SPECTRUM, HELIOTROPIC REACTIONS 27 



The number of experiments was limited but they indicate 

 an optimum between 495 and 450^)", in this respect agreeing 

 with the results on Avena. 



We were anxious to know whether for the heliotropic reactions 

 of sessile animals the optimum is situated in the same region 

 of the spectrum. The number of sessile animals which are 

 sensitive to light is rather limited and we had to make use of the 

 hydroid Eudendrium, which also served in the experiments of 

 Loeb and Ewald. 



II. THE HELIOTROPIC REACTIONS OF EUDENDRIUM 



The newly formed polyps are positively heliotropic to light 

 and they react by bending towards the light. The bending 

 occurs in the region near the stem; the method of procedure 

 was as follows: 



Immediately after the colonies were brought into the labora- 

 tory good stems with from 4 to 8 or more polyps were selected. 

 The polyps were cut off and the stems put into glass troughs 

 filled with sea-water, where they were held in position by being 

 fixed in little holes of a layer of paraffin, on the bottom of the 

 trough. The troughs had plain parallel walls. The stems were 

 exposed during the first day to ordinary light — since light is 

 necessary for the regeneration of polyps^" — and were then put 

 into the dark-room. In the dark-room the polyps developed 

 during the next day. These newly formed polyps are very 

 sensitive to light and were used for the experiment. The trough 

 was then exposed to a carbon arc spectrum, the visible portion of 

 which was about 20 cm. wide. The spectrum was in a dark- 

 room and all precautions were taken to guard against any 

 reflected or other light from reaching the polyps. The stems 

 were in a row and each one was exposed to a different part of 

 the spectrum. The position of each individual poylp was marked 

 in a diagram at the beginning of the experiment and the polyps 

 were exposed to the spectrum for times varying from five min- 

 utes to five hours. Then the polyps were put into the dark again 



'° Loeb, J. Einfluss des Lichtes auf die Organbildung bei Tieren. Pfliiger's 

 Archiv, Bd. 63, p. 273, 1896. 



