Question

why ulva is commonly callled sea lettuce and is not placed in kinhdom planta

Answer 1

Ulva is an algae belonging to family Ulvaceae. Ulva is an edible green algae.

It is not included in kingdom Plantae because it does not have well defined vascular system and it is a gametophyte; whereas plants are sporophyte(spore producing) and have well defined vascular systems for transport of nutrients and water throughout the plant body.