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



Two conditions of growth which were different in the two environ- 

 ments were tension in the stipe of the plant produced by the buoyant force 

 of the pneumatocyst, and light intensity. To determine if either is a 

 controlling factor in elongation it was necessary to experiment first on 

 the effect on the rate of growth of various tensions in the stipe, the light 

 intensities remaining equal ; and second, on the effect of different light in- 

 tensities, the stretching of the stipes being equal. 



The different tensions in the stipe were secured by weighting the 

 holdfast end with rocks of various sizes. The head of the kelp was at- 

 tached to the raft by means of a padded wire ring just below the bulb. 

 A cord was slipped through this ring and tied to a staple on the raft, thus 

 allowing the kelp to hang in the water with the whole weight of the rock 

 at the lower end pulling on the stipe. The loose wire rings were used 

 instead of muslin strips which were tried at first, because of the rapid 

 decay under the cloth when in intimate contact with the stipe. The ring 

 touched the stipe in only one place, and although eventually it began to 

 cut in, decay was much less rapid. The bulbs and fronds were all about 

 5 cm. below the surface of the water. The weights pulling downward on 

 the stipe produced tensions equal to those which would be produced by 

 the upward pull of bulbs whose buoyant force equaled the pull of grav- 

 ity on the rocks. 



The kelps in each series of experiments were chosen as nearly alike 

 as possible as to age, length, extent of growing region, condition, and 

 number of fronds. One kelp in each series was weighted with a small 

 rock of about 200 grams which was carefully chosen to balance the up- 

 ward pull of the bulb as nearly as possible. Other plants were stretched 

 by weights of 700 grams and still others by as much as 2,000 grams. It 

 was expected that those stretched by the heavier weights would elongate 

 most rapidly, and that those which were not stretched would grow more 

 slowly. However, data from a sufficient number of experiments was se- 

 cured to show that there is no relation between rate of growth and me- 

 chanical stretching in the stipe, for those plants in which the pull of the 

 bulb was just balanced by a small weight elongated as rapidly as those 

 stretched by the heavy rocks, and in some cases even more rapidly. 



A second set of experiments had for its object the determination of 

 the effects of darkness and of bright light on the rate of elongation of 

 similar kelps, the tensions in the stipes being equal. Equal tensions were 

 secured by picking out plants for each series whose bulbs by virtue of 

 their equal size were of equal buoyant force. By sinking one kelp of 

 each series about 4.5 meters and the other 5 cm. below the surface, the 

 stimulating effect of darkness was demonstrated. Both kelps grew in 



