88 PLANT PHYSIOLOGY 



may carry vials of pure colors correlative to the light and dark parts of the 

 chlorophyl spectrum, as was the case when the accompanying photograph 

 was taken, the meaning of which will be later explained; or it may be re- 

 placed by a $X 7 negative for GARDINER'S striking starch-printing experiment 

 (later mentioned). The arrangement does not, of course, permit as free 

 access of carbon dioxide as the uncovered leaf enjoys, but this is only a mat- 

 ter of degree and does not affect the result when sufficient time is allowed 

 for the experiment. 



The smaller screen (Fig. 19), of a lesser range of general usefulness, 

 though ample for demonstration of the present subject, is made upon the 

 same principle, except that it is arranged to clasp a portion of a leaf. A 

 spring clip holds a glass disc against the upper surface of the leaf, which is 



FIG. 19. SIMPLE NORMAL LIGHT SCREEN; X-J. 



supported below by a grating of threads stretched across the top of a venti- 

 lated dark box. The glass is removable from the clip and may carry a tin- 

 foil screen cut with a pattern and gummed to its under surface as in the figure, 

 or it may be used to hold a photographic negative (film or glass) against the 

 leaf. 



Simpler forms, embodying the same principles, may be adapted from 

 common materials, and even make-shift forms are possible. For demon- 

 stration purposes it is not essential that the under side of the leaf shall be 

 darkened, especially if the background is dark, but, for the most part, the 

 results are better when this is done. It is possible to adapt a fair normal 

 screen from the leaf -clasp described later under Transpiration, by placing 

 the pattern upon a glass held by one ring against the leaf, and attaching a 

 ventilated paper box to the opposite ring. 



DEMONSTRATION. This important experiment can be shown to an entire 

 class by following upon a large scale the general methods given in this and 

 the preceding sections. The large-leaved plant, after a day or two in dark- 

 ness, followed by two or three hours' exposure to bright light with normal 



