150 BOTANICAL GAZETTE [FEBRUARY 
a method which cuts off light without cutting off the carbon dioxid. Most 
leaves have all, or most, of their stomata on the under surface, so that if this 
is left free, the upper surface may be covered by screens as closely as desired. 
By means of the broken ring a tinfoil screen, with a pattern cut therein, 
(preferably backed by a thin glass), or a photographic film or similar device 
may be attached to the inner edge of one chamber, this is then applied to 
the upper face of a leaf, with the lower chamber-ring pressing the leaf 
mly against it. The access of light is now shut off from the lower sur- 
face by a simple accessory diaphragm arrangement (not shown in the 
figure), which does not impede gas passage; and such an arrangement 
gives a perfectly logical, and beautifully clear and conclusive demonstra- 
tion of the necessity of light for starch formation. Of course, this end may 
be attained by simpler make-shift devices; the virtue of this instrument 
consists simply in the facility and certainty with which the end may be 
reached. As shown by the figure, a universal joint and a screw-joint 
fitting over any upright support permit the apparatus to be adjusted at any 
desired height, plane, or angle. 
V. LEAF-AREA CUTTER. 
The most striking and conclusive way of demonstrating the funda- 
mentally important fact of increase of organic substance through photo- 
synthesis is by Sacus’s method of comparing the morning and evening dry 
weights of equal areas of similar green tissue; but it is rarely employed 
because of the inconvenience of the manipulation. The leaf-area cutter 
here figured (fig. 4), and described below, is designed to permit all parts 
of this valuable experiment to be performed with exactness and facility. 
The cutter works on the principle of a punch; the steel dies, operated by 
proper handles, cut disks cleanly from a leaf between them, the disks then 
dropping into a perforated aluminum cup screwed below the lower die. 
The diameter of the punch-dies is as nearly as possible 1.128°™"s and 
hence every disk is nearly 1°™? in area. In use the arms of the punch 
are slipped above and below the leaf, when the disks may be cut very 
conveniently and rapidly, in any desired number, care being taken t0 
avoid the larger veins. The larger the number taken the better, since 
local variations in thickness, etc., may thus be compensated, and 100 is a 
fair number. The cup is then unscrewed and covered by its own screW 
cap, which projects sufficiently to allow the cup to hang near the top of 
an ordinary test tube, as shown in the figure. Water in the bottom of the 
test tube is then boiled in the gas flame (a convenient holder for the test 
tube being shown in the figure), and the steam enters the perforations of 
