190 Puget Sound Marine Sta. Pub. Vol, 1, No. 17 



to base of fronds of only 23 cm. The largest one measured was 55 cm. 

 These kelps are mature ; and that they have ceased growing is evident 

 from the dark color and tough texture of the stipe (9). They are kept 

 stretched out nearly horizontal by the rush of water upstream or down- 

 stream, but they do not elongate as a result of it, although the super- 

 elongation of a specimen of Porphyra perforata f. lanceolata was accounted 

 for by Hus (14) as due to the stretching to which the plant was subjected 

 every time a wave rolled in or out. However the explanation for the 

 under-development of the Argyle kelps may be found in the bright light 

 in which they grow since the shallowness of the water means almost un- 

 diminished illumination. The fact that the plants growing in currents are 

 longer than those in quiet water of the same depth may be very easily 

 accounted for as conditioned by light intensity when one sees how much 

 more deeply the bulb of any kelp is submerged when a strong tide is run- 

 ning than it is at slack tide. The current tends to hold the kelp in the 

 direction in which it is flowing. The stipe will take the position in the 

 water which might be represented as the resultant of two forces, the one 

 the current pulling horizontally from the attachment, the other the buoy- 

 ant force of the bulb pulling upward. By taking this diagonal direction 

 the kelp is in darkness or dim light for a much longer time than is the 

 erect kelp in quiet water. 



That more light enters water when its surface is smooth was shown 

 experimentally* by agitating the surface of a tub full of water which had 

 a piece of glass 45x12 cm. in the bottom, and roughly measuring the light 

 coming through by its effect on ordinary photographic printing paper. 

 The source of light was a 40 watt mazda incandescent globe held by a 

 clamp to a piece of iron pipe fastened at one side of the tub and bent 

 over it so as to form an arc, every part of which was 30 cm. from the sur- 

 face at the center of the tub. The tub was 60 cm. in diameter and the 

 water was 22 cm. deep. The whole apparatus was 25 cm. from the table 

 so that the sensitized paper could be exposed directly under the glass. 

 To guard against reflected light affecting the paper, all near-by surfaces 

 were painted black including the tub, and the room was completely dark- 

 ened. The luxometer would not give results because the flickering on the 

 illuminated surface made an accurate judgment of the intensity impos- 

 sible ; so the total amounts of light transmitted through the water in equal 

 intervals of time as evidenced by the darkening of sensitized paper were 

 compared. Exposures were made with the light 45° from the edge of 

 the tub, which corresponded to the sun's position in mid-afternoon, and 



*This experiment was suggested by Dr. F. A. Osborn, head of the physics 

 department of the University of Washington. 



