The leaves look like aponogeton crispus---no bulb--they could be young plantlets. I have raised A. crispus seedlings and it took a while for a bulb to form. I have also gotten Tiger lotuses (Ny. maculatus) without bulbs---with good care, a bulb formed. Give them time.
Thank you, lowcoaster. I found some pictures of A. crispus and my plants look like the ones in the pictures. Color is not a good criterion, because my plants just came from the LFS. Certainly the long stems and leaf shape suggest A. crispus. In pictures, the wavy edges of A. ulvaceous look very different from those on this plant. Like you said, though, it's a bit early to tell.