Final answer:
Corn plants, with a single cotyledon and parallel leaf veins, are monocots, whereas tomato plants, with two cotyledons, are dicots.
Step-by-step explanation:
Based on the information provided about cotyledons, we can determine the classification of corn and tomato plants. A corn plant is a monocot because it produces a seedling with one cotyledon, shows parallel veins in its leaves, and has specialized structures such as a scutellum which channels nutrition to the growing embryo.
On the other hand, a tomato plant is a dicot because it produces a seedling with two cotyledons, and its cotyledons serve to absorb nutritive substances released from the endosperm during germination. Therefore, the correct answer is: A corn plant is a monocot; a tomato plant is a dicot.