222 Dr. F. E. Fritsch. On the Suhaerial and [Jan. 2, 



little way behind high-water mark. Towards the end of the dry season even 

 the larger shallow tanks are very much reduced, being surrounded by large 

 areas of meadow-land, marshy in character near the water but dry and 

 bleached beyond. In the deeper tanks {e.g., Mineri, Nuwarawewa) the sinking 

 of the water is not so noticeable, although the bleached algal remains on the 

 exposed stonework of the dams bear testimony to the height of the water in 

 the wet season. The rise of the water-level can take place very rapidly, and 

 the change produced by a single night's heavy rain is remarkable.* As soon 

 as the rains commence the shallower tanks are surrounded by a wide stretch 

 of shallow water devoid of any algal growth ; this latter will, of course, put in 

 an appearance subsequently, but the few tanks that I was compelled to 

 examine in the (beginning of the rainy) season gave quite barren results in 

 this respect. That an abundant growth of Alga? and other water-weeds does 

 often take place in the later portion of the rainy period is evidenced by the 

 decaying remains left behind on the sinking of the water-level in the dry 

 season. Where the irrigation-works are in working order they tend to 

 prevent the water-level from rising above a certain height. 



More important than the actual variation in the height of the water-level 

 are the consequent sudden changes in the concentration of the matter dissolved 

 in the water ; this cannot be without effect on a group so delicately constituted 

 as that of the fresh-water Alga?, and possibly this is a cause for the poor 

 development of the algal flora in some of the tanks. 



Whereas the three preceding conditions (light, temperature, and change of 

 water-level) are practically the same in every piece of lowland fresh-water 

 (especially the last two), we have now to consider those factors which differ 

 from tank to tank and are in all probability frequently the cause of the minor 

 variations in the algal flora ; these are : chemical composition of the dissolved 

 matter in the water, degree of movement in the water, nature of the sub- 

 stratum, and muddiness of the water. These (together with the conditions of 

 illumination) are just the same features as are probably the main cause of the 

 variation in the algal flora of temperate waters, but in the tropics the 

 dominant conditions (light, temperature) are the determining factors for the 

 general character of the fresh-water flora and, consequently, the minor 

 conditions have a rather different basis to work upon. My time was too 

 limited to enable me to collect many data regarding these minor factors and 



* According to Mr. McPhail, the local engineer, the water-level in the huge expanse of 

 Lake Mineri may rise as much as 10 feet in the course of one night. This tank is supplied 

 by a number of mountain streams, which, when the rainy season commences, pour large 

 masses of water into it. At Lake Kantelai I observed a rise of several inches in the 

 course of a single night. 



