172 BOTANICAL GAZETTE [august 



medium intensity of the stimulus produces no reaction; also the positive curv- 

 ing occurs in the zone of most rapid growth, while the negative takes place in the 

 region of greater maturity. The quantity of stimulus law already established 

 for heliotropism and geotropism 8 is confirmed by this work. The quantity of 

 stimulus necessary for a negative response is about iooo times that necessary 

 for a positive response. 



Jost 9 takes up the several positive arguments that have been offered in 

 favor of the starch statolith theory, and with some partisanship shows their 

 shortcomings. He observes that the negative argument is often used; that 

 while many facts do not aid in substantiating the theory they at least do not 

 disprove it. This statement holds, he asserts, because the theory itself has 

 experienced a gradual process of adaption to the demands of newly established 

 facts, w r hich makes the theory of 1909 quite a different thing from that of 1900. 

 In its earlier form the starch must actually fall on the Plasmahaut and lie there 

 for some time to induce the reaction, while in the later form movement of the 

 starch without geo-perception is explained by lack of irritability of the plasma, 

 and geo-perception without movement of starch is explained by saying that 

 actual displacement of the starch is not necessary for perception. 



The author has studied the response of the root on the Piccard centrifuge 

 and the effect of the removal or injury of various regions of the root tip on 

 geo-perception and geo-response. The results on the Piccard centrifuge agree 

 with those of Haberlandt, 10 though the author gives them a different inter- 

 pretation, which he believes accords better with all the facts known. Any 

 injury that leaves the root tip attached or removes o . 5-0 . 7 5 mm. gives a wound 

 effect that hinders geo-response for some hours. Removal of 1 mm. or more of 

 the tip hinders geo-response for many days. Jost believes removal of 1 mm. 

 or more of the tip affects the response in three ways: by wound shock, by 

 removing a highly sensitive geo-perceptive region, by removing a region of great 

 tonic significance in rendering other regions sensitive. His main evidence for 

 the tonic effect of the tip 1 mm. is the fact that on the Piccard centrifuge the 

 tip must extend over the point at least 1 . 5 mm. to give a reaction in favor of the 

 tip, showing considerable sensitiveness in the growth zone; while removal of 

 only 1 mm. of the tip renders the growth zone ineffective. The author believes 

 that Nemec's conclusion that statolith starch is necessary in the tip for geo- 

 perception lacks evidence, and that such a conclusion was drawn because 

 Nemec failed to recognize the important tonic effect of the tip 1 mm. Jost 

 believes that the meristem of the tip, along with the cap region immediately 

 bordering on it on the one hand and the growth region on the other, are the 

 regions of the maximum sensibility, while other regions may perceive but give 





8 See review of Blaauw in Bot. Gaz. 49:238. 1910. 



9 Jost, L., Studien liber Geotropismus. I. Die Verteilung der geotropischen 

 Sensibilitat in der Wurzelspitze. Zeitsch. Bot. 4:161-205. 1912. 



10 See review in Bot. Gaz. 47:482. 191 2. 



