IRRITABLE RESPONSE 233 



so rapidly as to introduce purely mechanical bendings of the young parts. 

 The method of determining the speed and force is given by Pfeffer, 

 "Physiology," 3, 170. Thefirst to use them was Knight, over a'century 

 ago. The modern forms ■ are usually driven by electric power, either by 

 a primary battery, as in the excellent form described and figured by Arthur 

 and supplied among his apparatus (Botanical Gazette, 22, 1896, 463), or 

 by a continuous power-current, as in Newcombe's very perfect form recently 

 described and figured (Botanical Gazette, 38, 1904, 427). The standard 

 electrical centrifuges sold for dairy and medical purposes (and of which the 

 Bausch & Lomb Optical Company issue a special catalogue) are no doubt 

 readily adaptable to this use, as Pfeffer has noted ("Physiology,'' 3, 170). 

 A form driven by a hot-air motor has been described by Hansen (Flora, 84, 

 1897, 352). Much simpler forms may be adapted, driven by a water-cur- 

 rent from a tap, as described by Stevens (Botanical Gazette, 20, 1895, 89), 

 by Detmer, 462, and by various others later, including one by Oster- 

 hout, 93, the latter's instrument being supplied ready for use by the Cam- 

 bridge Botanical Supply Company. 



Since centrifugal force undoubtedly results in throwing the 

 heavier particles contained in a cell away from the center of 

 rotation, thus producing a condition of one-directioned strain 

 in the protoplasm, and since gravitation certainly produces the 

 same effect, it seems probable that it is this condition of strain, 

 through the downward pulling of the heavier cell contents, which 

 gives the line of direction to the protoplasm which thus "per- 

 ceives" the stimulus. 



It is always important if a physiological process can be 

 grounded definitely in chemistry or physics, and a good case of 

 this is found in Czapek's chemical test of a geo tropically stimu- 

 lated root, on which the student should now inform himself. 



We come finally to consider geotropism from the ecological 

 point of view, and ask why it is so extensively used as a guide 

 by the parts of plants. In this connection it is to be noted that 

 no direct value of geotropism to any function of the plant has 

 been found, and that moreover it may even lead the parts into 

 fatal positions, as is shown by the following: 



Suggested Experimem. — Prepare a trough of paraffined wire netting, 

 and place on its bottom some soaked seeds. Then fill the trough with 

 earth, hang it up in a moist-chamber, keep the soil moist, and note the 

 result. 



