22 g PLANT PHYSIOLOGY 



and rod horizontal in the dark, and observe positions taken by the leaf 

 blades. Plants with a pulvinus to the leaf (e.g., String Bean), and stem- 

 less plants like Oxalis, give marked responses to a corresponding treat- 

 ment. Select a potted plant which has a cluster of irregular flowers (e.g., 

 Larkspur, Garden Nasturtium); bend over the tip of the cluster until it 

 is inverted and tie it in that position; keep in darkness, and observe posi- 

 tions of the opening flowers. Or shoots, preferably cut under water and 

 kept in water in a dark room with tip inverted, give good results. The 

 flowers of Narcissus, and the buds, in relation to the open flowers, of Pop- 

 pies, are important objects for this study. 



Select a young potted twining plant; observe the method of twining; 

 lay it down horizontally and note the twining phenomena. Also, if practi- 

 cable, revolve such a plant upon the klinostat, and note and interpret the 

 effect upon the twining. 



The student should now work out, from all his sources of 

 information, a good exposition of the occurrence of geotropism 

 in plants, including geotaxis and prominent cases of positive, 

 negative, lateral, transverse, and other special manifestations of 

 geotropism, with their significance in the economy of the organism. 



In the course of his observations the student will have noticed 

 some cases of rather rapid responses, and it is important to know 

 how rapid the response may be. This can readily be determined 

 thus: 



SUGGESTED EXPERIMENT. Arrange 2 or 3 strong-growing seeds as 

 for the first geotropism experiment, and, when the roots are i or 2 cm. long, 

 turn them into a horizontal position, preferably projected against a back- 

 ground ruled in small squares. Keep the chamber at a temperature of 

 22 to 25, and observe the roots frequently, noting the time needed to show 

 the first sign of bending, and also that required for complete turning. (A 

 special chamber for this purpose is that of SACHS, mentioned under Moist- 

 chambers, page 220. The experiment works well if the seeds are held 

 in a wire-and-sphagnum cage placed inside a flower-pot moist-chamber.) 



Place a slender-stemmed actively growing plant on its side, and note 

 the time needed for a first trace of bending and for complete bending. 



The student should now follow through the literature our 

 knowledge of this subject, including with it the related matters 

 of after effects, and especially the correlations existing between the 

 geotropism of main and of side roots or stems. It is quite possible 

 to study these correlations experimentally by using SACHS' simple 

 device of severing the main root some distance behind the tip, 

 and observing the effect upon the neighboring lateral roots. 



