234 Brown : Algal periodicity 



crystallizing dish in which there was a small amount of Cladophora 

 growing. By filling the vessel with water every few days the des- 

 mids were kept growing for several months and a careful study 

 was made of them. The vessel was kept in the greenhouse where 

 the air was damp and the temperature moderately high and steady. 

 At certain times these desmids were found to divide so as to repro- 

 duce themselves vegetatively much more abundantly than at other 

 times. Different causes seemed to induce this. If the water be- 

 gan to get low in the vessel so as to make the solution more dense 

 and richer in salts, division appeared to be more active ; if placed 

 on agar saturated with normal culture -solution, they divided more 

 abundantly ; if placed in a small crystallizing dish on the oven, so 

 as to change the temperature suddenly to a relatively high tem- 

 perature, 26 C, in 48 hours, one half of the number were in the 

 process of division. A sudden change, or hard conditions seemed 

 to induce cell-division. The cells that divided on the agar prepara- 

 tion, and the ones that divided in the vessel on the oven did not 

 develop into two healthy, full-sized specimens ; the ones in the 

 vessel kept in the greenhouse required from six to eight days after 

 cell-division could be first noticed for the daughter semi-cells to 



o 



reach full size. 



Experiments similar to the above were tried with Closterium 

 acerosum, with similar results. 



A very few specimens of Cosmarium were present in all the 

 ponds and streams throughout the year, but in no case were they 

 found in any abundance. On April 30 a few Cosmarium zygo- 

 spores, were found in pond no. 1 . Docidium Trabecule was rare 

 in pond no. 3 during October and November, and a few specimens 

 were obtained from pond no. 2, January 14. Hyalotheca and Staur- 

 astrum were present in the spillway of pond no. 4 in very small 

 quantities in December. A few specimens of Penium intemiptum 

 were found in the lower part of stream no. 1 during June, 1907. 



This species appeared in the water-works reservoir about the same 

 time. 



The Oedogoniales ranked second in abundance, considering 

 quantity, and they were rather generally distributed in the ponds 

 and streams under observation. Oedogonium and Bidbochacte, rep- 

 resenting the two genera in the family Oedogoniaceae, were found, 

 Oedogomum being quite common and Bidbochacte rather rare. 





