the small cells were still fewer in number and in parts formed a single transverse row, and in other parts short longitudinal rows of only two or three.  Nevertheless they sufficed to attract the eye, when the whole petiole was viewed as a transparent object beneath the microscope.  In these seedlings there could hardly be a doubt that the pulvinus was becoming rudimentary and tending to disappear; and this accounts for its great variability in structure and function.

In the following Table some measurements of the cells in fairly well-developed pulvini of O. corniculata are given:—­

Seedling 1 day old, with cotyledon 2.3 mm. in length. 
Divisions of Micrometer.**
Average length of cells of
pulvinus..................................................6 to 7
Length of longest cell below the
pulvinus..................................... 13
Length of longest cell above the
pulvinus...................................... 20

Seedling 5 days old, cotyledon 3.1 mm. in length, with the pulvinus quite
distinct. 
Average length of cells of
pulvinus.................................................. 6
Length of longest cell below the
pulvinus..................................... 22
Length of longest cell above the
pulvinus...................................... 40

Seedling 8 days old, cotyledon 5 mm. in length, with a true leaf formed but
not yet expanded. 
Average length of cells of
pulvinus.................................................. 9
Length of longest cell below the
pulvinus..................................... 44
Length of longest cell above the
pulvinus...................................... 70

Seedling 13 days old, cotyledon 4.5 mm. in length, with a small true leaf fully developed.  Average length of cells of pulvinus.................................................. 7 Length of longest cell below the pulvinus..................................... 30 Length of longest cell above the pulvinus...................................... 60

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* Longitudinal sections show that the forms of the epidermic cells may be taken as a fair representation of those constituting the pulvinus.

** Each division equalled .003 mm. [page 121]

We here see that the cells of the pulvinus increase but little in length with advancing age, in comparison with those of the petiole both above and below it; but they continue to grow in width, and keep equal in this respect with the other cells of the petiole.  The rate of growth, however, varies in all parts of the cotyledons, as may be observed in the measurements of the 8-days’ old seedling.