MOVEMENTS OF IRRITATION. 511 



We cover the bottom of a large zinc box with moist sand, heap 

 up part of it to form a wall against one of the sides of the box, 

 into which we can stick the lower end of the object under investi- 

 gation, so that the rest of it does not touch the sand and is 

 directed horizontally. We experiment with structures which 

 under normal conditions clearly exhibit plagiotropism, and first 

 select for examination young stems of Pyrus Malus, or runners 

 of Potentilla reptans or Ajuga reptans. Several specimens must 

 always be taken, as like one another as possible, and about 15-20 

 cm. long. After their leaves have been removed, they are placed in 

 the zinc box, some with their upper side uppermost, others with 

 their under side uppermost. We now place a cover on the box, 

 and leave the stems in the dark space for a good time (e.g. 

 twenty-four hours). We then find that all of them have curved 

 upwards, those however placed with their lower side uppermost 

 more strongly than the rest, as may be more exactly determined 

 by ascertaining the radius of curvature. This upward curvature 

 is in each case the result of negative geotropism, but when the 

 stems are in the normal position it acts in opposition to epi- 

 nasty, while the stronger curvature in the reversed stems must 

 be regarded as the resultant effect of geotropism and epinasty 

 acting simultaneously in the same direction. Epinasty, i.e. the 

 more vigorous growth of the upper side of plant structures, is to 

 be referred, according to H. de Yries, to internal growth deter- 

 minants, a view to which we shall return. 



The flower-bearing shoots of Atropa Belladonna, directed under 

 normal conditions horizontally, are remarkably epinastic. If we 

 cut off such shoots, place them in water, and leave them in the 

 dark for say twelve hours, we shall find that the shoot ends, at 

 the commencement of the experiment vertical, have directed 

 themselves horizontally (see Fig. 171). 



I have also examined young ends of plagiotropic shoots of 

 Corylus Avellana, sticking them in the manner already described 

 into moist sand contained in a zinc box. In these experiments, 

 frequently repeated in the course of the summer, I always 

 found, what is not in harmony with the results of H. de Vries, 

 that shoots deprived of leaves, and placed upper side upwards, 

 curved downwards, while shoots fixed lower side upwards curved 

 strongly upwards. In this case, therefore, we have strong epinasty 

 and weaker geotropism co-operating. 



According to my experiments, shoots of Corylus humilis placed 



