EFFECTS ON TROPISTIC RESPONSE 219 



not be attributed to loss of turgor.* In two other cases failure of 

 corn seedlings to grow erect followed an exposure of the dry grain 

 for 27 hours to the same radium-tube. f 



These observations merely show that, under the conditions of ex- 

 posure, the irradiated seedlings failed to grow upright as normally 

 under the stimulus of gravity, and an examination of the tissues 

 showed that, even if the stimulus has been perceived, response would 

 have been practically impossible on account of the slight development 

 of mechanical tissue, and other histological abnormalities. | 



Czapek^ has proposed a chemical test for the perception of gravi- 

 tational or other tropistic stimulus. If the stimulus is perceived, an 

 anti-ferment is developed in seedlings of Lupiyius albus and Zea 

 Mays, which inhibits the complete action of tyrosinase. This anti- 

 ferment is produced whether the organ responds by a curvature or 

 not, as when a seedling is rotated horizontally on the clinostat. 

 Even such a test would make it difficult to determine whether radium 

 rays caused a loss of geotropic sensibility, for the rays might directly 

 affect the ability of the protoplast to elaborate the anti-ferment when 

 subjected to gravitational stimulus. 



No experiments were performed with the express purpose of 

 studying the effect of the rays on phototropism, but observations of 

 all exposed plants in other experiments failed to indicate any loss of 

 sensitiveness to the unilateral stimulus of light rays. Shoots of seed- 

 lings from seeds exposed to radium rays of varying degrees of activity 

 and for various durations of exposure, manifested phototropic re- 

 sponse. This is in full agreement with observations reported by 

 Koernicke,^ who found that both roots and shoots of Via'a Faba, 

 Lupinus albus, and Ptsum saiiviun, grown from seeds exposed to 

 radium rays, were both geotropically and phototropically sensitive 

 so long as growth continued, but not afterward. Radium stimulus, 

 therefore, does not seem to interfere with sensitiveness to any other 

 stimulus except in so far as it lowers or raises the general vitality of 

 the protoplast as a whole. 



2. Can Radium Rays cause Tropistic Response? 



Experiments made for the purpose of detecting the existence of a 



radiotropism, or power to respond tropistically to a unilateral stimu- 



*One of the seedlings, and also one exposed for only 7 hours and a third exposed 

 for 27 hours showed a partial loss of chlorophyll from the leaves. 

 t See p. 218. 

 J For details see Chapter XVI. 



