RELATION BETWEEN VEGETATIVE VIGOR AND REPRODUCTION 535 
parts in the mat have taken on something of a resting condition. 
If such a mat is cut up and the pieces placed in another solution, new 
growth must take place in that solution before sporangia or oogonia 
can be formed, but if the mycelium is used earlier the hyphae not only 
grow but respond at once to the stimulus of the new environment by 
the production of appropriate organs. It was found that the most 
advantageous stage at which to use a mycelium was just as the mat 
started to form, for at that stage masses of activeh^ growing h3/phae, 
all in uniform condition, can be readily secured. 
Temperature. — Temperature is a factor of the utmost importance. 
Klebs states that a temperature of about i8 degrees Centigrade is the 
optimum for 5. mixta, but this is certainl}^ not true for S. ferax. It 
has not been possible to determine the optimum for each of the 
various species studied, but enough was learned to warrant the state- 
ment that the temperature requirements of the different species differ. 
In Achlya racemosa a high temperature favors the production of 
oogonia and inhibits the formation of sporangia, while the tempera- 
ture at which oogonia are produced with certainty by S. ferax is in 
the neighborhood of 15 degrees. S. monoica seems to do about as 
well at 18 degrees as at 15 ; but at 20 degrees or over there is a distinct 
decrease in the number of oogonia. Achlya prolifera seems to prefer 
a temperature of 18 to 20 degrees; at lower temperatures as well as 
at higher, 22 to 24 degrees, the production of oogonia is uncertain. 
Unfortunately it was not possible to determine the optimum tempera- 
tures from lack of apparatus by means of which constant low tempera- 
tures could be maintained. For the purposes of the present work, 
however, it was sufficient to maintain the temperatures near the 
optimum for each species. It is interesting in this connection to note 
that Peterson ('10) in his study of the Danish freshwater Phyco- 
mycetes states that A. racemosa produces oogonia with difficulty, and 
only at low temperatures. In the writer's experience A. racemosa 
produces oogonia very readily and particularly so at relatively high 
temperatures. 
Method of expressing the number of oogonia. — Klebs, in comparing 
the value of different substances for the production of oogonia ex- 
pressed the number of oogonia by the Roman figures I, II, and III, 
the relative number of oogonia present being in that order; Kauffman 
and Obel merely stated that few or many oogonia were present, while 
Horn ('04) followed the precedent set by Klebs. Such a method 
