24 



Goto Turesson 



side is thus converted into the morphological under side. Atrijjlex conforms in this 

 respect entirely to the psychroclinic spring plants investigated by Lidforss. 



The results obtained from the experiments with Atriplex prostratum Bouch. dis- 

 cussed above will suffice to justify completely the conclusion that the movements 

 induced in this plant by direct sunlight are geotropic in nature. The above-ground 

 organs are placjiogeotropic in direct sunlight but negatively geotropic in darkness, as 

 was also the case with the rest of the plants investigated'. By way of comparison 

 the conclusion is drawn that the plagiotropic orientation of the above-ground organs 

 so commonly seen in shore plants and so readily produced by exposing orthotropic 

 cultures to direct sunlight also in these plants is caused by geotropic stimuli in 

 response to bright light. 



It was stated above that the cultures were rotated on the horizontal clinostat 

 axis for a time of three weeks in order to neutralize the effects of epinasty. In fact, strong 

 downward curvatures result as soon as cultures are rotated on the clinostat. It 

 was found in both A. prostratum Bouch. and A. patulum that upright branches of 

 plants rotating on the horizontal axis often made angular movements of ISO**. How- 

 ever, upward movements soon followed and the branches came, finally, to rest in 

 an intermediate position. 



Recent investigations by Lundegåbdh (15) have made clear the nature of 

 epinasty. It is shown that positive geotropism accounts for all epinastic curvatures 

 in lateral branches investigated by him. As the opposing reactions of positive and 

 negative geotropism are thus seen to be the primary factors in the regulating 

 of the position of the branches, the conclusion seems to be justilied that in bright 

 light the balance is lost in favour of the geopositive reaction. In obscure light tlie 

 geonegative reaction becomes predominant. It is hard to believe that this deter- 

 mining effect of the light conditions on the nature of the geotropic response is re- 

 stricted to the plants discussed above and to the few additional ones to be mentioned 

 in the following. Probably a much larger number, perhaps the vast majority of 

 plants, will be found to behave in a similar wa_y. 



4. Psychrocliny and photocliny. 



The similarity between the above discussed movements in the shore plants and 

 the movements described by Lidforss (12, 13) as characteristic of many spring 

 plants (Holosten »1, Laniium purpureum, CorydaUs, Viola tricolor etc.) has already 

 been pointed out. However, in the latter plants the movements are released by 

 variations in temperature. The stems take up a horizontal position and becoine 

 pressed against the soil at a low temperature (about 3 — 6 " C above zero). At a higher 

 temperature (20 — 30 ° C) a change in the geotropic response results, and the stems 

 bend upwards and take up a vertical position. These variations in the position of 

 the stems, when submitted to changes in temperature, were first described by Vöch- 

 TiNG (29) in Mimulus Tillingii (syn. M. Intens) under the name of psychrocliny. The 



' For furtlier analysis of the phenomenon of plagiogeotropisni see LundegÅrdh 15. 



