570 Darwin . — On Geotropism and the 
mitted stimulus — in fact, imagine that our apogeotropic organ 
has the qualities which actually exist in roots. What will 
happen ? The normal plant in which the apex is free will 
behave like the above described specimens — it will bend 
upwards, and when the sensitive apex is vertical it will cease 
to be stimulated, and will therefore cease to transmit a stimulus 
to the bending region. 
Now take the other case in which the tip is fixed and 
the base free : the tip being horizontal is stimulated, and 
an influence is transmitted to the motor region, so that the 
basal end begins to rise. But now observe the difference 
between the two : the curvature of the normal specimen 
brings the tip nearer and nearer to, and finally into the 
vertical position, the position of equilibrium. But no amount 
of movement in the inverted specimen has any effect on the 
tip, which remains irritated because it remains horizontal. It 
is clear, therefore, that the motor part of the inverted specimen 
ought to continue curving ad infinitum. 
I had concluded that this ought to take place with roots, 
and made many fruitless efforts to demonstrate the fact, but 
was foiled by the difficulty of fixing the root by its slippery 
tip and also by the difficulties caused by the weight of the 
cotyledons. It was only when I turned to grass seedlings 
that I succeeded. Rothert 1 has shown that in the seeds of 
the grasses Setaria , Panicum , and Sorghum the localization 
of the heliotropic sensitiveness is more definite than in Avena 
or Phalaris . The structure of these seedlings is remarkable 
from the presence of what may be called a hypocotyl-— 
a relatively long stalk between the sheath-like cotyledon 2 
and the grain. This hypocotyl is the part which bends, but 
according to Rothert it is not sensitive to light, while the 
cotyledon is sensitive to light but does not bend. These 
1 Cohn’s Beitrage, loc. cit. 
2 I use the word cotyledon (in the sense in which it was employed in the Power 
of Movement) to mean the hollow sheath-like leaf-structure which precedes the 
true leaves. Rothert has adopted the term cotyledon in this sense, leaving on one 
side all questions as to the morphology of the part in question. 
