MOVEMENTS OF IRRITATION. 443 



The following experiment, which I frequently repeated, is in- 

 structive. We stick the lower end of a leafy shoot of Hippuris 

 vulgaris into the moist sand of our zinc box. When the shoot has 

 been kept in a horizontal position for one to one "and a half hours 

 at a high temperature, a distinct though still not very strong 

 curvature is already perceptible. We now place the shoot in a 

 vertical position with its lower end in moist sand, cover with a 

 bell-glass, and screen from the light. To our astonishment we 

 observe after a time that the curvature initiated when the stem 

 was in a horizontal position becomes greatly intensified now that 

 it is in a vertical position. We have here a phenomenon de- 

 pending on geotropic after-effect, and therefore also soon replaced 

 by another. The curvature, viz., which at first became more 

 marked when the stem was placed in a vertical position, gradually 

 disappears completely ; the shoot directs itself straight upwards, 

 since, after the geotropic after-effect has passed off, gravity acts 

 in the usual manner on its curved end. The phenomenon of geo- 

 tropic after-effect can be made out also with other shoots. 



In most plants, the power of carving under the influence of 

 gravitation is confined to the terminal internodes. The fully 

 grown parts of the shoot, lying further back, no longer react to 

 the action of gravity. So much the more remarkable, therefore, is 

 the geotropic behaviour of the haulms of grasses. Between the 

 successive, sharply differentiated internodes, are situated, as is 

 well known, the nodes, which are easily recognised by their colour 

 and swollen form. These are nothing but the basal portions of 

 the leaf-sheaths. While the parts of the haulm lying between 

 them may already have become mature, rigid, and hard, these 

 nodes retain their youthful character for a comparatively long 

 time, and hence also are easily able to effect geotropic growth 

 curvatures. This power of growth, it is true, finally dies away in 

 the cells of the nodes also. The younger nodes of grass haulms 

 are capable of more vigorous geotropic curvatures than the older 

 ones, since -their parenchyma still turgesces very energetically, 

 and since their cells are still capable of very active growth. If, 

 e.g., we cut out from the haulm of a flowering rye or barley plant, 

 a number of pieces each about 10 cm. long, and each with a node 

 at its middle, and then at once dispose them horizontally in our 

 zinc box, we shall find that the younger pieces, say at the end of 

 twenty-four hours, have curved much more vigorously than the 

 older ones. By measuring the angle, the extent of the curvature 



