No. 390.) THE DISTRIBUTION OF MALLOMONAS. — 491° 
were 156 percc. At a depth of fifteen feet they were more 
numerous, but the actual number per cc. was not determined. 
On July 14, after a high wind, the surface water contained 
420 per cc. 
The reasons for the peculiar vertical distribution of Mallomo- 
nas are not wholly apparent, but the reactions of the organism 
to light and temperature offer some suggestions. 
Mallomonas possesses a strong heliotropism. This has been 
shown by laboratory experiment. A brass tube having glass 
ends was filled with water containing 3360 Mallomonas per cc., 
and so placed that one end was exposed to the light, while the 
other was covered bya black cap. After standing for forty-eight 
hours in a horizontal position, portions of the water were care- 
fully withdrawn simultaneously from each end of the tube and 
examined. At the dark end of the tube only 810 Mallomonas 
per cc. were found, but at the light end there were 9480 
per cc. 
From this tendency to move towards the light, it would 
appear that the Mallomonas had tried to get as near as possible 
to the surface where the light was strongest, but that the warm, 
agitated water above the thermocline did not offer favorable 
conditions for growth. During the winter Mallomonas does 
rise to the surface, and is usually more abundant there than 
elsewhere in the vertical. 
Apparently Mallomonas prefers to live where the light is 
strong, where the temperature is low, and where the water is 
quiet. No doubt the long setæ (and possibly the balloon-like 
bodies referred to above) help to keep the organisms from sink- 
ing in quiet water. These become broken off when the water 
is violently agitated, and after that the organisms sink more 
readily. This may partially explain why Mallomonas does not 
develop near the surface during the summer. 
