Final answer:
The root-shoot axis of plants is determined by the orientation of the first zygotic cell division. Root and shoot growth are products of primary growth from the activity of apical meristems, resulting in elongation but limited radial expansion of these structures.
Step-by-step explanation:
The root-shoot axis of plants is an interesting aspect of plant development and structure. It is determined by the embryonic axis which consists of the plumule, the radicle, and the hypocotyl. Root growth begins with seed germination, where the radicle of the embryo forms the root system. The root tip, protected by the root cap, undergoes a series of changes in the zones of cell division, elongation, and maturation and differentiation. These zones are all part of primary growth, which is the result of cell division in the apical meristem, contributing to the lengthening of the root-shoot axis.
Shoot growth is also a result of apical meristem activity, which is located at the tip of the shoot. As these cells divide and expand, the plant elongates. This type of growth leads to plants getting longer and bushier with branch roots and shoots emerging. However, the width of shoots produced by primary growth is limited since cells do not expand significantly in the radial direction.
Therefore, the correct answer is: A. is determined by the orientation of the first zygotic cell division, as this initial division sets the stage for the subsequent establishment of the root-shoot axis and the development of apical meristems at the tips of the embryonic root and shoot.