and Laboratory Methods. 2319 



roots is best observed when the roots are allowed to grow against glass. The 

 preparation must be kept in the dark if the roots are to follow the glass. 



Fill a box with sloping glass side with moist sawdust and plant against the 

 glass three seedlings of the pea {Fismn sativutn') with only the main root grown 

 (Fig. 15). Place one seedling in the normal position, one in the inverted posi- 

 tion, and one in the horizontal position, all with the roots showing against the 

 glass. 



When the lateral roots appear their orientation may be observed and com- 

 pared with that of the main root. After this observation has been made, stand 

 the box on end and notice how the primary, secondary, and tertiary roots orient 

 themselves with respect to the attraction of gravity. 



XII. HELIOTROPISM. 



1. Heliotropism of Stems and Roots. When exposed to the one-sided action 

 of light, the hypocotyls and roots of seedlings afford a good means of studying 

 the heliotropic responses of the different parts of plants. Fasten four or five 

 seedlings of white mustard {Si/iapis alba) or buckwheat {Fagopyru7n esculenfum) 

 3 to 6 cm. long to a small wooden bar by means of blotting paper and rubber 

 bands, and suspend them in a rectangular glass jar. Instead of lining the jar 

 with filter paper, fill it with water up to the lower surface of the bar supporting 

 the seedlings. The three sides and top of the jar are to be rendered opaque by 

 a covering of black paper; or, better still, set the jar containing the seedlings 

 into a box, the interior of which is painted a dead black, and which has a win- 

 dow on one side for the admission of light at the level of the seedlings. By 

 means of the latter method one is absolutely certain that all side and top-lights 

 are excluded. The open side of the jar is exposed to a bright sky, but never to 

 direct sunlight. If the sky is very cloudy, an artificial light may be used as a 

 source of stimulation, but should not be so close that the heat produced by the 

 light will affect the plant. 



The hypocotyls will show a strong, positive curvature, while the roots will be 

 curved negatively. 



2. Latent Period. Raise in the dark room a small crock of seedlings of 

 German millet {Sefaria Italicd). When the seedlings are 1 to 2 cm. above the 

 earth, if all are vertical, make rapidly the following preparation : Thrust two 

 pieces of soft iron wire into the earth, one on either side of a straight seedling, 

 bringing the seedling and wires into the same vertical plane. Set the crock into 

 the box which admits light only on one side. Make the observations for curva- 

 ture at intervals of ten minutes. 



Conduct the experiment at the same temperature as in the test for latent 

 period with geotropism and compare the length of the periods obtained with the 

 two stimuli. 



3. After Effect. Allow the plant used in the preceding experiment to acquire 

 a curve of 20° to 30°, then readjust the two wires in such a way that they have 

 the same inclination as the plant under observation, and revolve the crock 

 horizontally through an angle of 180°. Observe at intervals of five minutes to 



