PO Le eT 
1909] BRIEFER ARTICLES 225 
for starch formation, it was found best to use some type of screen such that 
a sharp contrast would show between the light and dark parts; and for this 
purpose light-screen boxes were used. .These boxes have been fully 
described by Professor GANONG in the Botanical GAZzETTE.? Briefly, 
they are small boxes made of white paper blackened inside, with a network 
of threads across the top to support the leaf, and holes near the bottom 
to allow the air to pass through freely. A glass plate covered with tinfoil 
having a pattern cut in it is held closely against the network by a wire 
spring. When in use the leaf is held between the glass plate and the net- 
work. The principal advantage of these screens is that while excluding 
all light, they allow nearly the normal access of carbon dioxid to the leaf. 
The caution, by the way, against using screens which cut off all carbon 
dioxid as well as the light has been made several times in recent years, but 
some of the new elementary textbooks are still copying the old and erroneous 
method of putting cork or tinfoil on both sides of the leaf. Several light- 
screen boxes were attached to the leaves which had previously been emptied 
of starch, and the plants were placed in strong diffuse light. Leaves were 
then taken off and tested for starch at 10-minute intervals. In order to 
compensate the effects of individual peculiarities, 5 plants of each species 
were tested. The results given in the table are for full-grown (except in 
: € three cases noted), but not mature, leaves on actively growing plants, 
M pots, in a greenhouse. In the first column is given the time in darkness 
Tequired to empty the leaves of starch; in the second, the time in diffuse 
light required to. make enough starch to show a pale but clearly defined 
figure with the iodin test; in the third, the time required to show a sharply 
defined, dark figure; while in the fourth is given the time required for the 
lodin to produce its full effect. 
The best leaves, obviously, for this study are those in which photosynthe- 
Sis is most active, from which starch disappears most rapidly in darkness, 
from which the chlorophyll can be extracted quickly leaving the leaf white, 
and which give the iodin reaction quickly. As shown by the accompanying 
table, these are Pelargonium hortorum zonale, Fuchsia speciosa, Senecio 
mikanioides, Impatiens Sultani, and young plants of Helianthus annuus, 
Ricinus communis, Phaseolus vulgaris, Zea Mais, and Cucurbita Pepo. 
On the other hand, some leaves are not good for this study. Begonia 
palmata, Oxalis Bowiei, and Pelargonium peltatum, when boiled to swell 
the Starch, partially disintegrate, so that the figure does not show clearly 
with the iodin test. It is possible, of course, to apply the iodin test without 
Previous boiling of the leaf, but it takes 24 to 48 hours according to the 
* Bor. Gazerre 432277. 1907. 
