472 FREDERICK KEEBLE. 
material (Czapek, 1905). But in the cases of the infecting 
cells of C. paradoxa and C. roscoffensis the develop- 
ment of assimilatory pigment appears to be associated with 
the increase in the amount of available nitrogen. And if this’ 
is the case it would seem probable that the colourless phase 
is brought about, not by excess, but by lack, of nitrogen. 
The suggestion is worth hazarding that the colourless sapro- 
phytic stages of such organisms as Diatoms, Euglena, etc., is 
a symptom of nitrogen-hunger. This hypothesis is at least 
as probable as that now prevalent. For, failing adequate 
supples of nitrogen, no amount of carbohydrate-photosyn- 
thesis will avail; indeed, the more the carbohydrate-photo- 
synthesis, involving, as it must do, the wearing out and recon- 
struction of the nitrogen-containing chlorophyll-machinery of 
pigment and plastid, the acuter will be the nitrogen-hunger ; 
whilst, on the contrary, a shutting down of the photosynthetic 
process will economise nitrogen, and so postpone the evil day 
of nitrogen-starvation. ‘Though the facts are not yet avail- 
able for a confident statement, the hypothesis may be 
proposed that saprophytism generally may depend for its 
inception on nitrogen-hunger. It is tempting to push this 
provisional hypothesis to its limits, and to imagine that the 
great saprophytic group of the Fungi owes its origin to the 
changed mode of nutrition enforced upon it by lack of 
nitrogen. Lacking nitrogen, the photosynthetic activity of 
a green cell is greatly reduced or brought to a standstill ; 
the chlorophyll machinery ceases to be worth its upkeep, 
and, wearing out, is too costly in nitrogen to be replaced. 
The organism will obtain directly from its environment as 
much carbon as is of use to it, together with as much nitrogen 
as it can get. It becomes a saprophyte. 
Should this hypothesis be established, C. paradoxa and 
C. roscoffensis will rank high in interest among animals as 
suggesting the route along which far reaching evolution has 
travelled. 
