Final answer:
The sideways growth of Billy's plant is likely due to phototropism and gravitropism, where the plant is responding to its light source and gravity. Mutant plants that lack the organelle for sensing gravity may grow in erratic directions, and different plant species have various growth orientations, such as vertical, horizontal, or prostrate forms.
Step-by-step explanation:
The most likely reason for the direction of the plant growth observed by Billy, where the roots and stem were growing sideways, can be attributed to the plant's response to its environment, specifically light and gravity. This phenomenon is known as phototropism and gravitropism.
Plants grow in response to light sources and gravity. If the light source is to one side of the plant, it may cause the plant to grow in that direction, which is known as phototropism. Plants also grow in response to gravity; roots show positive gravitropism by growing downward, while stems show negative gravitropism by growing upward. However, in the absence of appropriate gravitational signals, as indicated by the mutant plant example, roots may grow in all directions due to the lack of the organelle that senses gravity, likely amyloplasts which are involved in gravitropism.
Furthermore, the orientation changes in the growth of plants can vary within a plant and between different species. Some plants have stems that grow upwards, some grow horizontally as rhizomes or stolons, and others exhibit a 'prostrate' growth form where they extend horizontally below the ground before growing up again.