76 



stant — for we could deduce several generally appliable rules 

 concerning these factors, by which could be explained the nor- 

 mally occurring cases of changing or remaining constant of the 

 number of green algae in sponge tissue (in light or in darkness). 

 But this fact does not at all exclude the possibility, that these 

 circumstances do change — as an exception. In such cases our 

 rules, deduced for normal circumstances, would not be binding 

 of course. 



In such a way one will have to explain the abnormalities 

 mentioned. When analyzing now the amount of algae present in 

 these cases (Table 6 B), we find: in green sponges in darkness 

 the multiplication to be normal — very slow — but the mor- 

 tality abnormally low ; in colourless sponges in light, on the 

 contrary, an abnormally slow multiplication and a normal mor- 

 tality. By these facts the abnormalities become already more 

 conceivable. 



VIII. The nature of the „symbiotic" association of sponge 

 AND green alga; its use to the alga and to the sponge. 



This part of my investigation has indisputably been the most 

 difficult one. Only after much groping, I succeeded in finding a 

 way out from the labyrinth of facts, which bear upon these pro- 

 blems ; not by lack of points of issue, but even more by their 

 great number, however partly leading — at first sight — in 

 opposite directions. 



I will begin with tlie use tvliich the ^^sijmhiotic''' association 

 offers to the alga. 



In the first place this question: Hoiv shall we state whether the 

 algae are in better conditions in one milieu — eg. a sponge — or 

 in another? We want to state the use of the „symbiosis" to the 

 alga, in other words: if the alga is in a better condition in the 

 sponge tissue than in the water. Which phenomenon on the alga 

 shall we take as critermn? 



