1918 Gail; on Fucus 141 



in place by laying a heavy rock upon each. It was also necessary to cut 

 a small slit in the top of each tub to allow the air to escape while the tide 

 was rising. Two tubs were used, but only one crate of each kind was 

 constructed. 



These shade-crates were placed over mature plants ; two weeks later 

 young plants were also placed under them. In this manner the Fucus 

 plants were grown under conditions as normal as possible, except for light. 

 Since Fucus grows in various latitudes, the slight variation in temperature 

 caused by the shade-crates was considered negligible. It is true that the 

 denser the shade the greater the atmospheric moisture. It is also probable 

 that bacteria are more abundant in darker habitats. On the other hand 

 diatoms, having chlorophyll, are likely more abundant in light. Diatoms 

 are among the most destructive parasites of seaweeds. While, therefore, 

 other factors may have varied some, these were avoided so far as possible. 



The crates were examined every week for nearly six weeks. No 

 change could be noticed until about the end of the third week. At this 

 time the plants growing in darkness had become a darker brown, and de- 

 cay had begun in spots, specially about the edges. The plants under the 

 crates which only l/g of the light penetrated were also darker in color, 

 but no indication of decay could be seen. By the end of the fourth week 

 these plants had also begun to decay. Under both crates, a considerable 

 number of the Fucus plants softened just above the hold-fasts and broke 

 loose. No difference could be observed in the plants under the crates which 

 more than % of the light entered. 



Tabl,e 1. Mature Fucus in natural habitat exposed for 6 weeks 

 to different light intensities 



At the end of the six weeks (table 1) all of the plants under the 

 crates that allowed less than % of the light to pass thru were dead or 

 were no longer growing. The conditions for the growth of Fucus were 

 as nearly the same as possible for all crates, except for the amount of 

 light received. Unfortunately the observations could not be continued 

 for a longer time; more conclusive results might have been secured con- 

 cerning the plants receiving more than 1/g of the total light. The results 

 show, however, that 1/g daylight is roughly the minimum limit for Fucus, 

 unless juvenile plants are more sensitive. 



