THE AÉRIAL ALGÆ OF ICKLAND 329 



observed by O. Muller (1899, p. 282) in various genera and species 

 in highly saline pools and springs in Egypt, and by Krasske 

 (1927, p. 270) in highly saline springs in Germany. In both cases 

 the salinily varied greatly, and Krasske thinks that craticular for- 

 mation occurs when the protoplast contracts away from the wall 

 in consequence of a strong concentration of the salt solution, hence 

 in a plasmolysed state. Geitler, too, observed craticular formation 

 in Anomoeoneis sphærospora (1927), but gives lack of nourishmenl 

 as the cause of the malformation. From the data there can hardly 

 be any doubt that the craticular forms are teratologic formations 

 produced by the cells, in a plasmolysed state, forming new valves 

 in the same way as the plasmolysed cell of a phanerogam can form 

 a new cell wall round the plasmolysed protoplast. In the aerial 

 Diatoms craticular forms are uncommon. In Iceland I have only 

 once observed such forms in Hantzschia amphioxijs. The craticular 

 forms cannot therefore be regarded as a kind of resting spores, they 

 must more probably be looked upon as teratologic formations arising 

 under unfavourable conditions. 



As aerial algæ I will then define all algæ that do not grow in 

 water, or which at any rate in periods are able to grow without 

 being immersed in water, even if they pass into a resting stage 

 during protracted periods of desiccation. 



