1916 Hurd; on Growth of Nereocystis 187 



light intensity as the season advances. Combes (4) (Rev. Bot. Centralb.) 

 reports that the optimum light intensity for Cystococcus and Chlorella is 

 much less than direct sunlight, and that Chlorella can live only in weak 

 light. 



The exact mechanism of the changes produced in plants by change 

 in light intensity is unknovpn. It has been shown specially by the work 

 on fungi that they are not to be accounted for through photosynthetic 

 changes. 



Mechanical stretching has also been said to cause superelongation. 

 Jost says that increase in growth as the result of tension is easily proved 

 by stretching a stem with a weight. However he reports Elfving and 

 Schwarz as finding that rate of growth is not affected by increasing or 

 decreasing the centrifugal force acting upon stems attached to a rotating 

 disc. Hus (13) accounts for an unusually long specimen of Porphyra 

 perforata f. lanceolata as being the result of continued pulling in one di- 

 rection by waves. 



The writer's experiments on Nereocystis luetkeana prove conclusive- 

 ly that those plants attached to the bottom grow more rapidly than those 

 floating on the surface of the water attached to a raft, although growth is 

 not affected by the detachment from the natural foothold (8, 28). This 

 was shown by artificially attaching one of two similar series of kelps to 

 rocks and lowering the rocks until the bulbs were about 4.5 m. below the 

 surface of the water. The corresponding series were fastened by their 

 holdfast ends to a raft and allowed to float on the surface. At various 

 intervals the lengths of the stipes were measured and recorded. A com- 

 parison of the rates of elongation of the kelps growing in the two posi- 

 tions (table 1) shows that the more rapid growth takes place in the sub- 

 merged, erect kelp. This same fact might be deduced from observation. 

 All those plants found growing attached to floating logs or docks, lying 

 continually on the surface of the water, were very short, rarely exceed- 

 ing a meter in length even when mature; while specimens submerged just 

 below them had the normal elongation for their depth of at least three 

 meters. 



Table 1. Comparison of the elongation of the stipes of submerged and 



floating plants. 



Submerged plant 1.85 m. in length elongated 14 cm. in 18 days. 

 Floating plant 1.88 m. in length elongated 1.25 cm. in 18 days. 



Submerged plant 1.65 m. in length elongated 40.5 cm. in 18 days. 

 Floating plant 1.35 m. in length elongated 12.0 cm. in 18 days. 



Submerged plant 2.65 m. in length elongated 91 cm. in 30 days. 

 Floating plant 2.71 m. in length elongated 30 cm. in 30 days. 



