82 BOTANICAL GAZETTE [FEBRUARY 
their normal green color. Extracts made from the algae showed 
that chlorophyll decreased while carotin increased in the algae as 
they browned. ARTARI (2) grew algae in the dark and found 
that with an organic source of nitrogen they remained green, but 
with potassium nitrate as the source of nitrogen the algae lost their 
green color. With potassium nitrate as the source of nitrogen the 
colorless algae would regain a normal green when placed in the light. 
Swart (48), working on spring and autumn leaves, found that 
just as the leaves were ready to fall they usually became yellow, 
and this was accompanied by a decrease in protein, nitrogen, 
phosphorus, and potassium. WILLSTATTER (53) noticed that in 
autumn, as the leaves yellowed, their chlorophyll content became 
less. 
Briccs, LyMAN, JENSEN, and McLANE (7) have summarized 
the suggested causes of mottling in various plants. Excess of lime, 
magnesium, organic matter, or some essential element, deficiency 
in lime, iron, organic matter, or some essential element, low humus, 
high nitrogen, inorganic manures, frost, poor drainage, wind, 
sunlight, irregular supply of plant food and moisture, fungi or 
bacteria, nematodes, a filterable virus, and other causes are claimed 
by some to produce mottling. Chlorophyll may disappear owing 
to the absence of some essential constituent in the leaf, or to the 
presence of some deleterious substance. They thought that the 
soil was the cause of the mottling of citrus leaves and consequently 
analyzed it. It was found that mottling diminished as the humus 
ratio increased. Most of the trees which bore mottled leaves grew 
in soil which had a low nitrogen content. They believed that the 
mottling of the orange trees was definitely correlated with the low 
humus content of the soil, for mottling diminishes as the humus 
content increases. Alfalfa and bean straw were recommended 
for use in mulching the citrus trees. 
JENSEN (25) analyzed green and mottled leaves, since it is 
known that organic matter attacks the soil minerals and sets free 
Ca, Fe, Mg, and PO,, because these elements are closely connected 
with the formation of chlorophyll. It was found that badly mottled 
leaves from orange and lemon trees always contained a higher 
percentage of iron, calcium, magnesium, and phosphorus than the 
