Final answer:
The words "ugly", "overgrown", "pig-weed", and "unsightly vegetation" create a negative and unattractive image of the grass-plot.
Step-by-step explanation:
The effect of the words "ugly", "overgrown", "pig-weed", and "unsightly vegetation" is to portray a negative and unattractive image of the grass-plot described in the passage. The use of these words creates a sense of decay and neglect, suggesting that the area is not well-maintained or aesthetically pleasing. The author's choice of descriptive language helps evoke a vivid image in the reader's mind and sets the tone for the description of the prison and its surroundings.