IXTRODUCTIOX. 



IN a previous paper on Ihe aerial algæ of Denmark (191"), p. 273) 

 I (lelined tlio concej)t »ai'rophilous algæ«. The definition an- 

 swered fairly ^vell lo ils puipose of delimiling a definile l)iological 

 group of algæ in Denmark. However, il had certain drawhacks, 

 and evidenlly did nol cover condilions in olher countries (juile so 

 well. Pnymaly has furlhermore laken exceplion lo the lerm ilself 

 and suggesls the designalion »aériens« for these algæ. I agree with 

 him, and in Ihis work I shall employ the term »aerial algæ«, thereby 

 furlher indicaling thai the delimitation of the biological group here 

 dealt with is nol the same as that employed by me in 1915. In 

 the lirsl place I here include Ihe Cyanophyceæ whelher or nol 

 they form resling spores, and also several Chlorophyceæ which 

 may likewise form special resling stages under unfavourable con- 

 dilions. Il has turned out thai several of the algæ included by me 

 under the true aerophilous algæ in 1915 (e. g. Zygoijonium ericetornm 

 (West and Starkey 1915, F'ritsch 1916), Hormidium flnccidnm 

 (Piercy 1917)) may, in faet, pass inlo more or less pronounced 

 resling stages and so survive unfavourable periods. Many olher 

 green algæ (e. g. Apatococcus lobatiis, Bolrijdiopsis arhiza) may deposit 

 a considerable amounl of fat and olher reserve food-slufTs in dry 

 jieriods, and pass inlo a resling stage wilhout the cell being es- 

 sentially altered. Il is then dilTicuIl to draw the line behveen whal 

 must be regarded as ordinary vegetative cells and resling cclls, hence 

 1 will nol here attempl lo dislinguish between algæ with resling 

 stages and the olher algæ. Whelher or not the Dialoms are capable 

 of forming resling stages we do nol know. The so-called craticiilar 

 forms, which have from olden times been observed in various 

 Dialoms (luinolia. Achnanthes, Melosira Dukiei, Navicnla, Surirella), 

 have, particularly by Ffitzer (1871, p. 103 f.), been regarded as a 

 kind of resling stage, a means by which the species survives un- 

 favourable condilions. More recentlv cralicular forms have been 



