568 
ADRIAN J. PIETERS 
When only one food, peptone, was offered in varying concentrations 
there was a steady increase in the dry weight with the increase in the 
concentration of peptone but the mycehum out of 0.2 percent, 0.5 
percent and i percent produced practically the same number of 
oogonia. See Series II, Table XIII and Series VIII, Table XV. 
When sugars or sugars and salts were added to the solution the absence 
of any fixed relation between weight of mycelium and the number 
of oogonia was even more marked. This does not mean, however, 
that a mycelium need not be vigorous in the sense in which this term 
seems to have been used by Klebs, Kauffman and Obel. They evi- 
dently meant that a mycelium must have been well enough nourished, 
and that a poorly nourished mycelium was to be guarded against in 
experimental work. It is very clear that a fungus can grow in a 
solution that will nourish too little to enable it to reproduce well; 
such a solution is to be considered as being below the minimum con- 
centration necessary for satisfactory growth. For 5. ferax a peptone 
solution containing o.i percent peptone, with or without sucrose 
provided none of the sucrose has been inverted, represents such a 
sub-minimum concentration. The minimum concentration for the 
production of a ''well-nourished " mycelium may vary with the species. 
For 6'. monoica o.i percent peptone is enough to produce a well nour- 
ished mycelium if the number of oogonia produced be accepted as 
determining whether or not a mycelium has been well nourished, 
while S. ferax needs a stronger solution. Solutions containing nutrient 
substances at higher concentrations than the minimum will enable the 
fungus to produce an increased yield of mycelium but not a propor- 
tionate increase in the number of oogonia. Above the minimum 
concentration an increase in the number of oogonia will only be 
secured by changing the quality of the nutrient solution. 
The Effect of a Given Environment May Not Become Evident Until the 
Plant Has Been Transferred to Another Environment 
Nutritive substances do not, however, all have the same importance 
for the development of the different parts of the plant. Pfeffer ('97) 
states this in general terms when he says (Oxford Ed., 1900, p. 387) 
"the importance of a substance to a plant is not to be measured solely 
by the amount of growth which it induces." 
While there is as yet no evidence that new species can be produced 
by changing the conditions, we can safely say that a plant is the 
