48 



9. In the green sponges the colourless algae are generally less 

 numerous than in the colourless sponges — of course one must com- 

 pare tissues in the same stage of development — ; this concerns 

 the colourless algae with structure as well as those without. For 

 the first ones this difference in tiumber is smaller than for the 

 last ones; and for the latter it still increases rather considerably 

 during the develojmient of the sponge tissue^ in order to probably 

 disappear almost entirely during or immediately after the gem- 

 mule stage. 



10. In the green and iti the colourless sponges the number of 

 colourless algae with structure is always much smaller than that 

 of those without structure. 



11. The total number of symbiotic algae jjresent in the green 

 sponges surpasses that in the colourless ones. 



Having stated this, we want to know, what is the reason of 

 alk this {point 1—11). In short, why in nature a {green) sponge 

 in light contains an excess of green living algae and a smaller 

 number of colourless dead ones, a {colourless) sponge in darkness, 

 on the contrary, an excess of colourless dead algae and a smaller 

 number of green living ones ; how both sponge types keep up their 

 „colour' ; and how they arise from each other {pag. 3.5). 



For that purpose we have to examine the factors ruling the 

 nmnber of the symbiotic algae in the sponge tissues. These factors 

 are 6 in number, viz. : 



A. the import of the algae from the surrounding water into 

 the sponge (the factor of import). 



B. the export of the algae from the sponge-tissues into the 

 surrounding water (the factor of export). 



C. the increase of number of the algae in certain parts of the 

 tissues by the reduction or death of other parts (the factor 

 of reduction). 



D. the decrease of number of the algae in a certain sponge- 

 volume by the growth of the sponge (the factor of growth). 



