Which organisms transform nitrogen to a form that useful to plants

nitrogen-fixing bacteria

Nitrogen-fixing bacteria are a type of organisms that transform nitrogen to a form that is useful to plants. These microorganisms are capable of transforming atmospheric nitrogen into fixed nitrogen.

Explanation:

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.