It is formed of a mass of small cells of an
average length of .021 mm.; whereas the cells in the lower part of the
petiole are about .06 mm., and those in the blade from .034 to .04 mm. in
length. The epidermic cells in the lower part of the petiole project
conically, and thus differ in shape from those over the pulvinus.
Turning now to very young seedlings, the cotyledons of which do not rise at
night and are only from 2 to 2 ? mm. in length, their petioles do not
exhibit any defined zone of small cells, destitute of chlorophyll and
differing in shape exteriorly from the lower ones. Nevertheless, the cells
at the place where a pulvinus will afterwards be developed are smaller
(being on an average .015 mm. in length) than those in the lower parts of
the same petiole, which gradually become larger in proceeding downwards,
the largest being .030 mm. in length. At this early age the cells of the
blade are about .027 mm. in length. We thus see that the pulvinus is formed
by the cells in the uppermost part of the petiole, continuing for only a
short time to increase in length, then being arrested in their growth,
accompanied by the loss of their chlorophyll grains; whilst the cells in
the lower part of the petiole continue for a long time to increase in
length, those of the epidermis becoming more conical.
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