Small seeded late maturing type of

Solution

Vigna mungo var. niger refers to the small-seeded late maturing type of blackgram. This variety of blackgram is known for its small seeds and takes a longer time to mature compared to other varieties of blackgram.  Vigna mungo var. viridis, on the other hand, refers to a variety of blackgram with green seeds, but it is not specifically associated with being small-seeded or late maturing. Vigna acotinifolia is a different species altogether and is not related to blackgram.