Final answer:
The energy source for a plant embryo inside a seed is the endosperm, which provides essential nutrients until the seedling can perform photosynthesis. The stored food within the endosperm is crucial for the development of the embryo during its early stages.
Step-by-step explanation:
The energy source for the plant embryo inside the seed is the endosperm. The endosperm is a tissue produced inside the seeds of most of the flowering plants following fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This nutrient-rich tissue serves as a food reservoir for the growing embryo and is essential for its development during germination.
Once the seed germinates, the plant embryo begins to grow, initially relying on this stored food within the endosperm until it is able to produce its own food through photosynthesis. During early development, the seedling does not have the capability to harness energy from sunlight, so it cannot rely on photosynthesis immediately. Instead, the endosperm provides the necessary nutrients for the embryo to establish itself until such a time when it can emerge from the soil and begin to synthesize its own food from sunlight.
Light availability can impact the ability of plants to perform photosynthesis. However, prior to reaching a stage where they can access light, the energy reserves of the endosperm are critical.