45 2 
PHYSIOLOGY 
they are also found in a considerable number of families allied to the 
Berberidaceae and Scrophulariaceae. 
Structure. The leaves of Leguminosae are usually much branched, 
and the primary motor organ, when present, is located at the base of 
the main petiole. In many cases there are also motor organs (secondary) 
at the origin of the secondary petioles, and if the leaf is ternately com- 
pound the petioiules or stalks of the leaflets are motor organs. Thus 
Mimosa has primary, second- 
ary, and tertiary motor organs 
(fig. 683); but the red and sweet 
clovers have only one set, the 
stalks of the leaflets. The 
motor organ consists of all or 
a portion of the petiole or peti- 
olule, modified by changes in 
the position of the vascular 
buncPes, and by an excessive 
development of the paren- 
chyma of the cortex. Through 
the greater part of the leaf 
stalk the vascular bundles lie 
at some distance from the 
center, surrounding a distinct 
pith, and within a cortex of 
moderate thickness. In the 
motor organ, however, they ap- 
FIG. 683. Leaf of Mimosa in open and closed proach one another so closely 
positions. From PART III. ^.u ^ ^.u i 
that there is scarcely any cen- 
tral pith, and they form a shaft, elliptical or kidney-shaped in section. 
Outside, the cortex is correspondingly larger, and its cells are usually 
somewhat different from the rest. As a whole the motor organ is some- 
times thicker than the other part of the petiole, but it is quite as likely 
to be smaller ; in all cases, however, the relative increase of the cortex 
in cross section gives the impression of a cushion of parenchyma. 1 
In this region the cells are rather regular in form, approximately 
cylindric, and with smaller intercellular spaces than in the nutritive 
regions. Intercellular spaces are present, however, at the junction of 
three or more cells. 
> This is the reason for a technical name applied to the motor organ, the pulvinus. 
