EFFECTS OF EXTERNAL CONDITIONS. 



109 



matching to the two surfaces of the glass, and blacken the edge of the 

 glass. Select five two-inch flower-pots, and pack them with sphag- 

 num in the pan at such distances apart that they will match the 

 open circles on the glass cover and will project a trifle above the rim 

 of the pan or pot. If now the cover be adjusted with the color 

 flasks in place, there will be beneath them five chambers, receiving 



FIG. 27. PURE COLOR CHAMBERS, THE MIDDLE ONE WATERED BY AN AUTO- 

 MATIC ARRANGEMENT. 

 One-fourth the true size. 



no light except of the pure color, readily kept moist (by water placed 

 in the saucer under the pan and soaking up to the pots through the 

 moss) and ventilated through the holes in the bottoms of the pots. 

 If in these small pots seeds be placed, and a strong light allowed 

 to fall upon them, the comparative growth under the colors may be 

 determined. Particularly advantageous for this are mustard-seeds. 

 If these be soaked, and ten be placed near the top of each pot (to 

 which they will adhere by their own mucilage), they will grow verti- 

 cally and their growth may be exactly measured. The light 

 should be thrown as nearly vertically upon them as possible, either 

 by special arrangements, or by fastening the flasks in place with 

 gummed paper, and tipping the apparatus to the proper angle. (A 

 thorough watering at the beginning of the experiment is all that is 

 needed.) 



This arrangement gives pure-color chambers available for other 

 purposes, such as photosynthesis and heliotropism. For special pur- 

 poses it could be made much larger, and the light thrown directly down 

 upon it from above with a mirror. If the tops of the smaller pots are 



