the greatest 

 barrier to the comprehensive use of the method. Some of the 

 best objects so far examined are : Spirogyra, Mougeotia, Desmids, 

 staminal hairs of Trade scantia, Myxomycetes, 1 the leaf-edge cells 

 of Elodea canadensis, root hairs, hairs of certain flowering 

 plants, 2 and the hyphae of Saprolegnia and other fungi. 



Only one or two filaments of the Algae, a single leaf of 

 Elodea as clean as possible, and so on, should be used to get the 

 maximum light effect. There is no need to use specially prepared 

 " ultra water " for this kind of work, ordinary distilled water 

 being free enough from particles, and in any case it is almost 

 impossible to prevent such from escaping from broken cells into 

 the mounting liquid. 



In many cases the appearance of a living cell when first 

 viewed by this method is undoubtedly surprising, especially if 

 no previous study of colloids by the method has been made. 

 Perhaps it may be said that a little study of the cell in this way 

 serves to emphasise more strongly than ever the fact that the 

 single cell is a system of great activity. This is the case for 

 some cells only, as will be seen below, and obviously we cannot 



1 Gaidukov, I.e. 



2 Price, S. R., "Observations with Dark-ground Illumination on Plant Cells," 

 Proc. Camb, PHI. Soe., vol. xvi. p. 481. 



