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229 
The outlines in Fig. 4 and 5 shew the usual change in the cur- 
vature of the pulvini which takes place as the leaf rudiment grows 
older but remains without contact. In an adult leaf directed 
horizontally upwards, the angle between the pulvini and the 
distal portion of the main axis is an acute one, but in leaves 
which slant downwards the pulvini bend upwards, the acute 
angle facing inwards. These curvatures are directly due to the 
action of gravity, and alter if the position of the leaf alters. 
They take place even when the leaflets are removed. The leaflets, 
chiefly by means of their own pulvini, even when the petiolar 
pulvinus is twisted around a support, soon assume their pro- 
per position, and one which varies according to the intensity 
of the illumination to which they are subjected. In fig. 5 the 
pulvini have a curiously hook like appearance, and it is at this 
stage, when the pulvinus is about */; its full length and the 
leaflets quite small and unfolded, that the pulvinar ,hooks” 
are most irritable. 
The irritability is somewhat more marked in the morning 
and early afternoon, than it is in the evening or at night. Cur- 
vature begins 5 minutes after contact has been applied, in '), 
hour may amount to '/; to '/: or more of a right angle and 
in | day may make one or more complete coils. If removed 
as soon as a distinct curvature has been produced, and dropped 
into alcohol, the curvature does not perceptibly alter. If then 
after some time placed in water, the curvature generally under- 
goes a slight but distinct straightening varying very much in 
different cases. The pulvini respond in 5 to 10 minutes to 
gentle rubbing at short intervals of time with a wooden rod. 
The terminal pair of pulvini are the most irritable, the next 
pair less but still markedly so, and the basal pair or pairs 
least of all. The curvature always takes place towards the 
side stimulated. The concave surface, when only a slight pri- 
mary curvature is present, is the most irritable, the beck and 
sides less so, responding to a similar stimulus of short dura- 
tion in a minute or two longer time, and by only '/, to '/; of 
the curvature that the concave surface performs. 
