Brown : Algal periodicity 243 







lower part, and in the smaller of the water-works ponds during 

 the greater part of the year. In stream no. I it grew on the 

 stones in the bottom, forming a tolerably dense stratum. A sim- 

 ilar stratum formed on the rocks at the outlet of pond no. 4 when- 

 ever sufficient water flowed over the spillway to keep them wet. 

 Around the edge of the smaller of the water-works ponds there 

 was usually a stratum covering the bottom in the shallow water. 

 Whenever sufficient oxygen collected in the meshes of a mass it 

 was loosened and floated on the surface. 



prince ps (0. Imp 



to be 



rather abundant in the most stagnant part of the smaller of the 

 water-works ponds during the latter part of September, October, 

 and the first of November, 1906. It appeared again the first of 

 June, 1907, and continued to be rather abundant until August. It 

 appeared also in ponds nos. 2 and 3, going through a similar 

 periodicity. 



Oscillatoria limosa ((9. Froelichii Kiitz.) appeared in all the 

 ponds and streams except pond no. 1 and stream no. 2, being 

 especially abundant in pond no. 2 and stream no. 1 from Decem- 

 ber to the first of May. It seems to thrive well in foul water. 

 It seemed to become more abundant in pond no. 2 when there 

 was considerable cow-manure around the edge of the pond. Cattle 



had access to the pond only a part of the time. 



Calothrix parietina (Isactis caespitosa Wolle) was abundant on 

 the stones in the bottom of stream no. 2 the entire year, changing 

 but very little. The conditions under which it grew were un- 

 changeable. 



Conclusions * 



th 



An alga growing under steady normal conditions continues, in 

 e region studied, to grow in a healthy vegetative state through- 



out the year. Nostoc 



P 



Vaucheria growing in stream no. 2 showed this, and also the 

 Myxonetna nanum, growing at the sewer outlet. 



A sudden change in external conditions checks the vegetative 

 growth of an alga and tends to cause it to enter a resting stage 

 form or to fruit sexually. 



For local observers I may state that stream no. I is known as Jordan Branch ; 

 ^ream no. 2, Stone Spring Branch ; pond no. I s Hill Pond ; pond no. 2, Faris Pond ; 

 Pond no. 3, Fees Pond; pond no. 4, Monon Pond. 



