and Laboratory Methods. 2429 



erect. During the night the stimulus must be continued by the use of an incan- 

 descent gas or electric lamp. Make observations 24 to 48 hours after the 

 beginning of the experiment, referring the behavior of the plants to geotropism 

 and heliotropism. 



13. Epinasty and Geotropism. — Gather some immature leaves of the dandelion 

 {Taraxacum officinale) and without allowing them to wilt, embed the proximal 

 end of each leaf in an embankment of wet sand in a tray. Only the basal 2 or 3 cm. 

 of each leaf should be embedded in the sand. Place some leaves in the normal 

 position and some in the reversed position. From other leaves cut out the mid- 

 ribs and insert the proximal end of each in the sand, some in their normal position, 

 some turned upside down. The preparations should be covered with bell-jars 

 lined with moist filter paper and kept in the dark. Observations are to be made 

 35 to 60 hours after the beginning of the experiment. Determine on what 

 environment the difference in behavior depends. 



Fig. 18. — Influence of Light on the Straightening of the Hypocotyl. The 

 lupine seedlings on the right were grown in diffuse light ; the other set, 

 planted at the same time, were grown in the dark. 



14. Influence of Light on the Straightening of the Hypocotyl. — In two crocks 

 of earth plant seeds of the lupine {Lupinus a/bus) placing one crock in the light 

 and the other in the dark. When the seedlings have grown above the earth 

 observe their behavior with regard to the curve of the hypocotyl (Fig. 18j. 



OTHER PLANT MOVEMENTS INFLUENCED BY LIGHT. 



1. Paraheliotropism. — (a) /// chloroplasts. Mount a leaf of Mnium on a slide 

 in tap water, supporting the cover-glass with capillary glass rods. During the 

 course of the experiment replenish as often as necessary the water lost by evap- 

 oration from the edge of the cover-glass. Clamp the slide to the stage of a 

 microscope and set in the dark room for 30 minutes. When the microscope is 

 removed from the darkness, examine the cells of the leaf immediately for the 

 position of the chloroplasts. Next set the microscope in diffuse light and exam- 

 ine the leaf at the end of 30 minutes. Lastly place the microscope in direct 

 sunlight and note the position taken by the chloroplasts. 



Repeat the experiment, using some filaments of Mesocarpus. exposing them 



