Problems in Aquatic Biology. 16 i 
periodically recurring factors, irregular periodicity of factors, which 
may occur at any time, although some are more likely to obtain at 
one season than at another. A brief consideration of these factors 
may be useful and we will first discuss the seasonal factors. Some 
of these act directly, others indirectly. Direct seasonal factors are 
the changes in light-intensity and temperature of water in spring 
and autumn. The indirect seasonal factors are more numerous; 
the most important are the seasonal change in dissolved oxygen 
owing to the seasonal changes in the temperature of the water, the 
increase in the amount of organic substance dissolved in the water 
in the course of the summer as a result of the decay of the cormo- 
phytic aquatics, 1 and of the fall of leaves into the water from the 
surrounding terrestrial vegetation (hence probably the frequent 
marked occurrence of Oscillaria and certain other Cyanophycete 
towards the end of the summer and in the autumn), the periodic 
changes in shading from the surrounding terrestrial and the cormo- 
phytic aquatic vegetation in spring and autumn, the influence of 
periodic changes in the fauna, and lastly periodic changes in water- 
level and in concentration of the salts dissolved in the water, 
wherever there are well-marked rainy and dry periods. All these 
seasonal factors are no doubt the determinants of the regular 
periodicity which is manifest in many cases. This regular periodicity 
is disturbed and often somewhat masked by the irregular factors; 
the more important of these are varying period of maximum and 
minimum temperature, abnormal periods of low and high tem¬ 
perature, occasional spells of drought (involving concentration of 
the water), heavy rainfall (having the opposite effect), periods with 
strong winds, abnormal spells of dull or bright weather, freezing in 
winter, and influence of man. In addition to the seasonal and 
irregular factors we have to distinguish a third set—correlated 
factors. These represent the influence of the diverse constituents 
of the vegetation in a piece of water upon one another and are not 
easy to determine ; some of them are seasonal, others are irregular. 
The most important of the seasonal correlated factors are the 
shading action of the Plankton on the submerged vegetation 
(probably a factor more pronounced in the Tropics than elsewhere), 
and the influence of the disappearance or appearance of one form 
upon the conditions of existence of another. The latter factor may 
sometimes also be an irregular one. As an example of a seasonal 
correlated factor of the former type I may mention the relation, 
1 Cf. Schmidle, Aus der Chlorophyceenflora der Torfstiche zu 
Virnheim. Flora, Vol. 78, 1894. 
