I 34 Geotropism of Grass-Haulms. 
NOTE ON GEOTROPISM OF GRASS-HAULMS. 
I N Sachs’ Arbeiten iii., p. 509, Noll described the curious effect 
of fixing a grass-haulm immovably in the horizontal position. 
Under these conditions the lower half alone grows, demonstrating 
Noll’s contention that the decisive feature in geotropism in increased 
growth on the convex, rather than inhibition on the concave side. 
The haulm being fixed, the growth of the lower side cannot produce 
curvature, and the effect, as Noll describes it, is the appearance 
“ of peculiar outgrowths and weals (Schwielen) on the lower side, 
in which the strong growth-tendency of the cells expresses itself.’ 
As this description implies, and as Noll’s figures show, the growth 
is principally at right angles to the longitudinal axis of the pul- 
venus. The tendency to growth must, however, be primarily in 
the longitudinal direction, and the figure (for which I am indebted 
to Miss Pertz) taken from a specimen obtained in class-work at 
Cambridge shows, in addition to a growth in thickness, the existence 
of longitudinal growth which has been sufficient to cause a rupture 
of the pulvinus. The result may, perhaps, be due to the method 
employed by us, and described in Darwin and Acton, Practical 
Physiology of Plants , Edit, iii., p. 166. Noll prevented curvature 
by inclosing the haulms in narrow glass tubes: we fix the grass- 
stalks firmly into a sheet of cork, placing them at right angles to a 
groove cut in it; the haulms being so arranged that the pulvini are 
over the groove. As far as the essential feature—the prevention 
of curvature—is concerned, our method is the same as Noll’s; but 
the pulvini not being in contact with a glass tube are free to pro¬ 
duce any form of outgrowth. 
Bot. Lab., Cambridge, 
June 11th, 1903. 
FRANCIS DARWIN. 
