Lemna gibba ?

Images by B. Rathinasabapathy (Id by P. Santhan) (Inserted by J.M.Garg) (For more photos & complete details, click on the links) 


Lemna gibba
(gibbous duckweed[1] or fat duckweed) is a species of Lemna (duckweed) with distribution in temperate areas in Europe, including Britain, to the Himalayas, Africa, South America and North America.  
This duckweed is one of Britain's most common small water plants, which forms familiar green mats covering stagnant water bodies.[2] It has a simple plant body, known as a thallus, which floats on the surface of the water and measures up to five millimetres in diameter. A single root hangs down into the water. Found in a wide range of still or slow-flowing water bodies, common duckweed can also occur on mud or damp rocks.
This species spreads mainly through vegetative reproduction,[4] but flowers are occasionally produced in shallow water exposed to the full sun. When covering the entire surface of a pond, it can make the water appear solid.
(From Wikipedia  on 25.8.14)

efloraindia: 12082014 BRS : 3 posts by 3 authors. Attachments (3).
Pl. find the attached file contain photos for id. confirmation.
Place : Adyar river, Chennai...
Date: 11.08.2014
Habitat : River
Lemna gibba L.(gibbous duckweed or fat duckweed)
Would you please tell me what KEY in these pictures determine it is Lemna gibba L.