Nucellus - Ovule
Enclosed within the integuments lies a homogenous mass of tissue which is termed as the nucellus. This is the tissue in which the female gametophyte differentiates and develops. At an early stage of ovule development, usually an archesporial cell or a group of cells differentiate in the hypodermal region of the nucellus. The archesporial cell may function directly as the sporogenous cell. In such situations, the sporogenous cell also remains hypodermal in position and is surrounded by a single-layer of nucellus, in the micropylar region. Such ovules are known as tenuinucellate. These are commonly seen in Sympetalae.
Figure: Tenuinucellate ovule of Elytraria acaulis. In c and d integuments are not shown.