The reader knows that the garden is not real because of the presence of artificial or genetically modified living beings. This foreshadows that Mrs. Swinton's son is also not natural, but a robot.
Explanation:
The question above refers to "Supertoys Last All Summer Long", a story about the utopian future where science has evolved so much as to create artificial or genetically modified beings that are very similar to natural beings and promote the same resources. These beings can be plants, animals and even humans. It is in this sense that we realize that Mrs.'s garden is composed of modified organisms, which makes this garden not, in fact, real. Just like Mrs. Swinton's son, we learn at the end of the story that he is not a real child but a robot.
