Here's why:

* Meristems are regions of undifferentiated cells that have the ability to divide and differentiate into various types of cells.

* This constant cell division allows plants to grow indefinitely, both in height (apical meristems) and width (lateral meristems).

* Animals do not have meristems, which is why they reach a certain size and then stop growing.

Therefore, meristems are essential for the continued growth and development of plants, allowing them to increase in size and complexity throughout their lifespan.