79 



green algae, ciiltivated in light in water from the conduit (so 

 excluding import), soon show a larger amount of green algae 

 than the water of the lake ever possesses. 



It is rather well conceivable, although not quite sure, against 

 what the sponge protects its algae. A priori one supposes this 

 protection to be against protozoa and other small fresh-water 

 organisms which will swallow the algae. Also in my cultures I 

 have got an indication for this, as shown in Table 9 B. There one 

 sees, among others, an algal-culturo in light in small concentra- 

 tion in water from the lake, which the 2'^d week consisted of a 

 ihin green membrane surrounded by a (newly formed) darker 

 green rim. During the Srd week, however, first the rim and next 

 also the membrane suddenly begins to disappear. Now, numerous 

 protozoa prove to have grown in the culture ; among which there 

 are some filled with the green sponge algae. About a week later 

 the whole algal-culture has almost been destroyed ; numbers of 

 degenerate algae remained, while the protozoa disappeared for 

 the greater part. 



Besides, there exists also a means of protection for the sym- 

 biotic algae within the sponge body against other enemies, not 

 enemies which will swallow them, but which cause their destruc- 

 tion in some other way : diatoms and ordinary algae. Above 

 (p. 43, 45 and Table 5) I treated already, how the sponge algae 

 were killed by an infection of diatoms or algae arising in the 

 culture. I do not venture to decide, to what this influence must 

 be ascribed ; one might suppose to (lack of food or to) poisoning 

 by products of metabolism, especially as the same thing happens 

 when mould-infection occurs (p. 43). It is obvious, now, that this 

 influence of the infecting algae and diatoms will be strenger in 

 light than in darkness; which, moreover, appears from Table 4. 

 So it might be possible, that a culture of symbiotic algae in 

 water and in light, with of course a high intensity of multipli- 

 cation, has in fact less increased after some time — by the hurt- 

 ful influence of luxuriantly growing algae and diatoms — than such 

 a culture in darkness, with but a small intensity of multiplication. 

 In Table 10 there are indeed some of such cultures to be seen. 



