51 



green algae are constantly imported from the surrounding water 

 in order to keep up their (small) quantity within the sponge 

 tissue (the factor of reduction is normally not acting). Therefore 

 the import is the only factor of increase of the number of green 

 algae in colourless sponges in darkness. 



When treating the factor of import I should mention another 

 remarkable fact. In winter, when the sponge tissue is reduced 

 to the gemmules fixed quite free in the skeleton, several normal 

 green symbiotic algae prove to be sticking to that skeleton; pro- 

 bably caught there in the course of the winter from the flowing 

 water. The gemmules, now, when germinating on their place in 

 the old skeleton — which happens very often — will immediately 

 find some green algae to their disposal. So the rapid increase 

 in number of the green algae in newly germinated gemmules, I 

 mentioned on p. 47 (3), has to be explained. 



Next one might ask if in nature the factor of import does 

 only concern the green algae, or the colourless ones too. The 

 latter would be realized, when also colourless algae occur free in 

 nature. This is certainly the case. Their number, however, always 

 proved to be but very small, compared to that of the green 

 ones ; what stands to reason (conf. pag. 42 — 45). Therefore the 

 factor of import may he neglected as far as the colourless algae 

 are concerned. 



Finally we may accept that the factor of import is equally 

 active in (green) sponges in light as in (colourless) sponges in 

 darkness-^ for the number of the algae in the surrounding water 

 will be equal for both of them in consequence of the continual 

 water circulation, and both sponges will probably possess the 

 same „filtering power" '). 



1) From an experiment it seems to result that the „filtering power" of green 

 sponges in light is greater than that of colourless sponges in darkness. In this case 

 also the import would be greater in the first sponges. We want, however, many ex- 

 periments to answer this question, and for the moment it is of no importance to us 

 (see the note at pag. 68). It will, however, be much more important when considered 

 in relation to a lack of O2, which possibly exists in (colourless) sponges iu darkness 

 (chapt. VIII). 



