1893.] 



A ciion of Light on Bacillus anthracis. 



43 



Fischer (' Beitr. zur Vergl. Morphol. d. Pollenkorner,' Breslau, 

 1890) says that most colours can be found in pollen, and that the 

 pigment usually, but not always, occurs in the exine, i.e., the outer 

 coat of the spore, and not in the contents, a fact very well in accord- 

 ance with the view that the colour acts as a screen, though Fischer 

 regards the coloured oily body as a sticky medium to facilitate the 

 adherence of the body to the insect. 



Among the comparatively rare pollen which is not yellow or orange 

 the following may be mentioned : — 



Pollen is white in Actcea, Bichardia, and other Aroidese, and some 

 species of Iris, as Professor M. Foster informs me. In the last cases, 

 however, Professor Foster adds that the white is a dead one. Actcea 

 s a shade plant, and in Aroidese the rule is for the pollen to be 

 formed and remain in the tube of the spathe, whence these cases 

 may not really be exceptions. 



The red pollens of Lilium chaloedonium and some other Lilies, Ver- 

 bascum, Glerodendron cermum, and G. Thomsoni (Fischer), and some 

 Geraniums, and the red-browns of other Lilies and of JEsculus, passing 

 into deeper browns in some Poppies, are not inconsistent with their 

 being screens against blue light. 



The cases which promise serious trouble, however, are the blue 

 pollens of certain Irises, Epilobiums, some Polemoniaccea, Scilla sibi- 

 rica ; some Poppies (e.g n Pajpaver orientale and P. bracteatum) are 

 almost pure blue, according to Fischer. 



Certain species of Linum, Anemone, Althcea, Agapanthus, Gilia, 

 JEchium, and a few Garyophyllacece are also said to be of various 

 shades of blue or violet, and they appear as witnesses against the 

 above generalisation. At the same time it must be borne in rnind 

 that blue pollen is rare, that we know little or nothing of the 

 spectroscopic and other optical properties of these pigments, and 

 that it may turn out that special peculiarities in the biology of the 

 plant are correlated with this colour. 



Fischer mentions (loc. cit., p. 10) some species of Geranium, 

 the pollen of which appears steel-blue ; this colour is due to the 

 presence of a very volatile blue oil, which rapidly disappears as the 

 pollen is exposed, and leaves the latter dark yellow in colour. If 

 many such cases occur, blue pollen may turn out to be easily 

 explained. 



Other Instances. 



It is obvious, however, that if the foregoing explanation of colour 

 screens is true, it will have a much wider application than I have as 

 yet given it. The following are probably examples in point : — 



Sporangia and spores of all kinds among the vascular Cryptogams 

 are usually some tint of orange or sienna — e.g., Ferns, Lycopodium, 



