GEOTROPISM 4;{1 



effect of weight of cell contents, whether of the sap 

 itself, or of the heavy particles contained in the cells, 

 exerting pressure on the sensitive plasma. The theory 

 of statoliths advocated by Noll, Haberlandt and Nemec 

 (in spite of certain tlitticulties which further work may 

 remove) is the only rational explanation hitherto offered 

 for gravi-perception. The sensitive plasma is the ecto- 

 plasm of the entire cell, and statoliths are relatively 

 heavy bodies, such as crystals and starch grains. 

 Haberlandt has found statoliths in the apo-geotropic 

 organs like stems.* When the cell is laid horizontal, 

 it is the lower tangential wall which has to support 

 the greater weight, and thus undergo excitation. In 

 the case of multicellular plants laid horizontally, the 

 excitation on the upper side is, as we have seen, the 

 more effective than on the lower side. This inequality, 

 it has been suggested, is probably due to this difference 

 that the statoliths on the upper side press on the inner 

 tangential walls of the cells while those on the lower 

 side rest on the outer tangential walls. 



When the organ is held erect, the action of statoliths 

 would be symmetrical on the two sides. But when it is 

 laid horizontal a complete rearrangement of the statoliths 

 will take place, anil the differential efl'ects on the upper 

 and lower sides will thus induce geotropic reaction. This 

 period of migratiun must necessarily be very short ; but 

 the reaction time, or the latent period, is found to be of 

 considerable duration, "Even in rapidly reacting organs 

 there is always an interval of about one to one and a 

 half hours, before the horizontally placed organ shows a 

 noticeable curvature, and this latent period may in other 

 cases be extended to several hours."t This great differ- 

 ence between the period of migration and the latent 



* Haberlandt — " Physiological Plant Anatomy " — p. tiOiJ. 

 t So9i—Ihid, p. 437. 



