The is the center of a stem and can store food.

The pith is the center of a stem and can store food.

Pith is the tissue that is located in the center of the stems of vascular plants. Pith is made up of soft spongy distinctive parenchyma cells which are large but have thin walls. The major function of pith is to convey nutrients to every part of the plant and to store nutrients in its cells.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.