Medical Officers of the Army in India.

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       Action in the primary pulvinus preceding centripetal action in the cut pinna
appears to be the normal phenomenon under certain conditions. It may be
looked for as almost certain to occur in young, heavy leaves, in which, from the
position and direction of the petiole, a special strain is evidently exerted on the
primary pulvinus in maintaining the erect position. In old heavy leaves, spe-
cially if loaded with dew or rain, similar phenomena frequently manifest them-
selves. They do not, however, occur with such constancy as in the case of the
young leaves. This is to be credited to the alteration in the character of the
pulvinar tissues accompanying increased age. In the leaves of mimosa pudica
the tissue of the under-half of the primary pulvinus is relatively weakest when
the leaves arey oung and becomes gradually stronger and more resistant with
increasing age. So much does this take place in certain cases that it is not
uncommon to meet with old but otherwise healthy leaves in which the primary
pulvinus has become rigid and in which accordingly petiolar depression is
impossible. This increase in strength in the normally weakest tissue with in-
creasing age is not peculiar to the pulvini of Mimosa pudica, much more con-
spicuous examples of it are present in other plants. For example, in Bauhinias
and in Albizzia Lebbek the young leaves exhibit extreme nyctitropic move-
ments which gradually diminish and ultimately absolutely disappear with increas-
ing age and thickening and rigidity of the primarily passively weaker portions
of the pulvinar tissues.

TABLE XIII.—Results of focussing the Sun's rays on the upper surfaces of the
primary Pulvini of Mimosa pudica.

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