MOVEMENTS OF IRRITATION'. 449 



shoots exposed to atmospheric air rapidly execute energetic geo- 

 tropic nutations, especially if the temperature is high. 2 



Researches have led to the result that when gravitation acts as 

 a stimulus it does not directly influence the growth of the organs, 

 but first affects the turgidity of their cells. We experiment with 

 the epicotyl of Phaseolus, with young shoot ends, 20 cm. in length, 

 of Aristolochia, Taraxacum, Plantago, Papaver, etc. The objects 

 are laid horizontally in a zinc box (see 173), so as to be in dark- 

 ness and in a moist atmosphere. When, after a time (two to four 

 hours) a distinct geotropic upward curvature is to be seen in 

 them, we lay them on a card marked with a series of concentric 

 circles. We seek the circle whose curvature coincides most nearly 

 with that of the object, and observe its radius. We now lay the 

 object in a 20 per cent, solution of common salt contained in a 

 crj'stallising glass. After some hours we place the dish on the 

 card, and, moving the limp plasmolysed structure with the for- 

 ceps, once more find a circle most nearly corresponding with its 

 curvature. The radius of this circle is greater than that of the 

 one found before plasmolysis. We see that the geotropically 

 curved structure retains a curvature after plasmolysis, and this 

 is due to processes of growth brought about by the influence of 

 gravity. That portion of the original curvature, on the other 

 hand, which can be removed by plasmolysis, must be ascribed to 

 conditions of turgidity. After the objects have been left in the 

 zinc box, in a horizontal position, for perhaps twenty-four hours, 

 their curvature is not reduced by plasmolysis ; it remains un- 

 altered when they are placed in the salt solution, since- it has been 

 fixed by processes of growth in the cells, and is therefore no 

 longer reversible. 



If we place in our zinc box in a horizontal position pieces of 

 grass haulms (I experimented, e.g., with Secale), 10 cm. in length, 

 and each provided with a node at its centre, sticking their lower 

 ends into one of the sand slopes, a considerable curvature quickly 

 appears in the node. For example, in one case which I observed 

 it amounted to 45. After completely plasmolysing the piece of 

 haulm the curvature was reduced to 25. Unmistakably, there- 

 fore, the co-operation of conditions of turgidity and growth in 

 the production of the geotropic curvature is here also to be re- 

 cognised. 3 



From what I have stated in my Lehrbuch der Pflanzenphysiologie 

 concerning the fundamental causes of geotropic nutations, it may bo 



P.P. G G 



