1916 Hurd; on Growth of Nereacystis 191 



with it at 90°. One half of a piece of Contrast Cyko paper was exposed 

 30 sec. with the water quiet, and then the other exposed for exactly the 

 same length of time with the surface very much agitated by rotary stirring 

 at the edge of the water. The paper was then developed in the ordinary 

 way and in every case that half of the sheet which had been exposed under 

 still water printed darker, showing that a rough surface keeps out some 

 of the light which would otherwise be transmitted. Also, there was great- 

 er contrast between the two when the exposures were made with the light 

 45° from the horizon, indicating that when the sun is below the zenith 

 rough water reflects more light in proportion than at noon. However, it is 

 by no means certain that the difference in the amount of light penetrating 

 smooth and rough water is enough to be a real factor in the explanation 

 of the elongation of kelps in currents. 



The better aeration of the water and larger amount of carbon dioxide 

 available where water is constantly in motion has been suggested as a 

 reason for the distribution of Nereocystis in swift tides and currents, and 

 its absence in quiet waters, noted by MacMillan (18), Frye (10), Setchell 

 (27), Rigg (24), and Peters (22). This factor cannot be the sole one 

 operating, or the kelps in the Argyle tide stream would be very long in- 

 stead of very short; for the water there is thoroughly aerated as it rushes 

 like a river over stones and other obstructions in its bed. MacMillan sug- 

 gests that the preference kelps have for moving water may be due to the 

 fact that when a tide is running the leaves are always submerged, and so 

 are protected from the friction resulting from impact with the surface. 

 It might be added that they are also more protected from exposure to heat 

 and drying. Frye attributes their absence in quiet water to the difficulty 

 of gas exchange which may be due in part to the suspended material de- 

 posited on the plant there. Peters notes the fact that in quiet water the 

 plants are often covered with a slimy coat of diatoms and other epiphytes 

 such as Ulva fasciata. Rigg refers to the demand for gases as bearing 

 on the question and points out the greater supply of oxygen and carbon di- 

 oxide available in moving water, Setchell says that Nereocystis will not 

 grow in quiet waters and "if their accustomed haunts are shut off from 

 full influence of the swells, they perish and are not renewed." He cites 

 a case at San Pedro, California, where a breakwater was constructed. 

 Since Peters has shown the destructive effect of the snails which infest the 

 plants of quiet water and cannot grow on those of swift currents, it seems 

 probable that the presence of epiphytes and parasites may be a large 

 factor in the problem. The unusually fine specimens found in the San 

 Juan channel which are free from those parasites which are so conspicuous 

 on the smaller plants near shore, lends color to this explanation. 



