214 



PLANT PHYSIOLOGY 



just 25 cm. beneath a sheet of glass supported at right angles to its 

 axis. Holding the eye a convenient distance (also about 25 cm.) 

 away from the glass, sight along the filament until the black spots 

 coincide, then make on the glass, in the line of sight, a small dot, using 

 for the purpose a needle with a wax tip dipped in India ink. Then 

 thereafter, at as frequent intervals as practicable for three or four 

 days, make similar records of the position of the filament, number- 

 ing each mark. Finally take off on tracing-linen a copy of the dots, 

 join them by fine straight lines, indicate the direction of movement 

 by arrow points, and state each time interval in minutes. 



Precautions. In attaching the filament the smallest efficient quantity 

 of shellac should be used. Further the plant must he screened from one- 

 sided light. This is accomplished automatically in a well-lighted green- 

 house, especially if the south side is shaded, but in a room it is necessary 

 to surround the plant up to near the glass by a paper cylinder, so that all 

 light shall fall from above, a small opening being left in the side for the 

 observation of the leaf filament. A good arrangement, where only the 

 movement of the main stem is desired, is afforded by placing the part under 



observation in the bottom of a 

 box, or even a large flower-pot, 

 on the top of which rests the 

 glass plate. While it is customary 

 to join the dots by straight lines, 

 there is some advantage in using 

 sweeping curves through the 

 points, though one system is about 

 as true, or untrue, to Nature as 

 another. While a support for the 

 glass plates may readily be adapted 

 from the ordinary laboratory sup- 

 ports and clamps, it is much 

 more satisfactory to use a tripod 

 made for the purpose, after the 

 plan shown by the accompanying 

 figure (Fig. 58), the whole being 

 placed upon a low table or other 

 support so that both plates may 

 readily be used. Instead of the 

 circle of paper above described, one 

 may use a triangle of paper with 

 the dot on one angle, or even may 

 place the dotted paper on a stick set 

 in the pot, — any arrangement which 



Fig. 58. — Arrangement for the study 

 or ctrcumnutation; Xf 



The upright glass on the right, for convenience 

 of the drawing, is shown too near the plant. 

 Both glasses are (preferably) circular. 



permits two dots to be seen and brought into line sufficing for the purpose. 

 But upon the whole the arrangement above described is the best, since it 

 permits the record mark on the glass to be made just where the filament, 



