Chrysanthemums are the central symbol of life, vital energies, sexuality in the story. Just like Elisa herself, they need nurturing and attendance. She lives a boring and unfulfilled life in a dull, loveless marriage, and she has never had a chance of finding her true self, except in the garden. That is why she commits to gardening so eagerly: because she couldn't bear to see those flowers withered. At the end of the story, she sees her chrysanthemums on the road, thrown away, "a dark speck" as Steinbeck says. They are wasted, just like Elisa's life, love, and happiness.
This is how to solve the problem:
We know that the growth rate is constant for both species of plants. So we can say that the height is a linear function of the number of days since d = 0.
Assumption (1 month = 30 days)
Species A:
Growth rate = 2 cm/month = 2/30 = 1/15 cm/day
Height on day d = 0: 12 cm
Height: H(d) = 12 + d/15
Species B:
Growth rate = 3 cm/month = 3/30 = 1/10 cm/day
Height on day d = 0: 10 cm
Height: H(d) = 10 + d/10
