WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 17 



Since in most land-plants the leaves are chiefly or wholly 

 on the lower surface, it is sufficient for rough purposes to 

 use a screen that shall allow free access and exit of gases 

 on the lower side, and 

 the " Normal Light 

 Screens " devised by 

 Prof. Ganong, and sup- 

 plied by the Bausch 

 and Lomb Optical 

 Company, are con- 

 structed on this prin- 

 ciple. 



The larger screen 

 (Fig. 42) is especially 

 useful ; in the figure 



it is shown fitted with Fig. 42. Ganong's Light Screen (large form). 



five tubes of coloured 



liquid (see 245). The screen consists of a box adjustable 

 for height and angle, black inside, adapted to take a fairly 

 large leaf. It is separated lengthwise into two compart- 

 ments, with a middle space for petiole and midrib. The 

 bottoms of the compartments are largely open but pro- 

 vided with diaphragms so that air can enter freely but 

 direct light cannot. Movable gratings of threads hold the 



leaf against the glass cover, 

 which may carry tinfoil cut 

 with any desired pattern and 

 gummed to its lower side, or 

 which may be replaced by a 

 5x4 photographic negative 

 (see 239). 



The smaller screen (Fig. 

 43) consists of a spring clip 

 holding a glass disk against 

 the upper side of the leaf, 

 which is supported below by 

 a grating of threads stretched across the top of a venti- 

 lated dark-box, the glass being removable from the clip so 

 that a tinfoil sheet cut into any pattern may be gummed 

 to its lower surface, 



Fig. 43. Ganong's Light Screen 

 (small form). 



