77 



Only two criteria are to be considered, as being most self- 

 evident: Ist the intensity of the multiplication of the algae, 

 2nd the total increase of an algal-culture within a certain time. 

 It appears less desirable to me to accept a criterion in the ap- 

 pearance of the algae, viz. in the more or less normal or dege- 

 nerate condition in which they are, as it must be very difficult 

 to apply. 



So there remains : tlte mtensiti/ of multiplication (the number 

 of stages of division per 100 green algae, see p. 54) and the 

 total increase of an algal-culture. Perhaps one will suggest 

 that both these criteria come to the same in fact ; but this is not 

 the case at all. The intensity of multiplication gives a waij of 

 measiiring the favourableness of the conditions imder which every 

 alga individually lives, for instance of the feeding milieu (apart 

 from factors which, in short, destroy those algae). The total in- 

 crease (or decrease) of an entire culture, however, is a measure for 

 tJie favourableness of all factors, which possihhj can be of amj 

 influence to the algae. So it may be that, though an alga finds 

 somewhere a favourable feeding milieu, by which it multiplies 

 rapidly, the whole culture is exposed to such a degree to the 

 gluttony of protozoa, that after all the number of algae is de- 

 creasing instead of increasing. 



So it is most important to apply both criteria in our investigations. 



Comparing a sponge (in light) and lake water (in light) with 

 regard to the question we are treating presently, we notice first 

 and premost the enormous difïerence in concentration of the green 

 algae. The sponge is grass-green or dark-green, the water from 

 the lake never has a green tint; the sponge contains numbers 

 of green algae in its tissue (one could take 3 X 10" green algae 

 per litre), the water from the lake only 4000 per litre (p. 28), 

 at the end of July. 



I have stated in chapter VII, that sponges in nature constantly 

 increase their number of green algae by import. So at any rate 

 the accumulation of algae in a green sponge is already partly 

 explained by this mechanical process. But import onl) is not 



