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Nr. 1] SUBAERIAL ALGAE FROM SOUTH AFRICA 
of Desmidiaceae, as the tropics, on the other hand, also harbour 
some few algae that especially seem to prefer warmer climates. 
Broadly speaking, there is, accordingly, this essential difference 
between subaérial and freshwater algae, that to the first mentioned 
the climatic conditions, especially atmospheric dampness 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 medium, while the climate is of comparatively secondary 
importance. Thus, among ihe subaérial algae one might lay down 
types of especially tropical and subtropical oceurrenee, as Phy- 
solinum monile (De WirLpeman) PrRINTz, Phycopeltis, and others, 
versus more temperate or arctic species, while the bulk of the 
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 distinet 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 consequence of their 
mode of living are subject to great variations in the conditions of life, 
above all in the amount of humidity. The extreme and uniform 
conditions under which all of them are living, naturally will stamp 
the types morphologically, and the epharmonic convergencee also 
here has evolved growth-forms that are in harmony with the 
natural surroundings. The exceedingly great variations and richness 
in bizarre forms which are to be found among species occurring in 
water — living under more favourable conditions — therefore are 
wanting among the subaérial algae. 
In virtue of just this circumstance, therefore, the subaérial algae 
are rather hand to determine, as systematically quite distinet or 
even alien types often have a morphological likeness, and in many 
cases cannot be diseriminated 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 conserved material, may be very hard to 
separate the various species, except in particular stages, 
especially as the individual species of subaérial algae appear to be 
very polymorphic and varying 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 
together. Thus, there is åa rule an abundance of material at hand, 
whereby the different phases of development are found more easily 
