What characteristic do grass and grass-like plants share?

Grass and grass-like plants are characterized by having parallel veins in their leaves. This vein structure is a key identifying feature of monocots, the class of flowering plants to which grasses belong. In contrast to dicots, which typically have leaves with a network of branching veins (or net-veined leaves), monocots exhibit a simpler vein pattern that runs parallel along the length of the leaf. This adaptation can influence the plant's growth and overall morphology, making it easier for grasses to thrive in various environments, particularly grasslands and prairies.

The other characteristics, while they may apply to some plants in various contexts, do not define grass and grass-like plants specifically. For instance, net-veined leaves are characteristic of dicots rather than monocots. Growing in clusters may apply to some species but isn't a defining feature of all grasses. Lastly, while many grass species do bloom in the summer, others may bloom at different times depending on their environmental adaptations. Therefore, the presence of parallel veins is a fundamental characteristic that clearly sets grasses apart from other types of plants.