THE frp:sh-water cyanophyceæ of ickland 257 



iodine whilst the sheats in f. typica are rather blueish if so treated, 

 and there is certainly no reason lo suppose that this case is unique. 

 Nevertheless 1 opine, that the introduction of the chlor-zinc-iodine 

 test is a great improvement which will certainly be of great import 

 in the revision of the Coccocjoneæ, that certainly is to be expected. 

 Later Giinther Schmid (1917, p. 342) has partly tested the 

 accuracy of the characters formerly employed and has partly at- 

 tempted to work out a new one. Within the individual species Giin- 

 ther Schmid proves the thickness of the trichomes to be very 

 constant, likewise he cites various observations from pure cultures 

 by Schindler and Pringsheim tending in the same direction. 

 The result of my observations is that in individual growths the 

 thickness is often but slightly varying, but if the same species is 

 obtained from different localities rather important deviations may 

 occur in this respect (cf. Gomont, Monogr. I, p. 287). Crow also 

 (1922, p. 86) adheres to the same opinion with regard to the size 

 of the cells of the Chroococcaceæ. However, there can certainly be 

 no question of various species on this ground, it is rather a case 

 of the individual growths being a "Clon" caused by the vegetative 

 reproduction of a single mother individual. 



I am therefore of the opinion that the thickness of the trichomes 

 is not of so great an importance as a systematic character as Schmid 

 appears to consider. 



It was evident to the old systematists that the characters based 

 on the colour might frequently be doubtful, but nevertheless such 

 characters were usually applied. 



According to our present knowledge there seems to be a number 

 of different external conditions that can change the colour of the 

 blue-green algae viz. light and nutrition. Nadson (1908) demon- 

 strated how direct exposure to the sun could bleach Phormidiiim 

 laminosum so that it assumed a yellowish colour. 



In a series of treatises Gaidukov (1902, 1903 MI, 1904, 1906) 

 demonstrated that the action of different coloured monochromatic 

 lights was able to cause certain blue-green algae (Oscillatoria sanda, 

 O. caldarioriim, Phormidiiim tenue) to change their colours, so that 

 they approached the complementary colour to the monochromatic 

 light used. Similar results were noted by Dangeard (1911) in re- 

 spect to Lynghya versicolor and Boresch (1919) respecting Phor- 

 midiiim foveolarum. But Boresch (1919) as well as Harder (1922) 

 have by means of culture experiments with a large number of species 



