Problems in Aquatic Biology. 151 
an algal species are intimately bound up with its reproduction, 
the formation of sexual organs very frequently preceding the 
disappearance or diminished abundance of the species involved (cf. 
the table on p. 164 , and also the chart). Klebs has brilliantly shown 
in the case of a few algal species 1 that reproduction depends on the 
realisation of certain external conditions, but at present no attempt 
has been made at their exact determination in nature; many of 
Klebs’ results depend on conditions which are never realised in 
nature, although many of them may be paralleled. A comparative 
study of the same algal species in various pieces of water shows 
that its period of chief abundance, although frequently coinciding, 
is not always simultaneous ; and the same applies to its reproduction. 
In some Algae no doubt these features (viz., maximum abundance 
and period of reproduction) are influenced by periodically recurring 
factors (such as the rise of temperature and increase in intensity of 
light in spring), so that reproduction and maximum abundance are 
definite periodical phenomena; in other Algae, however, the factors 
determining these features are of a more casual kind ( e.g ., sudden 
rapid evaporation and consequent concentration of the water) and 
consequently they do not evidence so regular a periodicity. 2 The 
only method of precisely determining the time of reproduction of 
different algal species and at the same time of obtaining accurate 
information on the natural phenomena which precede and are the 
cause of reproductive activity, is that of prolonged periodical 
observation of the aquatic flora of large numbers of different pieces 
of water, com'bined with careful notes on the prevailing external 
conditions. The data thus obtained can then be confirmed by the 
experimental method, introduced by Klebs, which will form a valuable 
auxiliary to observations in nature. The method of periodical 
observation, however, not only enables one gradually to determine 
the factors influencing reproduction and relative abundance in 
different algal species (biological factors), for if we take the external 
conditions influencing any piece of water collectively we obtain an 
idea of the factors determining the character of the algal vegetation 
as a whole (ecological factors). The preceding lines merely indicate 
1 Die Bedingungen der Fortpflanzung bei einigen Algen und 
Pilzen, Jena, 1896. 
2 Thus in the case of the algal flora in the pond at Telscombe 
which I am using as an illustration in the present paper 
Colcochactc scutata and Cliara sp. are forms in which repro¬ 
duction is a definite periodic phenomenon, whilst in Oedogonium 
invcrsum this is not the case. Oscillaria tenuis affords an 
example of a form in which maximum abundance is definitely 
periodic (cf. chart on p. 160). 
