47 



extensively in the relict pools of the emerging bottom-lands, a seed- 

 bed for re-stocking the waters of overflow is formed with each declin- 

 ing flood, and this seed-bed becomes potent only when floods return. 

 The absence of an autumnal overflow and the minor part that the 

 autumnal overturning plays in our shallow waters whm 39.2 is 

 passed, may alike tend to suppress here the autumnal or midwinter 

 pulses which occur elsewhere in deeper water. 



The occurrence of the vernal pulse of Asterionella in the last days 

 of April brings it into close relation with the major volumetric pulse 

 of the year (Pt. I., PL IX.-XIL). It is not only an important con- 

 stituent of this spring maximum, but it is one of the most prominent 

 primal sources of food of the Entomostraca Bosmina, Daphnia, 

 Cyclops, and Diaptomus, all of which exhibit an increase in numbers 

 at this period. It shares with Cyclotella the claim to the first place 

 quantitatively among the synthetic organisms upon which the 

 early spring plankton depends for its development. 



Our records are all based upon the catches of the silk net, through 

 whose meshes the isolated cells of Asterionella readily escape. Filter- 

 paper catches give much higher numbers except during the period of 

 maximum, when the numbers by the two methods do not materially 

 differ. This seems to be d.ue to the fact that isolated cells are rela- 

 tively much more abundant after the maxima than they are be- 

 fore them, and especially at the time of their appearance. These 

 diatoms form arcs, circles, or whorls, of a varying number of cells. 

 During the vernal pulses of 1898 the average number in these clus- 

 ters in the middle of March was three or four, and at the time of the 

 maximum on April 26 it rose to five or six, often reaching sixteen or 

 more. A fortnight after this maximum the average fell to 1.4, rising 

 again with the second pulse, on June 14, to 8.4, and declining in three 

 weeks, with the fading out of the pulse, to 1.2. 



Asterionella is frequently infested with great numbers of a minute 

 craspemonad flagellate protozoan which appears in thick-set rows 

 upon the ray-like cells, a single cell sometimes bearing a score of 

 these organisms. This diatom exhibits considerable variation in size 

 and proportions. The longer and more slender cells appear at the 

 times of the maxima. 



Cocconeis communis Heib.* Average number, 520,000, but more 

 than three times as abundant in 1897. This diatom occurs some- 

 what irregularly in the filter-paper collections, and has been recorded 



