PELAGIC PLANT LIFE 



317 



secretion pores. These filaments act as an effective suspen- 

 sion-apparatus (see Fig. 219). During unfavourable conditions 



Fig. 218.— Chain of Ch.f.toceras decipiens ('f-"). 



of existence, especially when there are considerable changes in 

 the salinity, sufficient mucilage is secreted to form a protecting 



Fig. 219. — Chain of Thalassiosira gravida (-f")- 

 Showing on the right five cells with filaments of mucilage. (Mangin. ) 



sheath round the cells. This I have myself observed in the 

 case of species of Thalassiosira on the Norwegian coasts. 



Adjustment of their organisms to the conditions of their 



