ORGANIZATION AND GROWTH 213 
polyp is formed sooner than the aboral, we should have to 
assume that the substances necessary for the formation of 
new polyps move in both directions in the Tubularian stem, 
but this movement occurs more rapidly, or earlier, in the 
direction from root to polyp, than in the opposite direction. 
8. I shall in conclusion mention the fact that under one 
condition the polyp may occasionally be formed sooner at 
the aboral pole than at the oral, even when both are subject 
to the same external conditions. This can happen when the 
formation of polyps is retarded, or made almost impossible, 
through external conditions. According to my former obser- 
vations, this can also happen when the pieces of Tubularian 
stem used for the experiment are very short. 
Vy. IRRITABILITY AND ORGANIZATION IN TUBULARIA 
We saw that the physical condition which determines the 
orientation of Antennularia in space has an effect also upon 
the position of its organs; the same relation also exists be- 
tween irritability and organization in Tubularians. Tubu- 
laria mesembryanthemum is neither heliotropic nor geo- 
tropic; light and gravity also have no effect wpon the origin 
of new organs in Tubularia. On the other hand I have found 
that the animal is highly stereotropic; that is, the orienta- 
tion of its organs is very clearly affected by contact with the 
surface of solid bodies. The root of Tubularia is positively 
stereotropic, that is, when brought in contact with the sur- 
face of a solid body it attaches itself to it and grows over 
the surface. Possibly through the slight friction a sub- 
stance is secreted which causes the root to stick. The polyp, 
however, is negatively stereotropic. When brought in con- 
tact with a solid body, it turns away from it until it is again 
surrounded by water, after which it grows in the direction 
determined by the orientation of the polyp. The same 
stimulus which determines the orientation of Tubularia in 
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