43 2 
Fritsch and Rich. — Studies on the Occurrence and 
against the effect of strong sunlight, and no conjugation will take place. 
Temperature has very little effect on the process except to accelerate it. 
We have no data as to the possible effect of the smaller amount of dissolved 
gases except that those species of Spirogyra , which grow in running (well- 
aerated) water, practically never show sexual reproduction. The preceding 
facts give us an insight into the causes leading to reproduction in the vernal 
phase ; these are the increased intensity of the light, the small percentage 
of dissolved salts in the water, the rising temperature, and (?) the decrease 
in amount of dissolved gases. Light is probably the most important, but 
its effect will be qualified by the increasing concentration of the water as 
the vernal period passes on ; and in the case of a dull spring (without much 
rainfall), the water may have attained such a degree of concentration by 
the time the light-intensity becomes adequate, that the latter is no longer 
able to stimulate the Alga in the direction of reproduction. In fact everything 
probably depends on each factor acquiring the proper degree of development 
at the proper time, and in co-ordination with the other influencing factors. 
It seems very likely that F. F. Blackman’s doctrine of limiting factors 1 can 
be applied to algal reproduction as to other physiological phenomena, and 
that in the event of the non-reproduction of a species of Spirogyra in the 
spring, one or more of the complex of influencing factors is limiting the 
process. In the case of a species like .S', varians (Hass.), Kiitz., the com- 
bination of factors necessary for reproduction is usually realized in nature, 
but in other species the reverse is the case (cf. 5 . rivularis , Rabh., in our 
table; see also Petit, loc. cit., p. 27 (S. fluviatilis, Hilse), and Klebs, loc. 
cit., p. 239). 
Our data, as will be seen by a glance at the table, give overwhelming 
evidence of the reproduction of Spirogyra in the vernal phase, and we are 
only able to mention one example to the contrary. From Comere’s and 
Petit’s observations (quoted on p. 425) we gather that matters are very 
much the same in the districts they investigated. Klebs, however (loc. cit., 
p. 244), gives a rather different account ; he says : ‘In der freien Natur 
pflegen viele Spirogyren im Friijahr und Friihsommer zu kopulieren, doch 
ohne irgend welche bestimmte Regel ; man fmdet Zygotenbildung ebenso 
im Sommer, bisweilen sogar im Friihherbst bei sehr sonnigem Wetter.’ 
This statement is not directly contrary to what we have found, but we 
think it unlikely that reproduction of Spirogyra in Germany is not just as 
predominant in the vernal phase as it is with us ; Klebs’s own observations, 
in fact (quoted on p. 428), point in that direction. The phrase ‘ ohne irgend 
welche bestimmte Regel ’ is perhaps therefore not quite exact. That 
zygospore- formation does go on occasionally in the height of summer and 
1 F. F. Blackman, Optima and Limiting Factors. Ann. of Bot., vol. xix, No. lxxiv, April, 
1905, pp. 281-295. 
