134 RELATIONS OF PLANTS TO LIGHT 



may have the effect of changing the character of the response 

 and the resulting curvature may carry the organism away from, 

 instead of toward the source of light. This is true of free mov- 

 ing organisms which have a low photosynthetic optimum. In 

 fixed forms the reaction consists in moving the organs to place 

 their surfaces more nearly parallel with the rays. 



The amount of increase in illumination necessary to constitute 

 a stimulus has been found to be very slight in the few organisms 

 in which it has been investigated. Results at hand are fairly con- 

 clusive that accretions to constitute a stimulus are in accordance 

 with Weber's law. 1 



203. Intensity of Illumination Necessary to Constitute a Stim- 

 ulus. Grow seedlings of Avena, or Phalaris, in a dark room, by 

 the use of germinating pans. When the shoots have reached 

 a height of two cm. place an ordinary paraffine candle at a dis- 

 tance of three meters and allow it to burn for an hour. The rays 

 of light should strike the upper part of the seedling and leave the 

 lower part in the shade. Extinguish the candle and examine the 

 seedlings at the close of a second hour. Repeat the test, mov- 

 ing the candle farther away from the seedlings every time until 

 no reaction is secured. It may be more convenient to use the 

 smaller candles sold for Christmas decorations or a micro- 

 gas-burner. The candles may be standardized as above, or 

 the gas-burner by means of a photometer in the physical labora- 

 tory. 



204. Negative Reactions to Light above the Maximum. Obtain 

 some mud and water from a ditch containing numerous specimens 

 of Euglena viridis, and place in an earthenware, or porcelain dish 

 and set near a south window for a few days. Remove the culture 

 to the middle of the room and take up a few drops of the liquid 

 and place a suitable amount containing numbers of the Euglena 

 on a large cover-glass, and invert over a stage moist chamber after 

 the manner of a drop culture. Examine with a low power and 



1 Massart, J. Recherches sur les organismes inferieurs. La loi Weber verifie pour 

 1'heliotropisme du champignon. Bull. Belg. Acad., 16 : 590. 1888. 



