MOVEMENTS OF IRRITATION. 445 



tissue. Thus when negative geotropic curvatures take place the 

 energy of growth of the cells, at any rate of the lower side, which 

 becomes convex, is very considerably increased. 



It is very instructive to undertake the following investigation, 

 which I made with specially good results using pieces of oat 

 haulms which had undergone vigorous geotropic curvature. We 

 prepare radial longitudinal sections of the node, and examine 

 them under the microscope. The cells of the parenchyma of the 

 underside are at once seen to be considerably elongated in the 

 direction of the long axis of the organ, while the cells of the 

 upper side are tabular in form. They are shorter in the longitu- 

 dinal than in the radial direction. The convexity of the lower 

 side of the geotropically curving grass nodes is due, therefore, to 

 vigorous stimulation of the growth of its cells. 



For further observations on the growth of geotropically curving- 

 plant structures, stems of Sida Napaea or Inula Helenium are 

 specially suitable. We use shoots deprived of their leaves and 

 end buds, composed of a few internodes, and 200-300 mm. in 

 length. They must be perfectly straight and require to be care- 

 fully selected. We take nine such pieces, cut them all to the 

 same length, and arrange them in groups of thi*ee. Those of 

 the first group we at once analyse by cutting off with a sharp 

 razor two strips of the cortex and determining their length. 

 Three other pieces we bring into our zinc box containing moist 

 sand, and place them horizontally. The last three we put in a 

 slightly inclined position into a glass cylinder, the bottom of which 

 is covered with moist sand and which can be closed, with a cork. 

 After twenty-four hours the second and third group are analysed. 

 We remove strips of cortex from their concave and convex sides 

 respectively, and measure their length. If now we compare the 

 average numbers in the second and third groups, it stands out 

 that, in the former, in which the stems were laid horizontally and 

 therefore curved vigorously, the growth of the now convex side 

 must have been greater, that of the now concave side less, than 

 that of the corresponding sides of the stems placed in the cylinder, 

 and hence not at all or only slightly curved. 1 



If we experiment on shoots with apical growth (e.g. epicotyls, 

 of Phaseolus), laying the shoot horizontally in the zinc box, after 

 marking it with fine ink-lines in the manner given on p. 385, in 

 order to determine the growth in the successive zones, then at a 

 particular time (after about twelve hours) we find the strongest 



