to the Plant Cell. 
535 . 
store most of their reserve food in the form of oil. Still more important is 
the work of Sullivan (’05) who showed that, in order to form lipochromes to 
advantage, certain bacteria required the presence of magnesium sulphate 
and a phosphate, preferably potassium phosphate. The presence of rela- 
tively large amounts of magnesium in oil-forming seeds and spores confirms 
what was said on a previous page regarding the necessity of magnesium for 
the assimilation of phosphorus. This necessity is the more evident when 
we remember that the formation of fat is usually preceded or accompanied 
by the formation of lecithin. 
The inability of Vaucheria cells to form oils in the absence of 
magnesium salts seems to prove that the foregoing hypothesis is the 
correct one. It seems possible that this function of magnesium may be 
its most important one. Probably all cells require at least a small 
amount of fatty material for the performance of all their functions. In 
the formation of these fats magnesium undoubtedly plays an important 
part. 
Spirogyra filaments which had grown for five weeks in the absence 
of magnesium were submitted to low temperature in the same manner 
as other cultures to induce cell and nuclear division. The nuclei of these 
cells appeared to respond to the stimulus more slowly than any of the others 
had done. In the cells which showed greatest injury no response what- 
ever was shown. Three hours and forty minutes after being placed in 
optimum conditions some cells and nuclei showed the phase of elonga- 
tion presaging division. It was nine hours after removing the cultures 
from the refrigerator that I found the first cases of actual division. The 
divisions appeared to be normal in every way, and left no doubt in my 
mind that the nuclei of Spirogyra are able, to divide mitotically in the 
absence of magnesium salts. 
V. Discussion of Results. 
Before proceeding further it may be in place to discuss the nature 
of the conclusions which may be legitimately drawn from experiments 
like those related in the foregoing pages. Since many of the methods 
which I have employed are capable of quite general application, it seems 
probable from the results obtained that they can be successfully applied 
to the study of certain types of physiological questions. 
Microchemical methods are observational in character, and, like most 
purely observational methods, can yield only qualitative results. When 
possible, they should be followed by quantitative work. This does not 
mean that qualitative methods of study are of less value ; on the con- 
trary, qualitative methods, being observational in character, are of the 
greatest value in all biological work, and will probably continue so for 
some time. A valuable feature of microchemical methods in physiological 
