Hi Mark, That's a good question. The degree to which the plant requires a transition period to switch from underwater to above-water growth depends entirely upon which kind of plant it is. There are a couple of instructive examples from this setup.
The left-most plant in this layout is
Limnophila aromatica. That one took some time to transition to emersed growth. In order to trigger the change I potted several stems in a riparium planter cup, which I then situated in the tank so that the the rim was just a centimeter or so below the water's surface. In this way the stems were maintained just barely underwater, but new growth was compelled to grow up above the water's surface. I use this same method for most kinds of aquatic plants can transition from immersed (underwater) to emersed (above-water) growth. As I recall the
L. aromatica took some time, several weeks, to show very much new growth above the water. It is a slow-growing plant anyway. By the way, the next shot shows the
L. aromtica the day that I received it. The immersed-form leaves are distinct (finer, longer and more deeply-colored) from the emersed-form leaves (more succulent, shorter, less richly-hued).
Other plants can change to emersed growth more quickly. In that same picture above there is a second carpeting plant,
Bacopa madagascariensis, with light green leaves just to the left of the red-patterned aroid (
Cyrtosperma johnstonii. It was somewhat easier to plant that one as riparium foliage. The underwater leaves of many
Bacopa are relatively succulent and somewhat resistant to drying. In the case of that plant I just took a number of underwater stems then planted directly directly into a riparium cup and began to train to grow across a foam trellis raft.
I did maintain that group of newly-planted stems in a humid, partially-covered tank for some time while new foliage began to grow, but now it is doing perfectly growing right in the normal room relative humidity.
This is a good questions. I have a couple of other examples that I should go dig up too.