INTRODUCTION 



155 



culture of the freshwater alga Spirogyro, marked increase in growth occurred when 

 ice water had been added to the culture medium, and H. W. Harvey (1933), working 

 with cultures of Nitzschia closterium, has shown that the addition of ice water and the 

 consequent increase in the proportion of trihydrol greatly stimulated growth. 



In considering periodicity it is interesting to note that the enormous increase in 

 diatom production is observed in areas which are under the influence of melting ice, and 

 it is probable that the extreme cold of the high latitudes maintains a high proportion of 

 trihydrol in the oceans which exerts a profound influence upon the phytoplankton. 

 Unquestionably the chemical and physical factors of the environment control the extent 

 and distribution of the algal flora and play an important part in the variation and evolu- 

 tion of the species. 



The samples were obtained by making vertical hauls with a plankton net through 

 depths of either 50, 100, or 200 m. The net most frequently used in the collection of 

 phytoplankton has a diameter of 50 cm., and is made of the finest bolting silk having 

 200 meshes to the linear inch. The net tapers to a diameter of 6 cm. where it joins a small 

 canvas collar for attachment to the collecting bucket, which is secured by a brass band 

 and a tightening screw. The entire length of the net is 5 ft. 5 in. A detailed description of 

 this net and the methods of working are given in Discovery Reports, 1, pp. 182-4 ( x 9 2 9)- 



The samples collected were preserved either in \-\b. glass jars fitted with metal screw- 

 caps or in a number of glass tubes plugged with cotton-wool and tissue paper. The 

 tubes, in numbers varying from two to ten, were placed in a large glass jar and packed 

 with absorbent cotton or tissue paper to prevent breakage. The material was preserved 

 by the addition of a solution of formalin, in proportions which varied with the density 

 of the catch. 



The following table, which shows in parts per cent the variation of the populations 

 found at 15 stations on the 30th W meridian, from 57 36' S to 14 27^' N, is taken from 

 Hart (1934). The enormous preponderance of diatoms in the polar waters and their 

 scarcity in tropical waters are clearly demonstrated. 



