Here's why:

* Transport of substances: Both systems are responsible for transporting essential substances throughout the organism. In plants, the vascular system carries water and nutrients from the roots to the leaves and sugars produced during photosynthesis from the leaves to the rest of the plant. In animals, the circulatory system carries oxygen, nutrients, and hormones throughout the body, and removes waste products.

* Specialized tissues: Both systems are made up of specialized tissues. In plants, xylem and phloem make up the vascular system. In animals, blood vessels and the heart make up the circulatory system.

* Direction of flow: The direction of flow in both systems is driven by pressure differences. In plants, water is pulled upwards by transpiration from the leaves, and sugars are pushed by pressure gradients. In animals, the heart pumps blood through the vessels.

While the specific mechanisms of transport and the structure of the tissues differ between plants and animals, the overall function of transporting essential substances is analogous.