Nr. 1] SUBAERIAL ALGAE FROM SOUTH AFRICA i^ 



of Desmidiaceae, as Ihe tropics, on Ihe other hånd, also harbour 

 some few algae tliat especially seem to prefer warmer climates. 



Broadly speaking, there is, accordkigly, this essential difference 

 between subaérial and freshwater algae, that to the first mentioned 

 the climatic conditions, especially atmosphenic danipness and tem- 

 perature are the deciding factors of their distribution, whereas the 

 freshwater algae are more dependent on the nature of the subsoil 

 and the involved physical and chemical conditions of the surround- 

 ing m.edium, while the climate is of comparatively secondary 

 importance, Thus, among the subaérial algae one miight lay down 

 types of especially tropical and subtropical occurrence, as Phij- 

 soliimm monile (De Wildeman) Printz, Phijcopeltis, and others, 

 versus more temperate or arctic species, while the bulk of tlie 

 freshwater algae, as it seems, are nearly ubiquistous. Also among 

 the subaérial algae there naturally occur species being to some 

 extent dependent on the substratum, especially so among litho- 

 philous forms. Thus there is a distinct difference between subaérial 

 algae from rocks abounding in lime, and rocks poor in lime, as 

 there are also comparatively few species being able to fasten 

 themselves to smooth surfaces by adhesive growth, e. g. the hard 

 and smooth stems of the bamboo, and the like. 



It is apparent that the subaérial algae in consecj[uence of their 

 mode of living are subject to great variations in the conditions of life, 

 above all in the amounl of humidity. The extreme and uniform 

 conditions under which all of them are living, naturally will stamp 

 the types morphologically, and the epharmonic convergence also 

 liere has evolved growth-fonTns that are in harmony with the 

 natural surroundings. The exceedmgly great variations and richness 

 in bizarre fornrs which are to be found among species occurring in 

 water — living under more favourable conditions — therefore ai'c 

 wanting among the subaérial algae. 



In virtue of just this circumstance, therefore, the subaérial algae 

 are rather hard to delermine, as systematically quile distinct or 

 even alien types often have a morphological likeness, and dn many 

 cases cannot be discriminated except in particular phases of their 

 development. These primitive forms are really in many cases so 

 slightly differentiated in point of morphology that it often, and 

 above all in dead and consei"ved material, niay be very hard to 

 separate tlie various species, except in particular stages, 

 especially as the individual species of subaérial algae appear to be 

 verj^ piolymorphic and varving under different external conditions. 

 Howewer, the subaérial algae have this advantage that in one and 

 the same sample usually occur one single or only a few species 

 togethcr. Thus, there is a rule an abundanoe of material at band, 

 whereby the different phases of development ane found more easily 



