Here is a confirmation of your idea, illustrating how well you hit.¹ I got a few seeds of Ipomœa pandurata, the species of Atlantic U.S. with a huge root. One seed only has yet come up. Its germination is same as of I. leptophylla, viz. caulicle remains shut and petioles of cotyledons lengthen & bring up the cotyledons.² The caulicle lengthens a little—is $\frac{1}{4}$ inch, the petioles are 2 inches long.

A. Gray