PHOTOSYNTHESIS 87 



use as light screens any discs of cork, tin-foil, black paper, etc., applied either 

 to one or to both sides of a leaf, partly because the lighted and unlighted 

 upper surfaces are not otherwise under the same conditions, but especially 

 because there is serious interference with the normal leaf-functions. Screens 

 correct in principle must allow for free access and exit of gases, but in their 

 construction advantage may be taken of the fact that in ordinary leaves the 

 stomata are either largely or wholly on the under surface, so that if this is 

 left free the upper surface may be covered as closely as desired. It is upon 

 this principle that I have constructed the two forms (described in the Botan- 

 ical Gazette, 43, 1907, 277) which are supplied among my normal apparatus 

 (page 46), and which, I believe, offer a logical and very efficient and con- 

 venient method of screening the leaf. The larger (Fig. 18), designed to take 



FIG. 18. NORMAL LIGHT SCREEN ADAPTED TO SEVERAL USES; 



an entire leaf of moderate size, consists of a wooden box readily adjustable 

 for height and angle, 5X7X1^ inches (internal), white without and black 

 within, separated lengthwise into two compartments, with an intermediate 

 space for petiole and midrib. The bottoms of the compartments are largely 

 open, and so matched by diaphragms that air can enter freely, but direct 

 light cannot. Movable gratings of silk threads hold the leaf firmly but 

 elastically against the glass cover, which may be either two separate strips 

 covering the compartments (and therefore the halves of the leaf) or a single 

 sheet 5X7 inches in size. The cover may carry tin-foil, cut with any desired 

 pattern, gummed to its under surface, thus forming a screen in which the 

 light and dark parts are under conditions otherwise practically alike; or it 



