— 128 — 



Experiment 7, fig. 3. In 51/2 hours curved through 65°; re- 

 versed and in 20 hours curved through 115'^. The first curve 

 scarcely visible. In all the above experiments the plants were 

 behaving rather sluggishly as shown by the small amount of cur- 

 vature produced per hour. 



These experiments show that Sorghum has a considerable power 

 of reversing its curvature but that the reversal is not complete, 

 since part of the original curve is generally visible. An Opponent 

 may assert that since the whole of the first curvature does not 

 disappear, Miehe's objection holds good. The value of Miehe's 

 Suggestion depends entirely on the assumption that the upward 

 curvature of the hypocotyl is due to its own geo-perception. The 

 facts given in the present paper are opposed to this view, we may 

 therefore leave Miehe's objection for the present. 



II. The method of the bent cotyledoii. 



The method here described is a modification of the well known 

 »glass boot« experiment of Czapek, i) It cannot perhaps be claimed 

 that the results are decisive, although they seem to me only ex- 

 plicable on the assumption that the cotyledon is the seat of geo- 

 perception. 



In my experiments the cotyledon was forcibly bent, and here 

 we departed from Czapek's plan, in which the curvature is slowly 

 produced by allowing the root to grow into a bent tube. 



We used Sorghums which had been grown under a toplight 

 which has the advantage of giving straightly grown seedlings, and 

 what is of especial importance, of giving plants with long cotyledons 

 and Short hypocotyls : the short cotyledon of the etiolated seedling 

 does not lend itself to manipulation nearly so well as light-grown 

 specimens. The seedlings are carefully extracted from the flower- 

 pot, the root is cut off, and they are allowed to lie on the table 

 for a few minutes to produce a very slight withering: this last 

 procedure is useful but not absolutely necessary. 



The extreme base of the cotyledon is fixed by pins (whiie 

 do not pierce the tissues) to a plate of cork. The free end of the 

 cotyledon may then be bent in a plane parallel to the cork-plate 

 and fixed to it by pins. 



^) See Pfeffer, Annals of Botany 1894; Czapek, Pringsheim's Jahrb. 

 1895, 27. 



