624 
BIOLOGY: H. S. WASHINGTON 
In plants, on the other hand, the active metabolic agent which en- 
ables the organism to assimilate carbon from the CO2 of the atmosphere 
is chlorophyll, and it is now well-known through recent researches^ that 
this is a protein compound which contains magnesium as an essential 
constituent. In this connection it is of especial interest to note that 
chlorophyll is very closely related chemically to hemoglobin, the formu- 
lae and apparently the constitution of the two being almost identical;^ 
except that the animal mxCtabolic agent is an iron compound and the 
vegetable agent is one of miagnesium, and that the former contains 
slightly more oxygen than the latter. In this similarity we are reminded 
of the association and mutual isomorphous replaceability of iron and 
magnesium among minerals. 
In plants again, in contradistinction to animals, of the two alkali 
metals, ^'potassium, unlike sodium, is essential to plant life."^ Water 
culture experiments have abundantly proved that, of the cations potas- 
sium, magnesium, and calcium are necessary to plant life, though a 
small amount of iron is requisite to the formation of chlorophyll, in 
which connection the close similarity between this substance and hemo- 
globin becomes of special interest. Sodium is not necessary to most 
plant existence. 
Although of subsidiary importance, the relative toxicity of the several 
cations to animal and vegetable life is of interest, as bearing in the same 
way as the facts set forth above. It must, of course, be understood that 
toxicity is largely a matter of concentration and that we have to deal, 
here very briefly and inadequately, v/ith the relative toxicities of ap- 
proximately similar concentrations of the several elements. It is also 
to be understood that we shall consider only the four cations (Fe, Na, 
Mg, K), which here concern us, and disregard others, as Ca, as well as 
the anions or acid radicals. 
To the animal organism the magnesium salts appear to be mildly toxic, 
or at least are very readily eliminated when ingested into the system, 
which is only true of the iron salts when taken in much greater amounts. 
That potassium is largely replaced in the animal organism by sodium 
has already been noted, and experiments by Loeb^ and others, which 
can be but barely alluded to here, show that to many animal organ- 
isms potassium is distinctly toxic, its ill effects being neutralized by 
immersion in solutions of sodium salts. 
To the higher plants, at least, the iron salts, especially those of ferrous^ 
iron, are distinctly toxic when present in the soil in much more than the 
small amounts needed for its (possibly catalytic) action in the formation 
