On the Localisation of Oeo-perception in the 

 Cotyledon of Sorghum 



by 



Francis Darwin. 



Mit 7 Textfiguren. 

 Eingelangt am 10. August 1907. 



I. Iiitrodiictory. 



Some years ago i) I showed that, when a seedling of Sorghum 

 or Setaria is supported in a horizontal position by the cotyledon, 

 the hypocotyl does not assume a vertical position under the in- 

 fluenae of gravitation, but curves into a series of rings or spirals. 

 This seemed to be exph'cabie only on the assumption that the 

 cotyledon is geo-perceptive and that since it remains horizontal the 

 hypocotyl is stimulated to continuous curvature. Miehe^) has 

 recently suggested that another explanation is possible; other 

 writers having moreover expressed doubts as to the validity of 

 the proof, it seems desirable to supply fresh evidence. Miehe's 

 Chief objection is identical in principle with the doubt expressed 

 in my paper on the localisation of geo-perception in the root-tip ^). 

 Miehe points out that the behaviour of the seedlings is explicable 

 with the aid of our knowledge of the intercalary character of the 

 growth of the hypocotyl. Assuming that the hypocotyl is geo- 

 perceptive, each new intercalary zone will bend upwards by its 

 own geotropism and will carry with it the older zones, in this way 

 the hypocotyl may be carried past the vertical. If the power of 

 growth is lost in the older zones with great rapidity, there may be 



') Annais of Botany 13, 1899, p. 567. 



'-') Frings hei ms Jahrb. 37, 1902, p. 584. 



'*) F. Darwin. Journal Linn. Soc. 36, p, 272. 



