104 



of green sponge algae iu liquid from a pressed sponge (Table 4), 

 cannot tell against this possibility of poisoning; for constant pre- 

 sence of those hurtful products of metabolism might always be 

 connected with the life of the sponge ; though then, of course, 

 they cannot be present in great quantity ; what is however not 

 necessary here.) Is there, finally, any reason to suppose, that this 

 „poisonous" influence of the products of metabolism is larger or 

 smaller in a sponge in the light than in the dark? This „poison- 

 ous" influence taken by itself does not give way to answer the 

 question affirmatively; but if one considers it as a reaction of 

 defence of the sponge against an intruder, then there is a reason 

 to suppose, at least to think it possible, that this influence is 

 stronger in darkness than in light. For the following reason : 

 As we shall see on p. 109 — 112, the production of 0^ by the 

 green alga in light is in fact the only function of life of the 

 alga, in which the sponge can have any direct interest. Of course 

 this function is stopped in the dark, and then the sponge has no 

 interest in the life of the alga any more, but probably will have 

 a reason to fight against it as a foreign intruder, and such more 

 vigorously than it would have done in light. As 3»"d cause for 

 the dying of the algae in the sponge cells comes into considera- 

 tion : lack of food, and as 4th cause : lack of O^ and accumulation 

 of C 0., in the algae themselves ; all of them only for algae in 

 sponges in darkness, of course. 



II. Let us now recall to our memory, what facts we have got 

 to know successively about the dying of the green sponge-algae. 



1. Continually algae are dying in the sponge tissue (p. 56 — 57). 



2. The intensity of the dying in all stages of development of 

 the tissue is constant, with a considerable lowering however in 

 the gemmule stage (p. 57). 3. Of the same number much more 

 algae die in a sponge in darkness than in light (p. 60). 4. The 

 intensity of dying in light is about equally high in a weak con- 

 centration of the algae in the sponge tissue as in a strong con- 

 centration ; in the dark, on the contrary, smaller in the first case 

 p. 61). 5. In free nature the number of dying algae (colour- 

 less ones with structure, p. 56) is always very small in a green 



