In seedlings one or two days
old, the pulvinus was so indistinct that we thought at first that it did
not exist; but in the middle of the petiole an ill-defined transverse zone
of cells could be seen, which were much shorter than those both above and
below, although of the same breadth with them. They presented the
appearance of having been just formed by the transverse division of longer
cells; and there can be little doubt that this had occurred, for the cells
in the petiole which had
Fig. 64. Oxalis corniculata: A and B the almost rudimentary pulvini of the
cotyledons of two rather old seedlings, viewed as transparent objects.
Magnified 50 times.
been dissected out of the seed averaged in length 7 divisions of the
micrometer (each division equalling .003 mm.), and were a little longer
than those forming a well-developed pulvinus, which varied between 4 and 6
of these same divisions. After a few additional days the ill-defined zone
of cells becomes distinct, and although it does not extend across the whole
width of the petiole, and although the cells are of a green colour from
containing chlorophyll, yet they certainly constitute a pulvinus, which as
we shall presently see, acts as one.
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