1921] SCHERTZ—MOTTLING 117 
STAHL also believes that the products of chlorophyll decom- 
position do not remain in the leaf, but diffuse through the veins to 
the stem. He gave as proof the fact that if the veins of a Ginkgo 
leaf are severed, the chlorophyll remains longer than it does in a 
control leaf. Similar experiments were conducted on the leaves 
of Coleus by the writer, but the leaves yellowed just thesame.* The 
types of leaves are entirely unlike, hence it is not surprising that 
the results were different. Stat believes further that, since the 
yellow pigments (carotin and xanthophyll) consist of carbohydrate 
materials only, they were not needed by the plant and so were 
left behind, but the green chlorophyll pigments which contained 
magnesium and nitrogen were decomposed, and these elements 
were carried away to meet the requirements of the plant. 
Swart has shown that it is unlikely that magnesium is with- 
drawn from the leaf. Table V shows that the plant does not lack 
magnesium, and consequently there would be no occasion for it to 
draw upon its chlorophyll supply for the very small quantity 
which is present in the chlorophyll molecule. On the other hand, 
all the data tend to show that nitrogen is the element lacking, and 
therefore it is quite possible that either the chlorophyll would be 
prevented from forming, or if formed would be decomposed, if the 
law of mass action plays any part at all in the process. 
The situation is summarized by MEYER (38) in the case of 
yellowing of Tropaeolum leaves. As the leaves age they become 
weakened. This weakening of the leaves results in curtailed 
assimilation, which is limited because the chloroplasts become 
smaller and the organs are weakened. .This weakening of the 
organs is a primary cause, while the decomposition of the protein 
follows because of this. Lastly, the decomposition of the protein 
accelerates the yellowing of the leaf. Meyer believes that the 
chlorophyll decomposes and is then borne away, while the yellow 
pigments remain as they were in the leaf, neither increasing nor 
decreasing in quantity. Since he made no quantitative deter- 
minations on the pigments of the leaves, it is easy to see how he 
might have deduced such a conclusion in regard to the yellow 
4On the leaves of Ginkgo, during the summer and autumn of 1920, at Washing- 
ton, D.C., the experiments of Stant could not be confirmed. 
